Sound System Reference Manual BOSCH

Sound System
Reference Manual
Public Address
Sound Reinforcement
Bosch Security Systems
Sound System Reference Manual
Introduction
Sound systems are used for amplification of speech or music to enhance intelligibility
or loudness by electro-acoustic means in order to serve an audience with a higher
degree of listening comfort.
A public address distribution system is designed primarily to carry live and recorded
messages, signal tones and background music (if required), from several different sources, to
a number of selectable remote areas. Common applications would be: hotels, restaurants,
railway stations, airports, factories, oil platforms, office buildings, schools, shopping areas,
ships, exhibition areas, etc.
A sound reinforcement system would normally be used to reproduce live voice (and often
music) to a number of people who are generally located in the same room or area as the
signal source. Typical applications are churches, lecture halls, political gatherings,
conferences, etc.
Sound system design is a comprehensive subject combining a chain of devices:
microphone, sound processing equipment, amplifier and loudspeaker, together with the
acoustic environment, into a single system.
The microphone converts the acoustical vibrations, caused by an audio source, into an
electrical signal. The processing equipment modifies the signal to compensate for
deficiencies in the source or environment. The amplifier increases the level of the signal to
one adequate for driving loudspeakers. The loudspeakers convert the electrical signal back
into vibrations, which are greatly influenced by the acoustic environment, and in turn
received by the ear of the listener.
This manual is intended to give readers with a technical background a reference to the
various aspects of audio engineering and sound system design.
TABLE OF CONTENTS
SOUND - THE THEORY...................................................................................................................................................... 1
1.0 BASICS............................................................................................................................................................................ 1
1.1 Speech........................................................................................................................................................................ 3
1.1.1 Dynamic Range ..................................................................................................................................................................... 3
1.2 Music ......................................................................................................................................................................... 4
1.2.1 Dynamic Range ..................................................................................................................................................................... 4
1.2.2 Musical Range versus Frequency .......................................................................................................................................... 5
1.3 Sound......................................................................................................................................................................... 6
1.3.1 Ear-characteristics.................................................................................................................................................................. 6
1.3.2 Weighting .............................................................................................................................................................................. 7
1.3.3 Sound Pressure Level............................................................................................................................................................. 7
1.4 Sound Propagation in Air........................................................................................................................................... 8
2.0 DECIBEL NOTATION........................................................................................................................................................ 9
2.1 Definition................................................................................................................................................................... 9
2.1.1
2.1.2
2.1.3
2.1.4
Logarithmic characteristics of the ear.................................................................................................................................... 9
Power ratios ........................................................................................................................................................................... 9
Voltage ratios....................................................................................................................................................................... 10
dB references ....................................................................................................................................................................... 10
2.2 Calculations............................................................................................................................................................. 11
2.2.1 Addition and subtraction...................................................................................................................................................... 11
THE SOUND SYSTEM ....................................................................................................................................................... 12
3.0 AN INTRODUCTION ....................................................................................................................................................... 12
3.1 Functional requirements ......................................................................................................................................... 12
MICROPHONES ................................................................................................................................................................. 13
4.0 MICROPHONES .............................................................................................................................................................. 13
4.1 Considerations when Selecting a Microphone ........................................................................................................ 13
4.2 Microphone Types ................................................................................................................................................... 13
4.2.1
4.2.2
4.2.3
4.2.4
4.2.5
Electrodynamic .................................................................................................................................................................... 13
Condenser ............................................................................................................................................................................ 13
Back Plate Electret............................................................................................................................................................... 13
Electret................................................................................................................................................................................. 13
Choices ................................................................................................................................................................................ 13
4.3 Pick-Up Response Patterns ..................................................................................................................................... 14
4.3.1 Omnidirectional ................................................................................................................................................................... 15
4.3.2 Cardioid ............................................................................................................................................................................... 15
4.3.3 Hyper-cardioid..................................................................................................................................................................... 15
4.4 Special Microphones ............................................................................................................................................... 16
4.4.1 The Lavalier and Lapel microphone .................................................................................................................................... 16
4.4.2 Noise cancelling microphone............................................................................................................................................... 16
4.4.3 Radio (Wireless) microphone .............................................................................................................................................. 16
5.0 TECHNICAL PRINCIPLES ................................................................................................................................................ 17
5.1 Directivity................................................................................................................................................................ 17
5.2 Sensitivity................................................................................................................................................................. 17
5.3 Installation Considerations ..................................................................................................................................... 17
5.3.1 Potential problems and causes ............................................................................................................................................. 17
5.3.2 Solutions .............................................................................................................................................................................. 17
6.0 MICROPHONE TECHNIQUE ............................................................................................................................................ 18
AMPLIFICATION AND PROCESSING ..........................................................................................................................19
7.0 MIXING CONSOLES .......................................................................................................................................................19
8.0 AMPLIFIERS AND PREAMPLIFIERS .................................................................................................................................21
8.1 The Pre-amplifier ....................................................................................................................................................21
8.1.1 Inputs ................................................................................................................................................................................... 21
8.1.2 Tone controls ....................................................................................................................................................................... 21
8.2 The Power Amplifier................................................................................................................................................22
8.3 Amplifier/Loudspeaker Interface.............................................................................................................................22
9.0 EQUALISERS..................................................................................................................................................................25
9.1 Equaliser Types .......................................................................................................................................................25
9.1.1
9.1.2
9.1.3
9.1.4
9.1.5
Basic tone controls ............................................................................................................................................................... 25
Band-pass filters................................................................................................................................................................... 25
Parametric equaliser............................................................................................................................................................. 26
Parametric triple Q-filter...................................................................................................................................................... 26
Graphic equaliser ................................................................................................................................................................. 26
9.2 Equalisation.............................................................................................................................................................27
9.2.1
9.2.2
9.2.3
9.2.4
9.2.5
9.2.6
9.2.7
Introduction.......................................................................................................................................................................... 27
The acoustic feedback loop.................................................................................................................................................. 27
Resonant acoustic feedback ................................................................................................................................................. 27
Principles of equalisation ..................................................................................................................................................... 28
Loop equalisation................................................................................................................................................................. 29
Loudspeaker equalisation.................................................................................................................................................... 29
Loudspeaker equalisation & Loop equalisation ................................................................................................................. 29
10.0 TIME DELAY ...............................................................................................................................................................30
11.0 COMPRESSOR/LIMITER ...............................................................................................................................................31
12.0 AUTOMATIC VOLUME CONTROL.................................................................................................................................32
13.0 TECHNICAL CONSIDERATIONS ....................................................................................................................................33
13.1 Specifications.........................................................................................................................................................33
13.1.1
13.1.2
13.1.3
13.1.4
13.1.5
13.1.6
Frequency Response .......................................................................................................................................................... 33
Power bandwidth................................................................................................................................................................ 33
Linear distortion................................................................................................................................................................. 33
Non linear distortion or clipping (THD) ............................................................................................................................ 33
Rated Output Power ........................................................................................................................................................... 34
Temperature Limited Output Power (TLOP) ..................................................................................................................... 34
13.2 Adjusting signal levels in a system chain. .............................................................................................................35
HARDWARE INSTALLATION ........................................................................................................................................36
14.0 GROUNDING AND SCREENING .....................................................................................................................................36
14.1 Earthing (grounding).............................................................................................................................................36
14.1.1 Safety and system earth’s................................................................................................................................................... 36
14.1.2 Earth (ground) loops .......................................................................................................................................................... 36
14.1.3 Microphone Earth Loops.................................................................................................................................................... 38
14.2 Radio and Mains Born Interference ......................................................................................................................39
14.2.1
14.2.2
14.2.3
14.2.4
Prevention of Interference.................................................................................................................................................. 39
Interference introduced via cables...................................................................................................................................... 40
Interference introduced inside rack unit............................................................................................................................. 40
Interference induced from 100 V loudspeaker wiring........................................................................................................ 40
14.3 Nineteen inch rack units ........................................................................................................................................41
LOUDSPEAKERS ...............................................................................................................................................................42
15.0 LOUDSPEAKERS ...........................................................................................................................................................42
15.1 Loudspeaker Types ................................................................................................................................................42
15.1.1
15.1.2
15.1.3
15.1.4
15.1.5
Standard loudspeaker cabinets ........................................................................................................................................... 42
Ceiling loudspeakers .......................................................................................................................................................... 43
Sound columns................................................................................................................................................................... 44
Horn loudspeakers.............................................................................................................................................................. 45
Full range high power loudspeakers................................................................................................................................... 45
15.2 Matching Loudspeakers to Amplifiers ...................................................................................................................46
16.0 TECHNICAL PRINCIPLES ...............................................................................................................................................47
16.1 Basic Principles.....................................................................................................................................................47
16.2 Detailed Considerations ........................................................................................................................................48
16.2.1
16.2.2
16.2.3
16.2.4
Resonant frequency............................................................................................................................................................ 48
Sensitivity .......................................................................................................................................................................... 48
Efficiency........................................................................................................................................................................... 48
Directivity (Q).................................................................................................................................................................... 49
THE ACOUSTIC ENVIRONMENT.................................................................................................................................. 51
17.0 OUTDOORS ................................................................................................................................................................. 51
17.1 Technical Considerations...................................................................................................................................... 52
17.1.1
17.1.2
17.1.3
17.1.4
17.1.5
17.1.6
17.1.7
Power ................................................................................................................................................................................. 52
Directivity .......................................................................................................................................................................... 53
Attenuation due to Distance............................................................................................................................................... 53
Variations of both distance and power............................................................................................................................... 54
Refraction .......................................................................................................................................................................... 54
Reflection........................................................................................................................................................................... 54
Ambient Noise ................................................................................................................................................................... 54
18.0 INDOORS ..................................................................................................................................................................... 55
18.1 Technical Considerations...................................................................................................................................... 55
18.1.1
18.1.2
18.1.3
18.1.4
18.1.5
18.1.6
18.1.7
18.1.8
18.1.9
Reflection & Absorption.................................................................................................................................................... 55
Reverberation..................................................................................................................................................................... 56
Reverberation time............................................................................................................................................................. 56
Calculation of Direct and Indirect Sound Fields................................................................................................................ 58
Calculation of Reverberant Sound Fields .......................................................................................................................... 59
Calculation of the early / late ratio..................................................................................................................................... 59
Speech Transmission Index (STI & RASTI) .................................................................................................................... 61
Subjective %ALcons and RASTI requirements................................................................................................................. 63
Converting RASTI to %ALcons ........................................................................................................................................ 63
19.0 DESIGNING FOR THE ACOUSTIC ENVIRONMENT ......................................................................................................... 64
19.1 Loudspeaker Placement and Coverage ................................................................................................................. 64
19.2 Summary of the Loudspeaker-design .................................................................................................................... 65
19.3 Speech intelligibility in churches & community halls ........................................................................................... 66
19.3.1
19.3.2
19.3.3
19.3.4
19.3.5
19.3.6
19.3.7
19.4
Small reverberant traditional church building.................................................................................................................... 66
Large reverberant monumental cathedral........................................................................................................................... 67
Small low ceiling auditorium............................................................................................................................................. 67
Large high ceiling auditorium............................................................................................................................................ 67
Wide low ceiling auditorium.............................................................................................................................................. 67
The total (church) sound system chain............................................................................................................................... 68
Predicting & calculating the performance of the church system........................................................................................ 68
System Calculation ............................................................................................................................................. 69
19.4.1 High ceiling area e.g. Large exhibition hall...................................................................................................................... 69
19.5.2 Sound Reinforcement System Calculation & EASE.......................................................................................................... 70
20.0 APPENDIX .................................................................................................................................................................. 73
20.1
20.2
20.3
20.4
DEFINITIONS ............................................................................................................................................................... 73
SYMBOLS AND UNITS ................................................................................................................................................. 74
EQUATIONS................................................................................................................................................................. 75
SURFACE MATERIAL LIST WITH ABSORPTION COEFFICIENTS ....................................................................................... 77
21.0 E A S E SOFTWARE................................................................................................................................................. 80
22.0 MEASURING EQUIPMENT .................................................................................................................................... 82
22.1
22.2
22.3
22.4
22.5
MLSSA - TRANSFER ANALYZER ................................................................................................................................ 82
NEUTRIK AUDIOGRAPH 3300..................................................................................................................................... 82
B & K SPEECH TRANSMISSION METER 3361 .............................................................................................................. 82
GOLD LINE AUDIO SPECTRUM ANALYZER DSP30..................................................................................................... 82
MLSSA - TRANSFER ANALYZER ................................................................................................................................ 83
Sound - The Theory
1.0 Basics
In order to design an efficient, effective and useful audio system, it is helpful to have a grasp of the way sound is
received, processed, transmitted, and perceived by the listener. Sound waves are generated by air particles
being set in motion by physical movement (such as the bow being drawn across a violin, the hammer hitting a
piano wire or a vibrating cone of a loudspeaker, etc.). Once the particles start moving, they begin a chain
reaction with other particles next to them. In this way, a movement of air is transmitted in all directions, by
expanding and compressing air. So sound is energy that is transmitted by pressure waves in air.
We can make a distinction in the waveforms:
1. Spherical waves originated from an in all directions radiating point source.
2. Cylindrical waves originated from a line source.
3. Plane waves originated from a plane source.
Spherical
( Sphere)
Single Speaker
Cylindrical
(Cylinder)
Array of speakers
1
Attenuation due to distance
2
acc. square of distance = r
1
acc. distance
=r
no attenuation
= r0
Plane
(Plane)
Multiple speakers
On relative long distances from the sound source (r >> source dimensions >> wavelength), we apply generally
the spherical wave attenuation r2 but regard the waveform as a plane wave. The justification can be seen in
above picture where the sound is moving in jumps of e.g. 1m. The total sound power (W) is in all cases the
same but the sound pressure (N/m2) or sound intensity (W/m2) is decreasing with r2.
Sound, as we refer to in this manual, generally consists of speech, music, alarm signals or attention tones.
Any kind of transmission or registration of sound, when converted into electrical signals, imposes a limitation on
the dynamic range, frequency response, intelligibility and natural quality. The limitation of the dynamic range is
the most prominent and the most important.
Dependent on our terms of reference dynamic range has different meanings:
1. In acoustics, the quietest sound level to the loudest one.
2. In music, the difference between pianissimo and fortissimo.
3. In sound engineering we prefer to express it as the difference between the maximum incidental peak
value and the minimum value of the converted electrical signal.
With different measuring instruments (oscilloscope, level meter, VU-meter, etc.) we can analyse the signal, but
dependent on the applied instrument’s characteristics (integration time) different levels will be shown.
Sparks =
= Integr. time 0 ms = Oscilloscope with memory screen
Peak
=
= Integr. time < 5 ms = Peak level meter (e.g. on mixing desks)
Fast
= Short Time Average = Integr. time 125 ms = Sound level meter (SLM)
VU
=
= Integr. time 270 ms = VU-meter (e.g. on amplifiers)
Slow
=
= Integr. time
4 s = Sound level meter (SLM)
LTA
= Long Time Average = Integr. time
30 s = Used for Amplifier cooling design
2
1.1 SPEECH
Speech consists of words and pauses. Words contain both vowels and consonants. Speech has loudness
variation and frequency variation. Dependent on the voice strength the frequency spectrum (which is the lowest
bass sound through to the highest treble one) is changing according the diagram. The lines in the graph
represent the average level per 1/3 octave.
Let's look at loudness first. The
vowels in a sentence have a
frequency spectrum below 1000Hz,
and they create the impression of
loudness. The human mouth
producing these sounds does so with
a wide opening angle and in indoor
situations can hit hard surfaces within
range like walls and ceiling etc, so
easily causing reverberation.
Male Speech Spectrum
80
Sound Pressure Level
dB
SHOUT
70
LOUD
60
RAISED
50
NORMAL
CASUAL
40
30
20
63
125
250
500
1k
2k
4k
8k
16kHz
Contribution to Intelligibility
5%
13%
20%
31%
26%
5%
7%
22%
46%
20%
3%
2%
Contribution to Speech Power
The consonants of the words in a
sentence, having a frequency
spectrum above 1000 Hz, provide the
articulation. The human mouth
produces these sounds with a narrow
opening angle and, because of this,
is rather directional.
Our principle aim is to deliver this
complete speech spectrum to the
listener’s ears as unchanged as
The loudness of speech as it relates to frequency.
possible. Unfortunately various
acoustical phenomena, which are discussed throughout this book, play their part in altering the speech
spectrum, at times making it impossible for listeners to understand what is being said.
Because of this certain techniques are employed to compensate for these phenomena in order to make the
speech intelligible.
1.1.1 Dynamic Range
The accompanying graphs show the speech pattern, versus time, of a trained announcer speaking at a fixed
distance from the microphone, measured using different instruments.
100
dB
Speech (male)
1
90
80
3
70
2
60
10
20
30
40
50
60
70
80
90
100 secs.
Curve 1: peak value, rise time 1 msec. decay time 2.7 sec.
Curve 2: r.m.s. value, integration time 270 msec.
Curve 3: r.m.s. value, integration time 30 sec LTA.
3
1.2 MUSIC
As with any sound transmission, the two most prominent features of music reproduction are the dynamic range
and the frequency response. If the dynamic range is limited, the music will appear emotionally flat, lacking both
subtlety and excitement.
If the frequency response is limited at the lower frequencies, the music will lack the depth to reproduce bass
instruments fully. If it is limited at the higher frequencies, harmonics, which are vital for instrument recognition,
will not be fully present, causing the music to sound dull.
1.2.1 Dynamic Range
The accompanying graphs show comparative dynamic ranges of different styles of music and speech,
measured simultaneously, showing: 1. Peak meter level. 2. VU meter level. 3. LTA level.
It can be seen that there is an average of 14 dB difference between the peak and VU level. Distortion during
very short peaks is almost inaudible, so in practice 6 dB peak clipping is permissible. Therefore 0 VU or100%
on a VU meter should correspond with a headroom of 8 dB under the distortion limit of the equipment.
100
Symphony no. 5 (Beethoven)
dB
1
90
2
80
3
70
60
10
100
dB
20
30
40
50
60
70
80
90
100 secs.
Speech (male)
1
90
80
3
70
2
60
10
100
dB
20
30
40
50
60
70
80
90
100 secs.
Military Band
1
90
2
80
3
70
60
10
20
30
40
50
60
70
80
90
100 secs.
Curve 1: peak value, rise time 1 ms, decay time 2.7 s.
Curve 2: r.m.s. value, integration time 270 ms.
Curve 3: r.m.s. value, integration time 30 s LTA.
4
1.2.2 Musical Range versus Frequency
63
63
80 100 125 160 200 250 315 400 500 630 800 1k
125
250
500
1k
0.34
50
0.68
40
1.36
31
2.7
Octave bands used
for sound
measurement
1K 1K6 2K 2K5 3K2 4k
25
2K
4k
5K 6K3 8k
10k
8k
Note
A B C D E F GA B C D E F GA B C D E F GA B C D E F GA B C D E F GA B C D E F GA B C D E F GA B C
0.043
0.085
Frequency (Hz)
to nearest 1.0
28
31
33
37
41
44
49
55
62
65
73
82
87
98
110
123
131
147
165
175
196
220
247
262
294
330
349
392
440
494
523
587
659
698
784
880
987
1047
1175
1318
1397
1568
1760
1974
2093
2350
3637
2794
3136
3520
3949
4186
Octave centres
0.17
Wavelength (m)
5.4
10.8
Third-octave
centres
Piano keyboard
(equal temperament)
FREQUENCY
25
31
40
50
63
80 100 125 160 200 250 315 400 500 630 800
1k
1K 1K6 2K 2K5 3K2 4k
25
5K 6K3 8k
10k
25
31
40
50
63
80 100 125 160 200 250 315 400 500 630 800
1k
1K 1K6 2K 2K5 3K2 4k
25
5K 6K3 8k
10k
VOCAL
Soprano
Contralto
Baritone
Bass
WOODWIND
Piccolo
Flute
Oboe
Clarinet (B flat or A)
Clarinet (E flat)
Bass Clarinet
Basset Horn
Cor Anglais
Bassoon
Double Bassoon
BRASS
Soprano Saxophone
Alto Saxophone
Tenor Saxophone
Baritone Saxophone
Bass Saxophone
Trumpet (C)
Trumpet (F)
Alto Trombone
Tenor Trombone
Bass Trombone
Tuba
Valve Horn
STRINGS
Violin
Viola
Cello
Double Bass
Guitar
KEYBOARDS
Pianoforte
Organ
PERCUSSION
Celeste
Timpani
Glockenspeil
Xylophone
FREQUENCY
5
1.3 SOUND
Sound is a series of vibrations compressing and rarefying the air. Loudness is the subjective experience of
sound level. Since, when we measure sound, we refer to changes in air pressure, a reference related to
pressure must be used.
The reference used is the level of sound at 1 kHz, which is barely perceptible to people with normal hearing,
being the quietest sound pressure that an average person can hear. This is called the 'threshold of hearing'.
which at 1 kHz is: 20 µN/m2 = 20 µPa = 2 x 10-5 Pa. (Pa = Pascal = N/m2). Sound Pressure related to this
reference level is expressed in dB (SPL).
As the sound pressure level is increased, a point is finally reached, just short of being painful to the ear, called
the 'threshold of pain', which, using the 1 kHz reference frequency, corresponds to 20 Pa.
Since the 0 dB (SPL) absolute reference is 20 µPa;
20 Pa
≅
20
20 log _______
2 x 10-5
≅ 120 dB (SPL)
1.3.1 Ear-characteristics
As is shown in the accompanying
graph, the Sound Pressure Level at
130
dB
the threshold of hearing varies with
(SPL)
Threshold of Pain
120
frequency. Because of this it would
require 60 dB (SPL) at 30 Hz to
110
produce the same impression of
loudness as 0 dB (SPL) at 1 kHz.
100
The threshold of hearing represents
the bottom limit of a series of ‘equal
90
loudness’ contours, which are also
shown. In studying the graph, we
80
notice two important factors.
70
Firstly, very much more
energy is needed to produce a bass
60
signal of a given loudness, when
compared with a 2 or 3 kHz signal 50
an important consideration in any
system used to reproduce music.
40
The second point is that if
30
noise, having a broad frequency
spectrum, with a level of, say, 20 dB
20
(SPL) is reproduced, the listener will
have an impression which corres10
ponds with the mirror image of the
Threshold of Hearing
20 dB equal loudness contour.
0
If the noise level is now raised a
different impression of the same
20
63
125
250
500
1k
2k
4k
8k
16k 20kHz
noise is received. This is due to the
Frequency
ear responding to the noise
Equal loudness contours
according to the changing curves of
equal loudness. In other words, all frequencies are present in the signal but depending on the level, they will be
heard in different relationships.
6
1.3.2 Weighting
In order to imitate this characteristic of the ear, a sound level meter often incorporates different filter curves
which corresponds with this subjective hearing. There are three types of curves internationally standardised
and they are called A-, B-, and C- weighting.
dB
A-WEIGHTING
10
0
-10
-20
-30
-40
80
125
100
160
200
250
315
500
630
800
400
1k
1.25
1.6
2k
2.5
4k
3.15
8k
5
6.3
10
Hz
12.5
A-curve
This weighting should theoretically be used only for measurements below 40 dB (SPL).
Many simple sound level meters though are equipped with an A-curve filter only, and nowadays the majority
of acoustic measurements are taken solely with A-weighting.
This is designated dBA (SPL).
1.3.3 Sound Pressure Level
The accompanying chart shows the sound pressures in dB(SPL), for several common sound sources.
threshold of hearing
threshold of pain
woods
dB 0
10
20
library
30
ticking
clocks
40
office
50
tearing
paper
60
70
conversation
7
traffic
80
pneumatic
drill
90
shouting
100
110
rock
band
120
130
jet
aircraft
(100m)
140
1.4 SOUND PROPAGATION IN AIR
Sound could most simply be defined as a series of vibrations compressing and thinning the air. To be
transmitted, sound relies on a vibrating object (vocal chords, loudspeaker, breaking window, etc.) which imparts
its motion to surrounding molecules or particles.
Important physical parameters, which influence the propagation of sound in air are: f = frequency (Hz),
v = velocity (m/s), λ = wavelength (m), p = pressure (Pa), T = temperature (K).
Velocity of sound
The velocity of sound is determined mainly by the temperature.
For normal conditions, in air, the velocity may be calculated by:
__
v = 20 √ T
where T is the temperature in Kelvin (0°C = 273 K)
This means that at 20°C
___
v = 20 √293
= 342,3
≅ 340 m/s
The relationship between frequency, wavelength and velocity is given by:
λ = v/f
Using these equations, it is seen that at 1 kHz at 20°C, the wavelength is
340
_____
λ =
= 0,340m
1000
Air Absorption
Listening to sound on distance makes us aware of a frequency dependant attenuation due to air-absorption,
the higher the frequency the more attenuation. This attenuation for a frequency of 500 Hz equals 0.3 dB per
100 metre, for 2000 Hz equals 1 dB per 100 metre and for 8000 Hz equals 7 dB per 100 metre (RH=70%).
Because the humidity effects the amount of water molecules in the air, also the attenuation of a sound signal is
effected. This means that a relative humidity (RH) of 20% attenuates a 4 kHz signal by 0.09 dB/meter, whilst a
relative humidity of 80% attenuates a 4 kHz signal by 0.02 dB/meter. The humidity level should certainly not be
discounted since its effect can be quite dramatic.
Reverberation time
The effect of reverberation time (RT60 ) in a room with volume (V) and surface (S):
RT60 = 0,161 V/(αS + 4mV)
with:
α = average absorption coefficient
m = attenuation constant (m-1 )
Room : 100x100x10 m
α = 0,1 V = 100.000 m3
S = 24000 m2
0
Relative Humidity = 60%
Temperature 20 C
Freq.
m [1-2]
4mV
RT60 ||
m [3]
m[3]
RT60
(m2)
(s)
||
RH=60%
RH=20%
RH=20%
(Hz)
(10-3 m-1)
------------------------------------------------------------------------------------------------------------------------------------125
0,12
48
6,62 ||
0,07
0,10
6,60
250
0,28
112
6,41 ||
0,15
0,23
6,46
500
0,51
204
6,18 ||
0,37
0,56
6,14
1000
0,78
312
5,94 ||
0,91
1,39
5,45
2000
1,49
596
5,37 ||
2,25
4,28
3,92
4000
4,34
1736
3.89 ||
5,6
14,5
1,96
8000
16
6400
1,83 ||
16,2
47,1
0,76
-------------------------------------------------------------------------------------------------------------------------------------
Example:
References:
[1] Room Acoustics (1991) , Kuttruff. [2] Handbook of Chemistry and Physics (1973)
[3] Absorption of Sound in Air versus RH and T (1967), Cyril Harris.
8
2.0 Decibel Notation
2.1 DEFINITION
The use of the decibel (dB) notation system is common in sound and communications work. This
system allows meaningful scale compression or expansion as required and greatly simplifies
computations involving large quantities.
Our human senses - touch, sight, hearing, sense of weight, etc. - all function logarithmically.
That is, in the presence of a stimulus the least perceptible change is proportional to the already
existing stimulus (Weber-Fechner law).
2.1.1 Logarithmic characteristics of the ear
Sine
wave
generator
To evaluate the ear’s behaviour in
respect to sensitivity for level
differences, we can experiment as
follows:
The diagram shows two identical
amplifiers and loudspeakers with a
signal generator switched to one then to
the other alternately. Initially the same
power is supplied to each loudspeaker,
e.g. 100 mW, and because of this both
their signals are of equal loudness.
mW
Stereo amplifier
mW
As the power to one of the loudspeakers is slightly increased no difference in loudness will be heard, whilst
continuing to listen to them one at a time. Only when one loudspeaker receives 26% more power it will sound
noticeable louder. At this point e.g. 126 mW is being fed to one loudspeaker and 100 mW to the other.
If the power of the other loudspeaker is also increased to 126 mW, the intensities will again be equal. If the
power to the first loudspeaker is once again increased, no noticeable difference will be heard until it receives
26% more power (26% of 126 mW = 32 mW), which brings the higher loudspeaker output to 126 + 32 = 158
mW. In this way, the noticeable increase in loudness is obtained by raising the level in a given ratio, not
by adding specific amounts of power. Increasing power in ten stages of 26% brings it to ten times its original
level. This is a logarithm increase not a linear increase.
A power increase of a factor 10 is one Bel, with each power increase of 26% being one tenth of a Bel and called
a decibel (dB). It must be appreciated that the dB is only a ratio, and that the ear hears the same difference
between 1 W and 2 W as between 100 W and 200 W.
2.1.2 Power ratios
The Bel is defined as: Log P1/P2 , so the decibel (dB) is defined as: 10 Log P1/P2.
1
2
3
1 2 3 4
0
power ratio
10
4 5 6 7 89
6 7 8 9
5
2
3
11 12 13 14
10
100
4 5 6 7 89
16 17 18 19
15
dB
1000
2
3
21 22 23 24
20
26 27 28 29
25
9
4 5 6 7 89
30
The amplification power ratio,
expressed in dB, is given in the
accompanying table. This shows, for
instance, that 3 dB amplification
doubles the power, and that a 100
times increase in power gives 20 dB
amplification.
2.1.3 Voltage ratios
When 10V is connected to a 10 Ω resistor:
I = U/R = 1A
Power dissipated (P) = U x I = 10 W.
When the voltage is doubled and still connected to the 10 Ω resistor:
I = U / R = 2A
P = U x I = 40W, i.e. 4 times increase in power.
This shows that, in this case, doubling the voltage results in a quadrupled power; or to put it another way, a
doubling of the power (3 dB increase) will not result in a doubling of voltage.
Since power is dissipated in the same resistor:
U12
U12/R
Ratio (in dB) = 10 Log P1/P2
= 10 Log _____
= 10 Log ____
= 20 Log U1/U2
U22
U22/R
Because 10 Log power ratio = 20 Log voltage ratio, a gain of 3 dB gives a 2 x power gain, but only a
1.4 x voltage gain. In the same way, a 6 dB gain results in a 4 x power gain but only a 2 x voltage gain.
1
10
2
3
2 4 6 8
0
4 5 6 7 89
12 14 16 16
10
voltage ratio
2
3
22 24 26 28
20
100
4 5 6 7 89
2
32 34 36 38
30
dB
1000
3
42 44 46 48
40
4 5 6 7 89
52 54 56 58
50
60
From this it can be seen that an
amplifier (or attenuator) having a
particular gain expressed in dB has
a different multiplicative effect
dependent on whether power gain
or voltage gain is being considered.
2.1.4 dB references
Though the decibel is only a ratio, it can be used to express absolute values if there is a given reference.
If, for example, a reference of 1 W is chosen, then 3 dB corresponds to 2 W, and 6 dB to 4 W and so on.
dBm - dBu
One of the common references used in the past, due to its frequent application in Telecommunications is
1 milliwatt (mW) across 600 Ohms, expressed as "dBm". (1 mW across 600 ohms = 775 mV.)
In practice however the resistance value is frequently ignored when dBm is quoted and the reference is
775 mV only, this makes this reference incorrect. In fact the dBu is referred to the 775 mV regardless of the
impedance and is still commonly in use in studio engineering.
dBV
This is the favourite and common reference for electrical engineering. The reference is 1 Volt regardless of the
impedance. Corresponding dB values are measured in dBV (e.g. 20 dBV = 10V).
dB(SPL)
There is another reference, which is used in the measurement of sound pressure levels. As we know sound is
basically a series of vibrations compressing and rarefying the air. Since, when we measure sound, we refer to
changes in air pressure, a reference related to pressure (the Sound Pressure Level) must be used.
The reference used is the level of sound , which is barely perceptible to people with normal hearing, being the
quietest sound pressure that an average person can hear at 1 kHz. This is called the 'threshold of hearing'.
At this point the threshold of hearing is very low: 20 µN/m2 = 20 µPa = 2 x 10-5 Pa. (Pa = Pascal = N/m2)
So Sound Pressure related to this reference level is expressed in dB (SPL).
As the sound pressure level is increased, a point is finally reached, just short of being painful to the ear, called
the 'threshold of pain', which, using the 1 kHz reference frequency, corresponds to 20 Pa.
Since the 0 dB (SPL) absolute reference is 20 µPa;
20
20 Pa ≅
20 Log _______
≅ 120 dB (SPL)
2 x 10-5
Other important levels are:
0,1 Pa ≅ 74 dB (SPL), and 1 Pa
≅ 94 dB (SPL)
10
2.2 CALCULATIONS
2.2.1 Addition and subtraction
When adding two unrelated sound sources, only their intensities (energy) should be added together.
Ls = 10 Log [10 L1/10 + 10 L2/10]
Two different noise sources both producing 90 dB (SPL) would be experienced as:
Ls = 10 Log [109 + 109] = 93 dB (SPL)
When subtracting two unrelated sound sources, only their intensities (energy) should be subtracted:
Ls = 10 Log [10 L1/10 - 10 L2/10]
The following graph shows how to add or subtract levels in dB's for non-related signals.
0
1
3
Nu
me
ric
2
al
Numerical difference between
total and large levels - decibels
3
dif
fer
en
4
ce
5
2
be
tw
ee
6
nt
wo
lev
els
7
8
be
ing
9
1.2
ad
de
10
1
d-
de
11
cib
els
12
0.6
13
0
3
4
5
6
7
8
9
10
11
12
13
To add levels of non-related signals.
Enter the chart using the numerical difference
between the two signal levels being added
(top right of chart). Follow the line
corresponding to this value until it meets the
curved line, then move left. The figure shown
on the vertical scale at the left of the chart is
the numerical difference between the total
and larger of the two signal levels. Add this
value to the larger signal level to determine
the total.
Example: Combine a 75 dB signal with one of
80 dB. The difference between these figures
is 5 dB. The 5 dB line intersects the curved
line at 1.2 dB on the vertical scale. This
means that the total value is 80 + 1.2, or
81.2 dB.
Numerical difference between total and smaller levels - decibels
To subtract levels of non-related signals.
If the numerical difference between the total and the smaller of the two levels is between 3 and 14 dB, enter
the chart from the bottom. Using the numerical difference, follow the line corresponding to this value until it
intersects the curved line, then follow the line to the left. The figure shown on the vertical scale at the left of
the chart is the numerical difference between the total and the unknown (the larger) level. Subtract this value
from the total to determine the unknown level.
Example: Subtract 81 dB from the 90 dB total. The difference is 9 dB. The 9 dB vertical line intersects the
curved line at 0.6. Deducted from 90 dB total, this leaves 89.4 dB.
If the numerical difference between the total and the larger of the two signal levels is less than 3 dB, enter
the chart from the left side. Then, at the intersection with the curved line, follow the line down to find the
numerical difference between the total and the smaller level.
11
The Sound System
3.0 An Introduction
When assessing the requirements of any sound system it is important to have a firm grasp of what tasks the
system will need to perform. Along with this, the acoustic environment will determine, to a great degree, what
equipment should be specified. It is vital therefore to clearly understand the characteristics of the equipment
available to meet these various needs.
This section contains a description of the basic components of the sound system, along with some technical
specifications and, at times, advice on the techniques involved in installing and using the equipment.
In certain applications, for example a small church needing only speech amplification, we can reduce the
equipment needed to a few microphones, one mixing amplifier and a few loudspeaker columns. The individual
microphone volume levels would be controlled on the amplifier, which also allows tone-control of the
loudspeakers. Once carefully set up, such a system should work without intervention, every time the amplifier is
switched on. Other situations, for example an oil platform, require both sophisticated routing and switching
systems, and a complete fail-safe redundancy backup system. Obviously, even though the sound quality should
always be adequate, the complexity of calculating the type and quantity of equipment required depends upon
the installation's requirements.
3.1 FUNCTIONAL REQUIREMENTS
Before starting to design a sound system it is vital to answer the following questions:
• Is the system required for speech alone, speech & music or music alone?
• Is the system required for announcements and/or for emergency purposes?
• How many calls must be made, at the same time, to different destinations?
• How many different music sources must be routed?
• What are the maximum and minimum ambient noise levels?
• What is the requirement in respect to loudness?
• What is the requirement in respect to speech intelligibility?
• What is the requirement in respect to annoyance due to excessive loudness?
• What is the requirement in respect to frequency response?
• What is the requirement in respect to sound orientation?
12
Microphone
s
4.0 Microphones
4.1 CONSIDERATIONS WHEN SELECTING A MICROPHONE
In any sound amplification chain, the first link is often the microphone, which converts acoustic vibrations into
voltage variations. Three types of element are generally encountered in microphones used in a professional
audio installation, Electrodynamic, Condenser, and Electret. The way an element is mounted in the microphone
body determines the microphone's pick-up response pattern.
4.2 MICROPHONE TYPES
4.2.1 Electrodynamic
The Dynamic microphone is based on the
principle of a coil moving in a magnetic field.
permanent
magnet
a.f. output
diaphragm
moving
coil
Sound pressure causes the diaphragm to
respond in rhythm with sound vibrations, so that
the coil moves inside the air gap of a permanent
magnetic field. This, in turn, induces a voltage in
the coil. The pitch and intensity of the original
vibrations determine the frequency and amplitude
of this voltage. This means that the higher the
frequency - the faster the coil moves, the louder
the sound - the further the coil moves.
4.2.2 Condenser
The basic elements of the Condenser microphone are a thin metal flexible diaphragm, which forms one plate
of a capacitor, whilst a solid metal plate forms the other.
The capacitance depends on the distance between the diaphragm and the plate. As the diaphragm moves,
the distance between the diaphragm and the plate varies, which causes the capacitance to change accordingly.
A steady D.C. polarising charge is maintained across the diaphragm and the plate. As the sound varies, this
causes the capacitance to vary, which in turn causes the voltage to vary, causing the subsequent current flow to
vary. A DC voltage, supplied by the mixing console or pre-amplifier unit, is carried on the microphone's standard
two core screened signal cable, and is called Phantom Powering. This provides the polarising charge and also
power for the microphone's FET amplifier.
4.2.3 Back Plate Electret
Though operating in a similar way to condenser microphones, the Back Plate Electret (BPE) range of
microphones feature a unique design. It is a combination of an uncharged, temperature independent,
diaphragm and a permanently charged back plate electrode (which is achieved by sealing electret material
onto a metal back plate).
4.2.4 Electret
Similar in operation to a condenser microphone, the diaphragm of the Electret microphone comprises a high
polymer plastic film with a permanent electrostatic charge.
4.2.5 Choices
Because the microphone is such a fundamental part of the amplification chain, great care should be taken when
making a choice. Normally a compromise must be made between reproduction quality and price, but it is wiser
to economise on other equipment than on microphones.
13
Until recently condenser microphones have been used primarily in recording and broadcast studios, and
rarely in public address systems. Having excellent reproductive qualities, condenser microphones tend to be
comparatively expensive, in some cases fragile, and generally require a fairly powerful phantom power supply.
Like condenser microphones, BPE microphones require a supply voltage, but because they do not need a
polarising charge, the current consumption is so low that up to four microphones can be powered by a single
IEC268-15A (DIN4559-6) standard phantom powered input. BPE microphones have very good speech
reproduction qualities, are rugged, and have low sensitivity to case noise, vibrations and hum fields.
The small FET amplifier contained within Electret microphones is often battery driven in consumer quality
models, and phantom powered in professional models. The current drain is so small that battery life is usually
several thousand hours. Though reproduction quality is lower than BPE microphones, the somewhat lower price
makes them a viable alternative to dynamic microphones.
Until recently Dynamic microphones were the most popular for general use, requiring no phantom powering,
being generally very rugged, and normally the least expensive. The lower sensitivity and, (in the case of less
expensive models) low reproduction quality, mean that particular care should be taken when selecting dynamic
microphones.
4.3 PICK-UP RESPONSE PATTERNS
The microphone shown in the accompanying illustration is sensitive to sound from any direction, responding
to a voice from the front in just the same way as to the sound from the audience at the rear.
The force on the diaphragm is determined by the
difference in pressure on its front and rear surfaces.
Because the back of the element is totally sealed, the
sound pressure variation leads directly to movement of the
diaphragm, irrespective of which direction the microphone
is facing.
FRONT
Because it is responsive to sound from all directions it has
what is called an "Omni-directional" response pattern.
diaphragm
Where the rear of the microphone is opened and the
diaphragm is exposed to sound waves from the back as
well as from the front, the polar plot is not omni-directional
as before, but results in a figure-of-eight directional
pattern.
0°
330°
30°
300°
270°
60°
-10
-20 dB
dB -20
-10
240°
90°
120°
Sound entering from the front will produce a frontal
pressure, which is greater than, and out of phase with, the
pressure due to sound entering the back. The difference
will generate a maximum signal.
A sound source situated to the side however, puts the
diaphragm under equal pressure from both sides and will
tend to cancel itself out.
If the opening at the rear is adjusted in size and character
by means of an acoustic filter, the polar response can be
varied between the extremes of omni-directional and
180°
figure-of-eight. A response approximately halfway between
these two is known as a Cardioid (heart shaped) response. The pattern known as a Hyper- Cardioid response is
particularly sensitive to sounds which are generated at the front, and on axis with the microphone body. Other
sounds, generated at the sides and back of the microphone are also picked up, but at a much reduced level.
210°
150°
14
4.3.1 Omnidirectional
Because of its construction, the Omni-directional microphone is sensitive to sound from any direction. It
responds to a voice from the front in just the same way as to the sound from the audience at the rear. Because
of their normally flat frequency response, irrespective of source distance, omni-directional microphones are
often used for recording and measurement. They are used in situations where sound coming from several
directions must be reproduced, and where either: a) the microphone is totally isolated from the loudspeakers, or
b) the microphone is in close proximity to the sound source, so that the comparative level of any amplified signal
it picks up is very small.
4.3.2 Cardioid
Unidirectional microphones with a Cardioid (heart shaped) directivity pattern are normally preferred in general
public address distribution applications.
The directivity factor is the power ratio of the
0°
transformed frontal sound when compared to an omni330°
30°
directional microphone with the same sensitivity for
diffused sound. For cardioid microphones the directivity
factor is max. 3 or the front to random sensitivity ratio 10
300°
60°
Log3 = 4.8 dB.
270°
-10
-20 dB
dB -20
-10
240°
90°
120°
210°
Careful tuning of the microphone ensures that whilst only
a small amount of extraneous noise is picked up from the
rear and sides of the microphone, the pick up pattern is
wide enough to pick up sound from a fairly wide area at
the front.
This allows a certain amount of freedom of movement
for the speaker, without large drops in volume level.
150°
180°
4.3.3 Hyper-cardioid
The hyper-cardioid microphone operates in the same
way as the cardioid microphone, but to a more extreme
degree. For hyper-cardioid microphones the directivity
factor is max. 4 or the front to random sensitivity ratio 10
Log4 = 6 dB.
0°
330°
30°
300°
270°
60°
-10
-20 dB
dB -20
-10
240°
90°
120°
210°
Because of the high directivity of hyper-cardioid
microphones, care should be taken in positioning to
ensure that the operator is consistently speaking directly
at front of the microphone.
Hyper-cardioid microphone characteristics present
difficulties to the designers of Lavalier (Lapel)
microphones, due to their sensitivity to local noise
generated by contact with the user’s clothing.
150°
180°
Both hyper-cardioid and, to a lesser degree, cardioid microphones have a strongly increased sensitivity to low
tones when the sound source is generated close to the microphone. This means that if an operator speaks very
close to the microphone, their voice will become unnaturally bass in character, at times making the message
unintelligible.
15
4.4 SPECIAL MICROPHONES
A large number of special microphones are available, ranging from broadcast, through to the individual
requirements of different musical instruments.
In the field of sound reinforcement and public address there are again several different types of microphone
likely to be encountered for specialist applications.
4.4.1 The Lavalier and Lapel microphone
These microphones have been specially designed to reproduce speech, and are small, light, and designed to
be worn (a) around the neck (Lavalier Microphone), or (b) clipped to a neck tie or jacket lapel (Lapel
Microphone) without causing discomfort. With this in mind, they are particularly sensitive to high frequencies in
order to compensate for the losses due to absorption by the user’s clothing and made insensitive to the low
toned noise caused when the microphone rubs against the clothing. The microphone capsules themselves are
specially mounted in order to absorb shocks and therefore reduce noise being transmitted though the
microphone due to movement on the speaker's clothes. Being omni-directional microphones, they are also
suitable for use in such applications where a wide area needs to be monitored, such as in a conference
recording system.
4.4.2 Noise cancelling microphone
This is essentially a hyper-cardioid microphone having an optimum speech characteristic, and is designed for
extremely noisy environments such as touring buses, factories, and supermarket floors. This type of
microphone must be held very close to the mouth, so filters have been built in to ensure that the frequency
response is flat when the sound source is close to the microphone, and also that the bass content of the
random noise is reduced.
4.4.3 Radio (Wireless) microphone
Great freedom of movement is provided for the microphone user by the use of a transmitter/receiver system. A
FM signal provides a link between either a hand-held or lavalier/lapel microphone and a receiver connected to
the sound system input. The hand held microphone has a built-in transmitter, while the lavalier model is
connected to a small pocket transmitter, allowing full hands-free use.
When two or more radio(wireless) microphones are used in the same location, care should be taken to ensure
that they each operate on a different transmission frequency, otherwise conflicts will occur.
16
5.0 Technical Principles
5.1 DIRECTIVITY
There is at times confusion between two terms of reference when microphones are being chosen for use in
difficult acoustic environments where the risk of feedback must be reduced.
The response of a typical cardioid microphone at 500 Hz, as shown in 4.3.2, indicates that the response at the
rear, on the 180° line, is some 23 dB less than that at the front. This is called the front-to-rear ratio. In 4.3.3 the
response of a hyper-cardioid microphone is illustrated. Though the front-to-rear ratio is only 14 dB it is far more
suitable for use in a very noisy environment. The reason is that the most ambient noise does not only come
from the rear, but from the reverberant or diffuse field which is picked up at the sides of the microphone, and it
is this field that the hyper-cardioid microphone, more than any other type, attenuates.
This is expressed in terms of what is called the front-to-random index where:
Fr = 20 log Sf/Sd dB
and Sd = average diffuse field sensitivity
where Sf = free field sensitivity at 0°
The cardioid microphone typically has a front-to-random index of about 4,8 dB and the hypercardioid
microphone has a front-to-random index of 5,8 dB.
5.2 SENSITIVITY
The sensitivity of a microphone is the output voltage for a given Sound Pressure Level at 1 kHz, in V/Pa.
Sensitivities vary considerably dependent on the type of design:
Studio Condenser
BPE
Electret
Dynamic
10 mV/Pa
3 mV /Pa
1,6 mV/Pa
1 to 2,5 mV/Pa
( - 40 dB rel 1V/Pa)
( - 50 dB rel 1V/Pa)
( - 56 dB rel 1V/Pa)
( - 60 dB to - 52 dB rel 1V/Pa)
5.3 INSTALLATION CONSIDERATIONS
5.3.1 Potential problems and causes
Problem
hum
oscillation
crosstalk
Cause
Mains power cables
100 V line output cables
other microphone cables
5.3.2 Solutions
The following steps help avoid these problems:
1.
Use only two-core screened (shielded) cable for individual microphone signal cables and extensions.
2.
Keep microphone cables away from mains power and loudspeaker cables. If it is necessary for the cables
to cross, try to ensure that they cross at 90°, rather than running along side each other.
3.
In installations with long microphone cables, use a cable transformer or line amplifier.
Also:Never position a microphone in the direct field of a loudspeaker, as this could cause acoustic feedback
(howl around), described in chapter 9.2.
17
6.0 Microphone Technique
Microphones in the Bosch product range, are of advanced design, are very sensitive, and reproduce the human
voice with great clarity. Many of these microphones have a hypercardioid response pattern, being particularly
sensitive to sounds, which are generated at the front, and on axis with the microphone body. Other sounds,
generated at the sides and back of the microphone are also picked up, but at a much reduced level. This
characteristic gives them a high front to random response index. Due to the fact that they are so directional,
hyper-cadioid microphones operate particularly well in difficult acoustic environments and in areas with high
background noise.
In order to optimise these, or any microphone, it is important to be aware of certain operating techniques.
1.
2.
The microphone should be pointing directly at, but placed a little below, the speaker's mouth. This is to
pick-up full spectrum sound including high frequencies and avoiding air blowing frontal on the
microphones diaphragm and causing “plops”.
The best distance from which to speak into a microphone is approximately 15 to 40 centimetres. If that
distance is reduced greatly, a phenomenon,
especially common to (hyper)cardioid
microphones, known as 'proximity effect' will
occur. This is a very noticeable increase in the
bass content of the signal, making the voice
muffled, and at times unintelligible.
3.
Speak at a consistent volume level.
4.
If the operator were to speak from a much
greater distance than that recommended, the
microphone would also pick up other sounds in
the room, effecting the overall clarity. This is
particularly unfortunate when the microphone
is in the same room as the loudspeakers, due
to the fact that the amplified signal could be
picked up by the microphone and amplified
again. If the amplification in this loop is allowed to continue, the disturbing phenomenon known as
acoustic feedback, or 'howl around', will occur.
5.
If feedback does occur, do not cover the microphone with your hand; this makes the situation worse. If
you are very close to the microphone, moving backwards sometimes helps eliminate feedback. The
operator should then reduce the amplifier volume slightly, or use a tone control or equaliser to attenuate
the offending frequency somewhat.
18
Amplification and Processing
7.0 Mixing Consoles
Certain installations involve a number of microphones, located in the same area, (for instance the stage or
platform of an auditorium), which need to be amplified at the same time. For simple speech reinforcement
systems a mixing pre-amplifier is fully adequate to fulfil the requirements.
More elaborate installations involving a larger number of microphones a Mixing Console (or Mixing Desk) is
the heart of this type of audio system, and is a device which takes the place of a simple pre-amplifier, being
the control unit where all the microphones, cassette players, etc. come together. It accepts these various
inputs and blends them together into one balanced whole. The final, mixed, sound is then sent to the input of
power amplifiers, tape recorder and/or monitor loudspeaker(s).
PHILIPS
Audio Mixer
1
2
3
4
5
6
7
8
AUX1
AUX2
AUX3
LEFT
RIGHT
Mixing consoles range from simple units which accept 4 microphone inputs, have basic tone controls, and
provide a mono output, to huge consoles having more than 60 input channels, each having very sophisticated
equalisation, feeding a large number of sub groups, which in turn feed a selection of main outputs.
The latter type tends to be accompanied by several banks of audio processing equipment and is very much
the domain of the professional mixing engineer.
In order to give the mixing engineer an undistorted judgement of the total sound, the favourite place for a mixing
desk is in the middle of the auditorium.
On the next page a sound reinforcement system for an auditorium is shown.
19
Sound reinforcement system
Passive
Speaker
Cassette Recorder
Multi cable
2
12x 1mV
4x 1V
Monitor
1
2
3
4
5
6
7
8
A
A
A
L
R
3
4
1
5
Monitor
6
Amplifier
Cassette Player
Speech & music in small auditorium
with recording & play back facilities
20
Passive
Speaker
8.0 Amplifiers and Preamplifiers
Although quite often presented as a single unit, the public address amplifier must be considered as two
separate sections: the pre-amplifier (voltage gain) and the output amplifier (power gain).
The pre-amplifier matches and amplifies the outputs of microphones, CD and cassette players, tuners, etc.,
to provide a voltage level suitable for driving the power amplifier. The pre-amplifier also normally incorporates
the tone controls, input sensitivity adjustments, and master volume controls.
The power amplifier, often available as a separate unit, is used to amplify the output power of a pre-amplifier,
distribution system, or mixing console to a level that will feed the loudspeakers properly. If necessary it is
possible to link power amplifier inputs together so that a single input signal can feed a large number of
amplifiers.
8.1 THE PRE-AMPLIFIER
8.1.1 Inputs
The pre-amplifier is normally used for matching and amplifying small voltages, to provide a voltage level,
usually 500 mV or 1 V, which is suitable for driving the power amplifier.
Typical inputs to the pre-amplifier may be:
moving coil (dynamic) microphone - 0,25 mV;
electret or BPE microphone - 1 mV
condenser microphone - 3 mV;
dynamic pick-up - 5 mV;
domestic source (tuner, cassette, CD, DCC etc) - 250 mV;
professional tape recorder - 1,5V.
From this range of input requirements two inputs are often chosen: a microphone input with a sensitivity of 0,5
mV to 1.5 mV; and a music input of 100 mV to 1,5 V.
Tone controls, input sensitivity adjustments, and master volume controls are usually built into the pre-amplifier.
8.1.2 Tone controls
Tone control circuits vary the frequency characteristics of an amplifier.
The bass and treble tone control circuits, with which most people are familiar, are basically amplification and
attenuation circuits, which operate over specific frequency bands. They operate as follows;
1.
If the bass or treble potentiometer is turned to the right, from its 0 ('flat') position, the gain is increased,
and the frequencies within its band of influence are amplified, giving an increase in volume of the
respective bass or treble frequencies. The 'lifting' of the treble frequencies is particularly useful when it is
desired to give speech greater clarity, helping it to 'cut through' noisy environments (see chapter 1.1 for
information regarding the speech spectrum).
2.
If the potentiometer is turned to the left; the respective bass or treble frequencies are attenuated. Bass
attenuation is particularly useful in large rooms, where long reverberation times at low frequencies cause
problems.
3.
Bass lift and treble attenuation is rarely required. Bass lift could be used when amplifying music in a
heavily damped room, where the bass frequencies would require reinforcement to give the music more
depth. Care should be taken though not to overload the loudspeakers when amplifying the bass content
of a signal.
+20
Please note that some lower
quality pre-amplifiers provide only
attenuation, giving no amplification
of either bass or treble frequencies.
+10
dB 0
-10
-20
63
125
250
500
1k
frequency
2k
4k
21
8k
16kHz
In contrast to this, all Bosch'
professional preamplifiers provide
both amplification and attenuation
(see example next page).
8.2 THE POWER AMPLIFIER
The power amplifier is used to amplify the output voltage of the pre-amplifier, distribution system, or mixing
desk, to a level that will feed the loudspeakers properly. Depending on the design philosophy of the
manufacturers, the input required to feed the amplifier at nominal full power can range from 100 mV to 10 V.
Many power amplifiers used in public address systems, and all amplifiers in the Bosch product range use what
is known as the 100 Volt line principle. This type of amplifier is favourable if long loudspeaker distances are
involved. (This principle is discussed in the following section) Other power amplifiers, often used in sound
reinforcement systems, provide a direct low impedance 2, 4 or 8 ohm output. If using the latter, make sure that
the impedance of the loudspeakers matches that of the amplifier, and that the amplifier power is always lower
than the loudspeaker power, so that the amplifier is not able to overload the loudspeakers.
8.3 AMPLIFIER/LOUDSPEAKER INTERFACE
As stated in 8.2, in order to interface loudspeakers with power amplifiers, all Bosch amplifiers utilised what is
known as the 100 Volt line matching principle, whilst certain amplifiers in the range also incorporate low
impedance outputs. If the load is always constant, the loudspeakers can be connected in a series/parallel
arrangement to exactly match the amplifier's low output impedance. However if the loudspeakers differ in power
and impedance, or if the quantity of loudspeakers changes, it is very difficult indeed to match them to the power
amplifier. In this type of situation, or in an application requiring long loudspeaker cable lengths, the 100 Volt line
matching system is used.
In the 100 Volt line matching system, transformers, which are mounted in the power amplifiers, are tapped to
step up the output voltage of the amplifiers from a low voltage to 100, 70 or 50 Volts. Transformers, mounted on
the loudspeakers, then reduce this again to the original low voltage, acceptable to the loudspeakers.
This system gives great flexibility in the design and use of public address systems for the following reasons:
1.
By increasing the output power voltage of an amplifier, the amount of current (measured in amps) involved
is reduced significantly. This means that even when high power amplifiers are used, line losses are kept
low, and heavy duty cabling is not required.
2.
Due to these low line losses, extremely long cable lengths are possible. This is a very important factor in a
public address installation.
3.
All loudspeakers may be simply connected in parallel.
So long as the total amount of watts
drawn by the loudspeakers is not
greater than the rated output power of
the amplifier, it does not matter whether
there is 1 loudspeaker or 150
loudspeakers connected to it at any
time.
100/70/50V
22
The 100V line principle can be compared to a normal domestic mains electricity power supply. In a mains
supply, a constant supply voltage is present, and it is necessary only to plug an appliance into the mains socket
for it to become operational. The amount of appliances plugged into a supply is irrelevant, so long as the total
amount of power (wattage) drawn is not greater than that available.
100V
70V
50V
0V
P
1/2P
1/4P
0V
amplifier
loudspeaker
100V
70V
P
P
P
50V
100V
1/2P
100V
1/4P
full power
1/2 power
1/4 power
1/2 power
1/4 power
When loudspeakers are connected to the 100V amplifier tap, their
full power is drawn, whereas if they are connected to the 70V tap,
only 1/2 of their rated power is drawn. This means that the 70V tap
enables the amplifier to power twice as many loudspeakers, with
each loudspeaker producing 1/2 of its potential power.
Similarly, the 50V tap allows loudspeakers to draw 1/4 of their rated
power, so that the amplifier is able to power 4 times more
loudspeakers, with each producing 1/4 of its potential power.
The transformers fitted to the loudspeakers have similar taps, but in
this case the actual power which the loudspeaker will draw (e.g. P,
P1/2, P1/4, or 6W, 3W, 1,5W), instead of the voltage, is printed
beside the "power" (+) tap. These loudspeaker transformer taps are
used in the same way as the amplifier transformer taps; matching
the power drawn (in this case by each loudspeaker) to the amplifier
power available.
When it is desired to reduce the power drawn by all of the
loudspeakers, it is of course simpler and more efficient to utilise the
amplifier transformer taps. It is possible though, by using the
loudspeaker transformer taps, to reduce the power drawn by only a
quantity of the loudspeakers, while the remainder draw full power.
Note:
When using the 100 Volt line matching system, the Rated Power of
the amplifier corresponds to the Rated Load Impedance of the
loudspeaker network. The total rated power required should be
calculated, by simply adding the Rated Power of the connected loudspeakers together, taking into account the
reduction in power drawn when using the loudspeaker power taps. It is important that this total should not
exceed the rated power of the amplifier.
70V
1/2P
1/4 power
23
Cable lengths.
Maximum Permissible Cable Lengths
The maximum permissible cable
lengths per size of cable are shown
in the accompanying graph.
1000
W
Example:
10
Assuming an amplifier of 100 W
tapped at 100 V and using a cable
of 2x0.75 mm2 . The length of the
cable should not exceed 250 m.
0
V
100
70
50
35
25
10
10
100
1000
10000
The values refer to a 10% voltage
drop, with the entire load
concentrated at one end of the
cable. The lengths can be doubled
when the load is distributed evenly
along the cable.
m
2 x 0.75 mm2
1000
Transformers.
Whilst considering the many
advantages of the 100 V line
matching system, it is important to
realise that by inserting transformers into the signal chain,
certain losses must occur.
W
10
0
V
70
50
100
35
25
10
10
100
1000
10000
m
2 x 1.5 mm2
The impedance of transformers also
varies with frequency, which of
course has an adverse effect on the
overall system frequency response,
and the demands placed upon the
amplifiers, especially when
reproducing bass frequencies.
1000
W
10
0
V
70
50
100
35
25
10
10
100
Any transformer has an insertion
loss. If for example, 10 W is
required at the loudspeaker
terminals, using a transformer with
an insertion loss of 1.5 dB would
require 14.13 W output from the
amplifier.
1000
10000
m
2 x 2.5 mm2
24
9.0 Equalisers
An Equaliser gives extensive control over the whole audio frequency spectrum by means of presence (gain)
and absence (attenuation) filters and can be used for optimising the frequency response of the sound system.
It can even equalise the complete audio chain, from microphone to ear. Used with care, this would guarantee
maximum amplification for the whole frequency spectrum, at the same time combating the problem of acoustic
feedback by reducing the level of frequencies which cause it.
9.1 EQUALISER TYPES
9.1.1 Basic tone controls
The bass and treble tone control circuits, with which most people are familiar, are basically amplification and
attenuation circuits which operate over a specific (though fairly broad) frequency band. They operate as follows:
1.
If the bass or treble potentiometer is turned to the right, from its 0 ('flat') position, the gain is increased,
and the frequencies within its band of influence are amplified, giving an increase in the volume of the
respective bass or treble frequencies. The 'lifting' of the treble frequencies is particularly useful when it is
desired to give speech greater clarity, helping it to 'cut through' noisy environments
2.
If the potentiometer is turned to the left; the respective bass or treble frequencies are attenuated. Bass
attenuation is particularly useful in large rooms, where long reverberation times at low frequencies cause
problems.
3.
Bass lift and treble attenuation are rarely required. Bass lift could be used when amplifying music in a
heavily damped room, where the bass frequencies would require reinforcement to give the music more
depth. Care should be taken though not to overload the loudspeakers.
+20
+10
dB 0
-10
-20
63
125
250
500
1k
frequency
2k
4k
8k
16kHz
These treble and bass tone control circuits are very basic units, operating over wide frequency bands, raising or
attenuating all of the bass or treble frequencies.
9.1.2 Band-pass filters
+20
+10
dB 0
-10
-20
63
125
250
500
1k
frequency
2k
4k
8k
16kHz
Bass and treble "Hi-Pass" and "Lo-Pass" (or "cut-off") filters are intended to restrict the frequency band. Their
purpose is to severely attenuate all signals below or above a fixed (normally very low or very high) frequency.
In situations requiring control over specific frequency bands, a variety of equalisers are available see next page:
25
9.1.3 Parametric equaliser
A parametric equaliser is a unit with 3 or 4 filters, and the possibility to adjust the frequency to be processed.
+20
+10
+
Q
gain
dB 0
-10
-20
63
125
250
500
1k
frequency
2k
4k
8k
16kHz
The processing consists of gain correction (+ , - ),and a selection of the width (Q) of the frequency band. This
makes it possible to alter, if necessary, a very small frequency band, without affecting the neighbouring
frequencies. Because only a few filters are used, the overall response tends to be quite smooth. (See 9.2.4)
9.1.4 Parametric triple Q-filter
Basically a parametric equaliser but with pre-set fixed (speech) centre frequencies e.g. 1-2-4 kHz. This filter
allows the operator to select the width & slope of the frequency band (Q) and presence or absence (Gain).
+20
+10
dB 0
-10
-20
63
125
250
500
1k
frequency
2k
4k
8k
16kHz
The unit is ideal for optimising the amplification of that part of the frequency-band that is responsible for speech
intelligibility, it adds clarity and compensates for air absorption. An adjustable bass cut filter provides smooth
roll-off of the bass content in the signal caused by e.g. speaking too close to a cardioid microphone.
9.1.5 Graphic equaliser
A Fixed Frequency or "Graphic" Equaliser often consists of 30 individual filter sections. Each control, which is
+20
dB
+10
0
-10
-20
63
125
250
500
1k
2k
4k
8k
16kHz
Frequency
often a sliding potentiometer or "fader", effects a narrow frequency band (third octave). The "peak", or
maximum effect is at the centre of each band, with the surrounding frequencies being effected to a
proportionately lesser degree. The total frequency spectrum is covered, allowing the signal to be sculptured at
several specific frequency bands as desired. To avoid excessive phase shifting, care should be taken to avoid
extremes of variation between adjacent controls with, for example, one control at full attenuation and its
neighbour at full amplification. The maximum level of both speech and music is in the 250-500 Hz frequency
range. The level at these frequencies should be kept as near to 0 dB as possible to avoid distortion due to a
general level increase.
26
9.2 EQUALISATION
9.2.1 Introduction
The increase in sound level which a sound amplification system can give to a performance in an auditorium is
limited by acoustic feedback.
In many cases, e.g. when the performance requires a long microphone distance or with a somewhat noisy
audience, the feedback limit prevents an adequate sound level being produced for comfortable listening. This
situation can be annoying to both performers and audience because of an insufficient sound level or when the
system amplification is increased, spurious ringing sounds.
The use of sound equalisation to reduce acoustic feedback contributes toward the comfort of both performers
and audience and will enhance the acoustic quality and increase the overall system gain.
Room conditions can also reduce intelligibility as they “colour” the sound by changing the frequency response.
This effect can also be corrected by equalisation.
Though the selection of microphones, amplifiers, and loudspeaker types is vital when creating a system with
smooth response, it is assumed that the sound system in question has already been optimised prior to
conducting any equalisation measurements.
9.2.2 The acoustic feedback loop
The total system loop contains basically a sequence of the following elements:
- microphone
- amplifiers with volume control and possibly a tone control (e.g. mixer)
- loudspeakers
- acoustic transmission link between loudspeaker(s) and microphone
The acoustic transmission link consists of one (or two) direct path between the loudspeaker(s) and the
microphone which is maintained by what is called the “direct sound field”, and many other paths caused by
reflections and multi-reflections which are maintained by what is called the “diffuse sound field”.
9.2.3 Resonant acoustic feedback
Acoustic feedback is spontaneous oscillation caused by the transmission of sound radiated by the
loudspeaker(system output) back to the microphone (system
Spontaneous oscillations input).
When the gain of the sound system is gradually increased, a point will be reached where spontaneous
oscillations (howling) start to occur.or resonant acoustic feedback can occur at any frequency for which:
a) the phase angle of the transmission through the acoustic feedback loop equals zero, and
b) the sound from the loudspeaker re-enters the microphone louder than the original sound (loop gain ≥ 1)
This cycle repeats itself, with increased amplification until the sound reaches the system’s maximum loudness
or until someone turns down the volume!
Even though a sound system is adjusted just below its critical gain, feedback will prolong the signal components
at this critical frequency, producing ringing or howling sounds. To avoid ringing sounds during speech or
musical performances, the gain has to be reduced to approximately 6 dB below the level at which spontaneous
oscillation begins, this is called Feedback Stability Margin (FSM).
27
9.2.4 Principles of equalisation
The ideal in any audio system is to obtain a flat frequency response over the complete audio frequency band.
When considering the sound system equipment alone, a flat frequency response can be achieved within very
fine limits, but when taking the sound system as a whole, with its associated acoustic link, changes are
introduced to the feedback frequency response by the very nature of the auditorium. The cancellation of these
changes in the frequency response, whether they be peaks or dips, is called “Equalisation”.
Using a measuring set-up as explained on the next page (9.2.5) we can obtain e.g. the following loop response:
50
dB
40
30
20
10
0
63
125
250
500
1k
2k
4k
8k
16kHz
Frequency
The dominant frequency where the acoustic feedback is likely to occur is 160 Hz and secondly 3.4 kHz.
50
dB
40
30
20
10
0
63
125
250
500
1k
2k
4k
8k
16kHz
Frequency
50
By equalising the loop response now with an “mirror imaged” filter response, the overall gain can be increased.
In order to maintain the optimum signal to noise ratio, increasing the gain at dips in the frequency response, as
well as reducing the resonant peaks, should be considered. This can be done manually by means of a graphic
or parametric equaliser or automatically by a so called intelligent feedback exterminator which work with a
number of narrow band filters adjusted dynamically at the critical frequencies and maintaining a FSM of 6dB.
50
dB
40
30
20
10
0
63
125
250
500
1k
2k
4k
8k
16kHz
Frequency
It is impossible to lay down hard and fast rules as to which equalisation method should be used, as the
requirements will be vary from one auditorium to another. The prime objective is to obtain a flat frequency
response of the loop to obtain max. possible gain for all frequencies and preserve the signal to noise ratio.
A listening test after equalisation is important because a flat loop response is not always a flat listening result, a
high frequency roll off is sometimes required ( 3 dB/octave > 1 kHz).
28
9.2.5 Loop equalisation
Power
Amplifier
PreAmplifier
Test Unit
In a speech reinforcement system facing the problem of acoustic feedback, we equalise the whole loop, which
consists of the system microphone(s) - amplification - loudspeaker(s) and room.
The test unit produces a 1/3 octave “warbled” tone, which glides from 20 Hz to 20 kHz, and is fed into the power
amplifier. The corresponding output, reflected by the room surfaces, is received by the system microphone and
plotted on the test unit recorder. Another method is to inject pink noise in the sound system and measure with a
1/3 octave Real Time Analyzer. This is a good method for adjusting a 1/3 octave graphic equalizer in the
system.
9.2.6 Loudspeaker equalisation
Power
Amplifier
Test Unit
In a system used for playing pre-recorded music, we concentrate our measurements on the loudspeaker
reproduction. In this case we use a calibrated measuring microphone at the audience position (averaged), and
equalise only the power amplifier, loudspeaker(s), and room. The most convenient method is to inject pink noise
in the sound system’s line input and measure with a 1/3 octave Real Time Analyzer on the audience position. A
1/3 octave graphic equalizer is the easiest to adjust but difficult to hide for unauthorised tempering.
9.2.7 Loudspeaker equalisation & Loop equalisation
For sound systems used for music reproduction and sound reinforcement, where there is a need to optimise
both, the loudspeaker equalisation should be carried out first, and secondly an additional equaliser should be
used in the system microphone channel.
29
10.0 Time Delay
When a sound reinforcement system in an large auditorium, with loudspeakers located at the left and right hand
side of the stage and dispersed at intervals along the length of the auditorium, a problem of timing becomes
apparent. When all loudspeakers produce their sound at the same time, the listener hears the speaker's voice
coming from the direction of the closest loudspeaker, instead of from the stage.
This conflict between the visual and audible experience is rather uncomfortable. To overcome this disturbing
effect, the sound from each (group of) loudspeaker(s) must be delayed using time delay equipment. If the timing
is set properly, (based on the speed of sound travelling at 5 meters per 15 milliseconds), and the sound of the
loudspeakers arrives later (5-15 ms) and not more than 10 dB louder than the original speakers voice, the
sound will appear to originate from the front of the auditorium or area, where the speaker is located.
Another problem occurs at railway stations, where the aural announcement origin is located at the closest
loudspeaker, but is then followed by arrival of sound from the other loudspeakers, causing echoes and
reverberation. To overcome this disturbing effect, the sound from each (group of) loudspeaker(s) must be
delayed using time delay equipment. If the timing is set properly, the sound will be synchronised with the
furthest loudspeakers and will benefit the intelligibility considerably. The most effective way of doing this is to
use the loudspeakers located in the middle of the platform as the starting point. The other loudspeakers which
should be pointing away from this centre position, should be delayed proportionally so that the sound appears
to come from this centre position. The loudspeakers should be selected carefully and angled for a minimum of
backward radiation.
Railway platform without delayed loudspeaker signals
Railway platform with correctly delayed loudspeaker signals
30
11.0 Compressor/Limiter
A compressor and a limiter are input signal dependent attenuators. The dynamics of input levels below the
threshold are not affected, but the dynamics of levels above are reduced. The attack time is 1 ms, while the
adjustable release time is dictated by the application, short for speech (100 ms), long for music (>1s).
Output
COMPRESSOR 1 : 3
LIMITER 1 : 30
Output
3V
3V
1V
1V
.3V
.3V
30dB
0.3V
Log scales
1V
3V
30dB
10V
30V
0.3V
Log scales
Input
1V
3V
10V
30V
Input
A limiter effectively restricts the output level to e.g.
1V for all input levels above the threshold level
without introducing distortion. A limiter is ideal for
mounting in call stations to guarantee a fixed maximum output level, independent of the person speaking (male/female/distance/loudness). To utilise this
maximum peak level with the full capability of the
sound system, it is necessary to align the rest of the
chain in such a way that also the maximum
undistorted output level of the amplifier is reached.
A compressor reduces input signal variations
above the threshold level to about one third (in
dB’s) without introducing distortion.
(30 dB input variation gives only 10 dB output
variation).
A compressor is ideal for background music
applications to reduce the (often unwanted) large
dynamic range of recordings or broadcastings.
The release time should then be set on >1s to
avoid music sounding unnatural (pumping).
31
12.0 Automatic Volume Control
Automatic Volume Control (AVC) regulates the loudness of a P.A. announcement relative to the ambient
noise level. This guarantees maximum intelligibility and minimum annoyance.
The ambient noise level is continuously measured by a microphone connected to the sensor input of the AVC
unit, which uses this measurement to set the attenuation of the signal path. During periods of low ambient
noise, the PA system gain is reduced by the AVC-unit, and during periods of high ambient noise, the PA-system
gain is restored to its nominal maximum. A blocking circuit ‘freezes’ the input sensor while an announcement is
being made, ensuring that the announcement itself is not measured by the unit as ambient noise.
The control range of the AVC, with attenuation values from 6 to 21 dB, depends on the maximum loudness of
the sound system. 80 dB(SPL) is regarded as a comfortable maximum listening level. If the loudspeaker system
is set up so that a maximum of 89 dB SPL can be achieved, then a control range of 9 dB would be the right
choice. An AVC unit with 21 dB control range would only be used in PA systems which can produce a maximum
level of 101 dB(SPL), being 21 dB above the comfortable listening level of 80 dB(SPL).
The AVC unit is factory pre-set, therefore only the sensing input microphone gain in the corresponding
loudspeaker-zone and the reset time (blocking) needs to be adjusted.
If a microphone is located inside the loudspeaker zone to which it is addressed, the gain should be carefully set
to avoid acoustic feedback. The system should be checked during periods of high ambient noise and low level
talking into the microphone in order to ensure that no acoustic feedback, automatic attenuation(AVC), or limiting
(Callstation) occurs.
32
13.0 Technical Considerations
13.1 SPECIFICATIONS
13.1.1 Frequency Response
+20
This graph illustrates the typical flat
response of an amplifier suitable for
music reproduction.
The written specification of this type
of frequency response should state
the frequencies at the points where
the curves have dropped by 3 dB.
In our example, the frequency
response is from 63 Hz to 16 kHz.
+10
dB 0
-3
-10
-20
63
125
250
500
1k
frequency
2k
4k
8k
16kHz
When this specification relates to power amplifiers the level at which it is measured should be 10 dB below the
rated output power.
Specifications should be read carefully. If a manufacturer chooses -6 dB points as reference, he is able to
quote a frequency response range which extends much wider than more ethical competitors.
13.1.2 Power bandwidth
The Power bandwidth is the frequency range in which the amplifier can deliver its rated power (-3dB) with a
maximum distortion level (THD) as stated by the manufacturer (0.5% for PA amplifiers).
13.1.3 Linear distortion
If an amplifier is not capable of amplifying the full frequency spectrum equally, the amplified waveform will be
altered in a similar way as when tone controls are used. This unwanted modification of the signal is called
linear distortion, which in its extreme could give rise to a guitar input producing a 'piano' sound output.
13.1.4 Non linear distortion or clipping (THD)
clipping
This graph shows an amplifier with too much input signal.
dynamic range
If the amplifier is overdriven, a clipping of the output voltage is likely to
occur. This effect, called non-linear distortion, happens when the
input signal exceeds the dynamic range of the amplifier. When the
voltage is clipped, the normal curve of the signal wave is squared off,
producing extra harmonics of the fundamental. This is commonly
referred to as Total Harmonic Distortion (THD).
The result is an audible change, making the sound uncomfortably raw.
clipping
Another problem occurs when the current continues to rise, causing too much energy to be fed into the
loudspeakers (beyond their Power Handling Capacity (PHC) limits), which could cause them to be damaged.
33
13.1.5 Rated Output Power
Rated Distortion Limited Output Power is the power which the amplifier is capable of dissipating in the rated
load impedance, at a given frequency or frequency band (1 kHz), without exceeding the rated Total Harmonic
Distortion (THD). This is defined in publication IEC 268-3.
Emotional speech, or certain passages of music, can cause pronounced audio signal peaks. Such
instantaneous features of speech and music have to be reproduced without distortion. Generally, allowance
must be made for speech attaining voltage peak values of approximately three times its average.
This may be expressed as 20 Log 3 = 10 dB. 10 dB, as a power ratio, means that the peak power is roughly 10
times that of the average power. This is called the 'rated power' of an amplifier. A 100 W amplifier, for instance,
having a input sensitivity of 100 mV, will produce 100 W output when the input voltage reaches 100 mV.
This 100 W is the maximum output power which the
amplifier can produce whilst still keeping distortion below its
specified limit.
V 1/3V
average
Under normal conditions, however, the average input
voltage will only be 33 mV (allowing up to 100 mV for peaks)
and the average output power will only be 10 W (allowing up
to 100 W for peaks).
This means that, on average, an amplifier normally operates
at only one tenth of its rated (or peak) value. In our example
this will be 10 W.
13.1.6 Temperature Limited Output Power (TLOP)
The IEC 65 standard states that an amplifier, running under worse case conditions, should at least be able to
run continuously at 12½% of its Rated Output Power without any components overheating.
This means that 100 W amplifiers, located in ambient temperature of 45° C, with + 10% mains over-voltage,
stacked on top of each other, in a 19 inch rack frame, should be able to run continuously for 24 hours per day
at 12.5 W average power without overheating.
34
13.2 ADJUSTING SIGNAL LEVELS IN A SYSTEM CHAIN.
1
Microphone in a Callstation is often combined with a pre-amplifier and limiter to optimise the signal in the
transport cable. The max. output signal is due to the limiter restricted to 0 dBV (=1V).
The potmeter affecting the gain before the limiter should be adjusted to the announcer and/or acoustic
feedback. The limiter is activated by the peaks in the signal therefore the average level of speech will be around
-8 dBV but the peak level is close to 0 dBV.
2
Routing controller (SM30 or SM40) has 0 dBV input sensitivity for 0 dBV output. The input adjusters
should always be in maximum position and only be changed in the seldom situation that you do not want the full
power out of the system for this corresponding microphone input.
The attention and alarm signals are separately adjustable to an average level of -8 dBV (can be checked as 0
VU on the amplifier). If signal processing is applied (tone controlling, equalising, time delay etc.) take care of
their gain settings to avoid unwanted gain or attenuation for speech/music (can be checked with pink noise).
3
Amplifier needs 0 dBV at the input in order to deliver 100 Volt to the rated load impedance. For this rated
outputlevel we specify THD - Power bandwidth - S/N ratio etc, acc. IEC 268-3 DIN45500 FTC etc.
The Temperature Limited Output Power acc.IEC 65 is specified as 9 dB below the rated output power under
extreme working conditions and is a measure for the cooling capacity of the amplifier power stages (heat-sinks
and/or ventilators). The VU-meter has an integration time of 240 ms and adjusted so that 40 Volt (sinewave
rms) reads 0 VU (=8 dB below 100 Volt); therefore in practice, speech and/or music should give not more than 0
to +3 VU readings as maximum in order to guarantee that short peaks in the signal (exceeding 100V) do not
cause unacceptable audible distortion.
4
Loudspeakers are (for reasons of electrical power transport and installer requirements) generally
connected via a 100 Volt line system. Matching of the loudspeaker requirements to the available amplifier power
is done via the tapping-down possibilities (1/2P-1/4P)(70-50V).
Generally the Power Handling Capacity acc. IEC268-5 will be greater than the Rated Power of the loudspeaker
in order to avoid damaging of the loudspeaker during excessive signal overload (acoustic feedback!).
Rated Power of the amplifier corresponds via 100 Volt to the Rated Load Impedance of the loudspeaker
network. Therefore the total rated power of the connected loudspeakers (taking the powertapping into account)
should not exceed the rated power of the amplifier.
5
Automatic Volume Control (AVC) regulates the loudness of the P.A. announcement in relation to the
ambient noise. This guarantees maximum intelligibility and minimum annoyance. During low ambient noise level
the PA-system gain is gradually reduced with 9 dB by the AVC-unit.
The 9 dB controlrange is chosen to assure a good performance for ambient noise levels upto 75 dB(SPL). The
loudspeakersystem should be set-up such that a calculated SPLtotal of 89 dB can be achieved, being 9 dB
above comfortable listening level of 80 dB(SPL). The AVC-unit is factory pre-set, therefore only the gain of the
sensing input should be adjusted to the applied microphone(s) in the corresponding loud-speakerzone and the
resettime (blocking).
The AVC-unit should be by-passed for announcement microphones placed in the addressed loudspeakerzone
for acoustic feedback stability. If not, the system gain will be reduced by the control range of the AVC during low
ambient noise. Acoustic feedback stability should then be checked during high ambient noise levels and low
level talking in the microphone in order to avoid any automatic attenuation or limiting.
The AVC-unit with 21 dB control range is only for those PA-systems which can produce a maximum level of 101
dB(SPL) being 21 dB above comfortable listening level of 80 dB(SPL).
35
Hardware Installation
14.0 Grounding and Screening
14.1 EARTHING (GROUNDING)
14.1.1 Safety and system earth’s
In order for a sound system to operate satisfactorily and safely, care must be taken to ensure that it is
adequately earthed (grounded).
The earth path provided by the mains cable is the 'protective', or 'safety' earth, which takes any
potentially hazardous positive voltage down to ground if an electrical fault occurs.
With professional audio equipment this must also act as a 'system earth', being connected to the
system's screening (shielding) network, and taking all of the interference, 'collected' by the
screening,
down to ground. It is therefore vital that this is a 'clean' or 'noiseless' earth.
Unfortunately the mains earth is often contaminated with interference, caused by other types of
equipment which use this common earth. If possible, an alternative clean earth should be
established. The best way to ensure this is to make a separate path to earth by driving a long copper
pole into the
ground, and connecting this to the amplifier or system rack(s) with an adequate earth
wire.
14.1.2 Earth (ground) loops
Incorrect earth wiring in a public address distribution system can cause malfunction of the equipment by
introducing hum, distortion, or instability. It can even result in an overload condition, which may cause complete
electrical breakdown of components. The main reason for the occurrence of these problems is the inadvertent
introduction of earth (ground) loops in the earth wiring. Earth loops exist where multiple connections to earth are
made from any one part of the system. Once the system is installed and these problems become apparent, it is
often very difficult to trace the source of the problem. Because of this, it is vitally important to design the system
so that an earth loop is not built in.
In a system, where several units are powered directly from the mains supply, earth loops can be caused by the
mains wiring. A typical example of this would be a tape recorder or professional DCC or CD player, mounted
in the 19 inch rack frame.
PLAY
a &^% $
connection
to rack frame
signal cable
screen
mains power
cable
In this case three different paths to earth will have
been established:
•
via the mechanical connection of the chassis
to the rack unit.
•
via the mains earth wire.
•
via the signal cable screen.
An earth loop would be the result.
Special measures must be taken to remove the earth loop, but at the same time, for safety reasons, the earth
connection to the units must be maintained.
36
To ensure that each unit within the system has only one path to earth:
1.
Even though many domestic, and some professional, music sources and auxiliary equipment have no
electrical connection to the mains earth, there is often the possibility of a mechanical connection when, for
instance, the chassis of the unit makes metal to metal contact with the rack frame.
To avoid intermittent contact, and guarantee maximum security, each amplifier and music source chassis
should be securely earthed (if necessary with short lengths of wire) to the rack.
2.
The earth of the power cable carrying the electricity supply from the mains should be securely connected
to the rack, and all the mechanical earth’s connected at that point.
3.
In any amplification equipment, two earth’s are present. One is the 'electrical' earth, connected to the 0V
side of the circuit. The other is the 'mechanical earth', connected to the unit's chassis.
4.
On amplifiers which are stacked, or mounted in a 19 inch rack, all electrical earth’s should be connected
to the mechanical earth at the same point. These earth’s should not be wired together, but an individual
wire should be run from each amplifier to the earth connection point.
4a.
Where a separate clean earth is available, these electrical earth’s should be wired to this ground
connection point.
In instances where the only available earth is the mains safety earth, all the electrical earth’s should be
connected at the point where the power cable carrying the electricity supply from the mains is connected
to the rack.
4b.
mains power
5.
When mains powered domestic music sources (CD players, etc.) are used in a system, a 1:1 isolating (or
"galvanic separation") transformer should be fitted in the signal cable.
6.
An alternative to this is to connect the signal screen to earth at one end of the cable only. This should be
connected at the amplifier or preamplifier input, but not at the output of the signal source (CD player,
cassette machine etc.).
37
14.1.3 Microphone Earth Loops
Earth loops are a common source of hum in installations with long microphone and connection cables.
To prevent earth loops it is wise to follow the next considerations:
1.
If it is necessary to use single screened cables, these should only be used for very short microphone
cables in noise-free environments. With such cables, the screen is used as the return connection for the
microphone, and because the screen is then earthed at the pre-amplifier input, any noise or hum induced in the
cable is included in the microphone signal and amplified.
2.
Normally twin core screened cable should be used. The screen, which is connected to earth at the preamplifier input, is not used as the return for the microphone. In most cases, no problems with hum pick-up will
occur if the microphone is wired in this way. In severe environments, with intense magnetic fields, it is possible
that noise or hum will be induced, not only in the cable screening, but also in the cable cores.
3.
For 100% security, a twin core screened cable, with its screen connected to the pre-amplifier’s earth,
should be connected to a balanced pre-amplifier input. This is a pre-amplifier fitted with a separating
transformer at its input. Such a transformer gives a common mode rejection greater than 30 dB to the
microphone signal lines and therefore cancels out any hum present on those lines.
Do not connect the cable screening wire to the metal body of the microphone connecting plug.
4.
Example how to connect electrical equipment in a chain, the screening is only on the receiving side of
the equipment connected to electrical earth, this is to avoid ground loops ( hum).
38
14.2 RADIO AND MAINS BORN INTERFERENCE
Bosch amplifiers and distribution systems contain extensive protection against external interference sources
and, in normal circumstances, will not be effected by them. However, extraordinary radio-born and mains
electricity supply conditions may cause problems which have to be solved individually.
Problems may be expected when:
•
An electrical field strength exceeds 1 V/m. This would be the case when the system is installed:
a. Within a 20 km radius of a 1 MW medium wave radio transmitter.
b. Within a 5 km radius of a 100 kW FM or television transmitter.
c. Within a 100m radius of a 0.5 W citizens band transmitter (e.g. 27 MHz),
depending on the directivity of its antenna.
d. Near medical equipment. For instance; within 100 m of a 27 MHz, 1 kW radio-therapy unit.
Frequencies above 200 MHz (like radar or relay connections >1 GHz) seldom cause problems.
A factor which normally decreases the interference influence, is the screening property of the building,
especially when metal construction materials or reinforced concrete are used.
•
When voltage spikes on the mains electricity supply exceed 800 V.
This can occur when highly inductive or capacitive loads are switched on and off on the mains network.
Problems of this kind can normally be solved by installing a good mains filter. A variety of special application
versions exist for this kind of situation.
14.2.1 Prevention of Interference
There are two basic methods of preventing radio born interference:
1.
Screening the source of radiation:
Generally the most effective method, but only feasible when the offending cause is located 'in house', as
would be the case with medical equipment etc. The equipment and the patient would have to be located
inside a Faraday cage, within which the 'radiation area' would be confined.
2.
Screening the effected equipment:
The 19 inch rack unit, within which the distribution system, and/or amplifiers of a sound system would
be mounted, makes an ideal screen for the electronic circuitry. The rack used must have a top and
bottom plate, and an all metal door. The only holes in the outside surfaces should be for ventilation,
and these should be in the form of louvers or small holes, rather than one large opening. The rack can
form a more efficient screen when all of the component parts (covers, construction bars, etc.) are
electrically connected.
top plate
earth bonding
wires
metal
door
cable
inlet
rear
ventilator
louvres
base
bottom
plate
39
This is done by using short lengths of wire to join each part to its neighbour. On large surfaces, such as cover
panels, these connections should be made in several locations ( e.g. 6 wires on side/rear covers). In extreme
cases it may be necessary to remove paint, and use self-tapping screws every 5-10 cm to make the cabinet
100% RF immune. See accompanying illustrations for examples.
14.2.2 Interference introduced via cables
Any cable, whether signal, loudspeaker, or mains, is a
potential antenna for radio born interference.
keep
short
ferrite ring
toroid core
type 4C6
Where feasible, disconnect and reconnect each cable in turn,
until the offending cable(s) is (are) found.
mains/loudspeakers
antenna/audio
The simple modification illustrated can effectively cancel this
problem. Experiment with the amount of windings to find the
optimum RF damping.
or ferrite rod
equipment
interior
14.2.3 Interference introduced inside rack unit
In some cases, hum can be induced into a signal line from the radiation effects of mains electricity voltage
cables and transformers. Care should be taken when planning the internal wiring of the rack unit, to keep input
wiring, where possible, away from mains wiring and transformers.
14.2.4 Interference induced from 100 V loudspeaker wiring
Signal wiring, both inside the rack, and in external cable ducts, should be kept separate from 100 V loudspeaker
wiring. If this is not done, inductive & capacitive coupling might occur, causing the system to oscillate.
40
14.3 NINETEEN INCH RACK UNITS
39HE
39
35
32HE
30
25HE
25
Bosch public address equipment, like distribution controllers
amplifiers, monitor panels and many auxiliary equipment, are
designed to be mounted in a cabinet with a standard front
panel width, called a "Nineteen Inch Rack". Some music
source equipment (background music players, CD players,
radio tuners, etc.) can be modified, using bolt on accessories,
to fit also into these 19 inch racks.
To simplify calculation of 19 inch rack space required, a
standard height 'HE' equal to 44.55 mm (1.75 inches) has
been chosen. Most power amplifiers for instance, are 3 HE in
height, requiring 133.65 mm of rack space. The use of this
height standard eases the problem of calculating the number
of amplifiers, modular distribution units, or panels that will fit
into a given rack.
Certain rules should be observed when planning the
equipment layout of the rack.
15HE
20
1. Cassette front loaders, tuner scales, and other frequently
used equipment should be mounted at a height which
makes the front panel clearly visible to the operator.
15
2. If power amplifiers are mounted beneath rack frames
containing microprocessor controlled distribution units, a
heat shield should be installed above them. This is
necessary to deflect hot air currents, which could otherwise
cause instability in the microprocessor units.
10
3. When several high power amplifiers are used, a fan unit
should be mounted in the bottom of the rack to ensure
adequate ventilation.
5
41
Loudspeakers
The loudspeakers used in the audio reproduction chain are a vital factor in determining the overall quality and
success of a sound system. Because if this, it is vital to understand the different types of loudspeakers
available, and their particular strengths and weaknesses.
Bosch offer a wide range of loudspeakers in their product range, but all are designed and rigorously tested to
reproduce speech clearly, and to provide a very high level of reliability.
15.0 Loudspeakers
15.1 LOUDSPEAKER TYPES
Cone loudspeakers are the most commonly used units, which in order to function properly must be mounted in
correctly designed enclosures (cabinets or boxes). Dependent on the enclosures in which they are mounted,
the characteristic of handling a wide frequency range makes them particularly suitable for the reproduction of
music and speech. Loudspeakers with larger cone diameters generally give better low frequency reproduction.
The fact that they are less efficient, and do not produce a high SPL, compared with diaphragm (horn) type
loudspeakers, limits their use in areas of high ambient noise, or where the loudspeaker must be mounted a
great distance from the listeners. The following units are based around cone loudspeakers:
15.1.1 Standard loudspeaker cabinets
Standard (infinite baffle) loudspeaker cabinets, are in principle a sealed box containing 1 cone loudspeaker, and
have a typically wide dispersion pattern. Their shape makes them convenient for mounting on walls or pillars, or
suspending vertically from the ceiling, to give a wide beam of sound.
The bass response of the sealed enclosure is very much dependent on its inside volume. Normally, a large
sealed enclosure will provide better bass response than a small one. In high quality reinforcement and
Hi-Fi installations, enclosures are "tuned" to the resonant frequency of the (bass) loudspeaker, often by building
in a bass opening or elongated port having very critical dimensions.
42
15.1.2 Ceiling loudspeakers
A ceiling loudspeaker is a cone loudspeaker,
mounted on a front panel, which may be
recessed into a ceiling or hollow wall. They
can be spaced at regular intervals to give a
fairly even coverage of sound.
Loudspeakers placed in a square pattern
Ceiling Lsp
X
-6dB
A common used calculation-method leads to
the mutual distance between the speakers:
D = 2 H tan (α/2)
( H = Ceiling height to Ear height
and α = opening angle at 4 kHz )
H
C
And the total number of the speakers:
n = Area / D2
D = 2 H tan (α/2)
n = Area / D2
D
Opening Angle at 4 kHz = 60 0
Ceiling Height
in m 3
Mutual Distance D in m 1,7
Covered Area
in m2 3
The accompanying tabel shows the level
variations which can be expected for
different opening angles.
Level variation = 4,5 dB
3,5 4 4,5 5 5,5 6
2,3 2,9 3,5 4 4,6 5,2
5,3 8,3 12 16 21 27
Opening Angle at 4 kHz = 90 0 Level variation = 5 dB
Ceiling Height
in m 3 3,5 4 4,5 5 5,5
Mutual Distance D in m 3 4
5
6
7 8
Covered Area
in m2 9 16 25 36 49 64
6
9
81
Opening Angle at 4 kHz = 120 0 Level variation = 7 dB
Ceiling Height
in m 3 3,5 4 4,5 5 5,5 6
Mutual Distance D in m 5,5 7 9 10,5 12 14 16
Covered Area
in m2 30 49 81 110 144 196 256
Note:
a: Caution should be taken when mounting
these units in particularly high ceilings (> 5
meters) and in noisy environments. The
level of sound reaching the listeners may be
unacceptably low, due to the distance
involved, and the limited maximum SPL
available from the units.
b: It is difficult to obtain good results from a
ceiling loudspeaker system in rooms with a
reverberation time of more than 2 seconds
(see chapter 18 for indoor acoustics).
D
α/2
-6dB
-6dB
H
-12dB
-12dB
- 9dB
0dB
-1
-6dB
0dB
-3
-4 -5 -6 -7 -8 -9 -10 -11
-12
The level variation for wide opening angles
however is due to the extra distance attenuation
more than for small opening angles.
A special ceiling speaker program CSP calculates
the actual levels under and between the speakers,
and can be set for 6-5-4-3-2-1 dB variation. For
every Bosch ceiling speaker type the required
number of speakers and mutual distance is
calculated.
α
-6dB
-2
-8dB
43
15.1.3 Sound columns
Sound columns are a group of (usually 4 to 10) loudspeakers mounted close
together in a vertical array. Due to an interesting acoustical phenomenon, though
the beam of sound emitted horizontally is approximately the same as a normal
cone loudspeaker, the beam of sound emitted vertically is narrow (10-150) and
therefore very directional, especially at higher frequencies. Column loudspeakers
are particularly useful in situations where a great degree of control is required over
the vertical spread of sound, and no spill of sound is acoustically allowed.
A typical instance would be in reverberant environments (e.g. churches) where it
is desirable to beam the sound down onto the listeners, without it reflecting off
hard walls and ceilings.
Unfortunately the bass frequencies are less directional than the higher
frequencies, and spread much wider than the useful loudspeaker opening angle.
In reverberant environments this wide spread of low frequencies can excite a
reverberant field, causing great problems with intelligibility. In situations where the
microphone is in the same room as the loudspeakers, this can also cause
acoustic feedback. This can be overcome by the use of equalisation (described in
chapter 10), reducing the volume of the bass frequencies in the signal. Though
this is acceptable for speech purposes, it would have an adverse affect on the
quality of music reproduction, so care should be taken not to completely eliminate
the bass content of the signal if music is to be amplified.
44
15.1.4 Horn loudspeakers
Horn (or 'diaphragm') loudspeakers, are different to cone loudspeakers in
that the sound produced is generated by a small, thin metal diaphragm,
and amplified by the shape and size of a folded horn. They produce a very
powerful, concentrated, beam of sound enabling them to reach listeners at
a great distance.
Because the diaphragms are normally mounted in moulded plastic or
metal folded horn enclosures, they can be easily rendered weatherproof,
which allows them to be used outdoors and in dusty and humid
environments. They may be mounted on masts or higher buildings and/or
arrayed in a column to produce a directional vertical beam.
The diaphragm loudspeakers used in public address installations have the
limitation of having a fairly restricted frequency range, giving a diminished output at low frequencies due to the
diameter of the horn and at high frequencies due to folding of the horn. This makes them generally unsuitable
for satisfactory music reproduction, but can to some degree be compensated for by combining them with cone
loudspeakers.
15.1.5 Full range high power loudspeakers
Loudspeakers with diaphragms mounted directly onto
the mouth of an exponential horn are often used as the
treble component of a "full range" multiple loudspeaker
enclosure. The audio signal is fed through a suitable
crossover filter which eliminates the bass content,
which could damage the diaphragm. These enclosures,
often grouped together in a cluster, are used in
installations to produce full range high power sound.
Combining the horn in the centre of the woofer
loudspeaker has the advantage of a compact stackable
or arrayable unit.
45
15.2 MATCHING LOUDSPEAKERS TO AMPLIFIERS
4Ω
8Ω
8Ω
8Ω
8Ω
8Ω
8Ω
8Ω
8Ω
8Ω
8Ω
4Ω
8Ω
Two systems are available for connecting loudspeakers to amplifiers:
-The direct low impedance system
and
-The 100 Volt Line Matching System
(which is normally used in public
address emergency & announcement systems).
8Ω
4Ω
8Ω
8Ω
4Ω
4Ω
4Ω
4Ω
8Ω
4Ω
4Ω
8Ω
4Ω
4Ω
4Ω
4Ω
4Ω
4Ω
4Ω
4Ω
4Ω
4Ω
The loudspeakers could be
connected in a series/parallel
arrangement, as illustrated, to
exactly match the amplifier's low
output impedance.
This is only a feasable solution if the
power leads to the loudspeakers are
reasonable short, otherwise line
losses are considerably.
matching loudspeakers to amplifier low impedance output
If the loudspeakers differ in power
and impedance, it is very difficult indeed to match them to the power amplifier. In this type of situation, or in an
application requiring long loudspeaker cable lengths ( e.g. public address systems), the 100 Volt line system
should be used.
When loudspeakers are connected to the 100V tap on the amplifier's
line matching transformer, their full power is used, whereas if they
100V
0V
are connected to the 70V tap, only 1/2 of their rated power is used.
70V
This means that the 70V tap enables the amplifier to power twice as
P
50V
1/2P
many loudspeakers, with each loudspeaker producing 1/2 of its
0V
1/4P
potential power. Similarly, the 50V tap allows loudspeakers to use
amplifier
loudspeaker
1/4 of their rated power, so that the amplifier is able to power 4 times
more loudspeakers, with each producing 1/4 of its potential power.
100V
P
full power
70V
P
P
50V
100V
1/2P
100V
1/4P
70V
1/2P
1/2 power
1/4 power
1/2 power
1/4 power
1/4 power
The transformers fitted to loudspeakers have similar taps, but in this
case the actual power which the loudspeaker will draw (e.g. P, P1/2,
P1/4, or 6W, 3W, 1,5W), instead of the voltage, is printed beside
each power (+) tap. A reduced loudspeaker volume can be set by
using these taps. For instance if the same type of loudspeakers are
powered from a common amplifier, and it is desired to have one of
them producing less volume than the others, then it is a simple
matter of connecting the signal to either the 1/2 or the 1/4 power (+)
tap. This would reduce the output of the loudspeaker by 3dB or 6dB
respectively.
Note:
When using the 100 Volt line matching system, the Rated Power of
the amplifier corresponds to the Rated Load Impedance of the
loudspeaker network. The total rated power required should be
calculated, by simply adding the Rated Power of the connected
loudspeakers together, taking into account the difference in power
drawn when using the loudspeaker power taps. It is important that
this total should not exceed the rated power of the amplifier.
Loudspeakers in the Bosch product range are manufactured with a Power Handling Capacity (PHC) according
to the IEC268-5 standard. These loudspeakers are actually capable of withstanding power input greater than
the PHC, which enables them to avoid damage during times of excessive signal overload (acoustic feedback! )
46
16.0 Technical Principles
16.1 BASIC PRINCIPLES
•
•
•
Loudspeaker power handling capacity is measured in watts (W). A 6W loudspeaker would be able to
accept a maximum of 6 watts from a power amplifier.
The 'sensitivity' of a loudspeaker is the Sound Pressure Level (SPL), expressed in dB, at 1 kHz, measured
at a distance of 1 meter, on an axis with its centre, when it has an input of 1 watt.
Each time the input power of a loudspeaker is doubled, the SPL rises by 3 dB. Therefore if we know the
sensitivity of a loudspeaker, it is a simple matter to calculate its SPL at any given power input. E.g.: If a
loudspeaker has a sensitivity of 99 dB (1W/1m), 2W would raise the SPL by 3 dB, to 102 dB; 4W would
increase it to 105 dB; etc., until it reaches maximum rated power.
80 dB
80 dB
86 dB
Total sound pressure level is raised by 6dB
80 dB
83 dB
80 dB
Total sound pressure level is raised by 3dB
112dB
106dB
1m
2m
• If 2 loudspeakers are placed side by
side and given the same input signal
(so that both are in phase, operating
as one unit) the SPL at the listeners
would be 6 dB more than the SPL of
a single speaker. Each time the
quantity of loudspeakers is doubled,
the SPL is increased by 6dB.
• If those same loudspeakers are
placed some distance away from
each other (even so small a
distance as 1 meter), there will
always be a shift in phase at the
ears of the majority of the listeners.
This causes the total SPL to
increase by only 3 dB, instead of
6dB, each time the quantity of
loudspeakers is doubled.
100dB
3m
4m
94dB
5m
6m
7m
8m
The sound pressure level is decreased by 6dB per distance doubling
•
As we move further away from the sound source the SPL drops. Again a simple rule is in force; Each time
the distance from the loudspeaker is doubled, the SPL drops by 6 dB.
For instance, assuming that we have a loudspeaker cabinet producing 112 dB at 1 meter; at 2 meters
distance the SPL would be 106 dB; at 4 meters 100 dB etc.
This rule only deals with direct sound, not taking into consideration any (indirect)sound returned from
reflective surfaces.
That problem is dealt with separately in chapter 18.
47
250 Hz
1 kHz
4 kHz
The opening angle is frequency dependent.
We need full spectrum equal coverage,
minimising the 4 kHz variation at ear level.
• All of these
examples so far
have dealt with
loudspeakers
producing a 1000
Hz tone, being
measured in line
with the loudspeaker's axis. By looking at the polar
diagram, we can
see that the SPL
differs depending on
the frequency being
transmitted, and at
what angle the listener is relative to the
axis (0o). This effect
will be used in the
formula for the
direct sound (LQ.)
see chapter 17.
The number of
degrees between the points where LQ = 6 dB is the opening angle normally defined for 4 kHz for clarity reasons.
In the polar diagrams, this is indicated with a grey shading. The opening angle upto 4 kHz is vital for the
intelligibility & clarity reasons.
16.2 DETAILED CONSIDERATIONS
16.2.1 Resonant frequency
At the resonant frequency the impedance is very high in relation to the average impedance. This varies from
cone loudspeakers (20 Hz to 300 Hz) to horn drivers (200 Hz to 1 kHz). The 'nominal impedance' is the
impedance of the lowest part of the curve above the resonant frequency (fo) - usually around 400 Hz. Damage
can occur to the loudspeaker if power is sustained at the resonant frequency. So where continuous alarm
signals are required, care should be taken to ensure that the frequency of the signal is well above the resonant
frequency of the loudspeakers used.
16.2.2 Sensitivity
The sensitivity level of a loudspeaker is the loudness expressed in dB (SPL) at 1 kHz and at a distance, on axis,
of 1 m with an input of 1 W. The importance of this figure may be illustrated by examining the effect of varying
the two main parameters, namely, distance and power.
Because the efficiency of loudspeakers, horn drivers, columns, etc., vary so much, it is impossible to define the
number of loudspeakers required for a room (and the amplifiers required to drive them) without first calculating.
Assume that it is required to produce a SPL of 80 dB at a distance of 32 m. To calculate the required power
for the loudspeaker (For simplicity an outdoor situation is chosen):
Reduction in acoustic level due to distance
= 20 Log 32
= 30 dB
To compensate for this reduction, 80 + 30 = 110 dB (SPL) is required at 1 m distance from the loudspeaker.
If the loudspeaker has a sensitivity of 100 dB (SPL) the missing 10 dB should be compensated for with an
extra 10W power applied to the loudspeaker.
16.2.3 Efficiency
A loudspeaker's ability to convert electrical energy into acoustical energy is defined as its efficiency, and can be
stated as a percentage figure (values between 0.5-10%). This value is required for calculations of the
reverberant sound field. See chapter 18 for details. Because this varies with frequency, Bosch specifies the
loudspeaker's efficiency per octave band, on the technical documentation.
48
16.2.4 Directivity (Q)
The directivity factor (Q) of a loudspeaker is the ratio of the mean squared sound pressure level at a fixed
distance, measured on axis (which is normally the direction of maximum response), to the mean squared sound
pressure level at the same distance, averaged over all directions. Q is therefore a measure of the response of
the loudspeaker in a three dimensional plane.
125 Hz
1 kHz
8 kHz
At low frequencies the radiation of a loudspeaker has a spherical form which becomes more directional as the
frequency increases. This indicates that Q is frequency dependent. Since readings are normally taken in 10°
intervals in a sphere, for each of seven octave bands, this requires the processing of more than 2000 readings.
Because of this only a few of the leading manufacturers actually quote figures for the directivity factor.
130
dB(SPL)
Q-Factor
100
120
110
100
10
90
80
70
1
125Hz 250Hz 500Hz 1kHz
1W, 1m(dB)
Max,1m(dB)
Q-Factor
Effic. (%)
Hor. Angle
Vert.Angle
76
87
3.3
0.02
180
87
98
3
0.21
180
97
108
5.1
1.24
180
70
97
108
9.8
0.64
180
40
2kHz
4kHz
8kHz
98
109
19
0.42
140
20
97
108
25
0.25
110
12
93
104
36
0.07
80
8
Standard format for loudspeaker technical specifications,
showing average performance for each of the seven octave bands.
Typical average Q values are:
Loudspeaker in sealed (infinite baffle) enclosure
Average male human speaker
Column Loudspeaker
Cardioid Column Loudspeaker
:2
: 2.5
:7
: 20
49
These specifications are measured in an anechoic room following the procedures defined below:
1.
The frequency response is measured on axis (0o) at 5 metres and calculated to 1 metre:
- with “slow” damping using a gliding tone and/or a 1/3 octave warble (woble) tone.
- in 7 octaves, using a stepped pink noise measurement signal.
dB(SPL)
0
-10
80
125
100
160
200
250
315
500
630
800
400
1k
1.25
1.6
2k
2.5
4k
3.15
5
8k
6.3
10
Hz
12.5
The effective frequency range is defined as being the range between those points at which the level
drops by 10 dB.
2.
For enclosures with single loudspeakers, polar diagrams are measured using pink noise. This is done
in octave steps with centres at 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz, and 8000 Hz.
For enclosures with asymmetrical or multiple loudspeakers the directivity balloon is measured using a
pan and tilt device. The definition is every 100 for all 7 octave bands.
3.
Using the measurements in 1. and 2., the software package EASE is used to calculate the “Q” and
“Efficiency” values for all relevant octave bands.
4.
Using the measurements in 2., the horizontal and vertical opening angles (-6 dB) are determined for all
relevant octave bands.
5.
The Power Handling Capacity is determined by applying the IEC pink noise test signal shown below
for 100 hours. After this test the loudspeaker should still be able to perform according its specification.
dB
0
Special noise signal acc. IEC 268-3
for Power Handling Capacity test
of loudspeakers (100 hours duration)
-10
-20
63
50
80
125
100
250
160
200
315
500
400
50
630
800
1k
1.25
2k
1.6
2.5
4k
3.15
5
8k
6.3
10
Hz
12.5
The Acoustic Environment
The characteristics of sound and the way it is transmitted are very much altered by the environment in which
it is generated. The same audio signal would sound quite different in a sports stadium as compared to a large
reverberant church or to a heavily damped lecture room.
In general, it is possible to differentiate between two situations: the outdoor and the indoor environment.
In both situations though we are striving primarily at:
1. Speech Intelligibility
- delivering the message to the ears of the listener clearly.
2. Quality of Reproduction
- delivering e.g. music to the ears of the listener as unchanged as possible.
17.0 Outdoors
In the outdoor environment several factors must be considered which influence sound reproduction and
reception:
•
•
•
•
•
•
•
•
•
•
•
Sensitivity
Power
Directivity
Distance
Reflection
Absorption
Refraction
Air absorption
Humidity
Temperature
Echoes
51
17.1 TECHNICAL CONSIDERATIONS
17.1.1 Power
Power
dB (SPL)
1W
2W
4W
8W
16 W
32 W
100 dB
103 dB
106 dB
109 dB
112 dB
115 dB
Each time the input power of a loudspeaker is doubled, the SPL rises
by 3 dB. The effect, at a distance of 1m, is shown in the table, which lists
the increase in SPL with doubling of power, from a nominal value of
100 dB (SPL)
Intermediate powers may be accounted for by: dB (SPL) at measured power = SPL 1.1 + 10 Log P/P0
where:
SPL 1.1 = sensitivity of loudspeaker in dB (SPL) for 1 watt at 1 meter
P
= power (W)
= reference power (1W)
P0
Using our reference of 100 dB(SPL), for an increase of 12 W the calculation is:
100 + 10 Log 12
= 100 + 10.8
= 110.8 dB (SPL)
1
2
3
1 2 3 4
0
power ratio
10
4 5 6 7 89
6 7 8 9
5
2
3
11 12 13 14
10
100
4 5 6 7 89
16 17 18 19
15
dB
1000
2
3
21 22 23 24
20
26 27 28 29
25
52
4 5 6 7 89
30
This SPL increase of 10.8 dB for a
power increase of 12 W can also be
seen in the accompanying table.
17.1.2 Directivity
Before attempting to calculate coverage, it is necessary to know a little about the different characteristics of
certain types of loudspeakers. One of the fundamental differences in loudspeaker types is their 'opening angle'.
This is the dispersion (measured as an angle) of sound which radiates from the front of the speaker.
Dependent upon the environment and the particular application needs, it may be necessary to use
loudspeakers with a wide opening angle, which disperse (spread) their sound over a wide area.
Alternatively it may be necessary to concentrate a beam of sound in a particular direction. This would be
important where an unnecessarily wide spread of sound is not only wasteful in amplifier energy, but could
reflect off nearby buildings, or disturb people in neighbouring areas. This is particularly vital when the
microphone is also outdoors, and exposed to sound coming from the loudspeakers. An uncontrolled spread of
sound could return a large amount of the audio signal into the microphone, which will be amplified again,
causing acoustic feedback or howl. Take care to place the loudspeakers in such a position that there is a "quiet"
area around the microphone location, if possible with the loudspeakers in front of, and pointing away from, the
microphone.
Even though certain types of loudspeakers produce a fairly wide spread of sound, by grouping several of them
in a vertical configuration, commonly called a column, the shape of the total beam of sound can be altered to
make it more directional. This is discussed in greater detail in 15.1.3.
In installations with low output level loudspeakers, mounted along the length of an area, spacing the
loudspeakers less than 15 meters apart will help minimise echo. See 10.0 for details of using a delay line in this
type of situation.
17.1.3 Attenuation due to Distance
When sound is reproduced in an outdoor situation, without any objects to cause reflection, the listener hears
only direct radiation. The sound pressure level drops by 6 dB(SPL) each time the distance is doubled.
The table below shows SPL decrease with the doubling of distance, from a nominal value of 100 dB(SPL)
Distance
dB(SPL)
1m
2m
4m
8m
16 m
32 m
100 dB
94 dB
88 dB
82 dB
76 dB
70 dB
Assume that a loudspeaker source has a sensitivity (SPL1.1) of
100 dB(SPL). An input of 1 W gives the following results:
For intermediate distances:
dB(SPL) at measured distance = SPL1.1 - 20 Log r/r0
where:
SPL1.1 = sensitivity of loudspeaker in dB(SPL) 1W;1m
r
= measured distance (m)
= reference distance (1m)
r0
Using the nominal value of 100 dB, the calculation of the SPL at 25 metres is:
100 - 20 log 25 = 100 - 28
= 72 dB(SPL)
1
10
2
3
2 4 6 8
0
4 5 6 7 89
12 14 16 16
10
distance in metres
2
3
22 24 26 28
20
100
4 5 6 7 89
32 34 36 38
30
dB
1000
2
3
4 5 6 7 89
42 44 46 48
40
52 54 56 58
50
53
60
The SPL decrease of 28 dB at a
distance of 25 metres can also be seen
in the accompanying table.
17.1.4 Variations of both distance and power
Assume that a loudspeaker has a sensitivity of 100 dB. To calculate the dB(SPL) at 26 m with an input of 10W:
At 26 m the loss in dB(SPL)
And at 10 W, gain in dB(SPL)
= 20 log 26
= 10 log 10
= 28.3 dB
= 10 dB
The total effect of both variations is simply their algebraic addition:
100 - 28.3 + 10 = 81.7 dB(SPL)
Generally we can calculate as follows:
Ldir
= Ls + 10 Log(Pel) - LQ - 20 Log(r)
Ls = SPL1.1 = SPL value for 1W at 1m on axis.
Pel
= power consumption of loudspeakers (W)
= on/off axis level difference
LQ
r
= distance from the source
17.1.5 Refraction
Refraction, or bending, occurs when sound passes from one medium to another. This effect is also noticeable
when sound passes through layers of air which have different temperatures and thus different sound velocities.
Cooler
The illustration shows the effect of refraction,
causing sound to bend upwards.
Warmer
17.1.6 Reflection
Although the effect of reflection is mainly of concern in an indoor situation, reflections from buildings outdoors
give distinct and very disturbing echoes. If the time delay between the original sound and the reflected sound
is more than 50 ms, the listener will be able to hear, and to recognise, a reflected sound as a whole "echo" of
the original. Knowing the speed of sound in air to be 340 m/s, then the time difference of 50 ms is equivalent
to a distance of 17 m. If the difference between the direct and the indirect distances is significantly shorter than
17 m, the reflected sound will have the effect of reinforcing the direct sound, rather than causing an echo.
17.1.7 Ambient Noise
The perceived quality from a sound reinforcement and/or public address distribution system can be particularly
effected by ambient noise. The constant sound of passing traffic, the rumble of heavy industry or even the hum
of conversation from a large crowd, can create a significant ambient noise level, which must be compensated
for.
When the sound level of a source is being measured in a situation where ambient noise is present, it is
necessary to subtract the ambient noise level reading from the combined (total) reading in order to find the
actual level of the source alone. If this is not done it is not possible to measure the source level accurately.
This is calculated by:
Ls = 10 Log [10
L1/10
-10
L2/10
]
where: L1 is the reading taken of the source and the noise combined (e.g. 60 dB(SPL)) and,
L2 is the reading of the noise alone, with the source shut off (e.g. 55 dB(SPL)).
In this example the level of the source is:
Ls = 10 log [106 - 105,5]
= 58,3 dB (SPL)
54
18.0 Indoors
18.1 TECHNICAL CONSIDERATIONS
When designing a sound system for indoors, the situation is made difficult by a number of problems which must
be taken into consideration.
Because the listener is often seated some distance from the source of the sound, high frequency signals are
absorbed by the air, while the lower signals activate reverberation as they bounce off hard walls and ceilings.
This means that, in a reverberant environment, with increasing distance, we encounter two problems at the
listeners:
•
A decreasing original (direct) speech spectrum (SPLdir), discussed in 17.1.3.
•
An reverberant low toned indirect/reflected speech spectrum (SPLrev). This means that the listeners may
hear everything loudly, but the consonants in the speech are hidden or masked by the reverberation,
causing low speech intelligibility, so that they cannot understand what is being said.
20
18
dB 16
14
12
10
8
6
4
2
0
-2
-4
-6
-8
-10
Direct Field
Reverberant Field
SPLrev
SP
L
di
r
0.1 0.16 0.25 0.4 0.63 1
1.6 2.5
4
6.3
10
0.125 0.2 0.315 0.5 0.8 1.25 2 3.15 5
8
DC
DL
D/DC
18.1.1 Reflection & Absorption
When a sound source is in a room and enclosed e.g. by walls and a ceiling, these surfaces will partly reflect and
partly absorb the sound. The intensity of the reflected sound wave (Iref) is smaller than the incident one (Iinc), a
fraction α of the incident energy is lost during reflection, or:
Iref = (1-α) Iinc
α is called absorption coefficient
Most of the building materials have measured absorption coefficients (α) and reflection coefficients (r).
α + r = 1.
If all the sound is reflected (r = 1), no sound is absorbed by the material (α = 0).
The list with absorption coefficients is provided (see appendix) for a selection of materials; a higher figure per
octave band = greater absorption. As can be seen, soft materials generally have more effect on higher
frequencies.
55
18.1.2 Reverberation
If sound is generated in a room, part will travel directly to the listener; more will arrive after having been
reflected, and still more after successive reflections.
The effect of these repeated reflections is called reverberation, which leads to the build-up of diffuse sound
throughout the room, called the reverberant field.
The actual level of the reverberant field is determined by three factors:
• the nature of the sound source
• the physical volume of the room
• the reverberation time.
18.1.3 Reverberation time
The reverberation time (T) of a room is a measure of the time taken for the sound level of the reverberant field
to fall by 60 dB. The following points regarding reverberation time are assumed:
• the reverberation time in a room is the same whatever the position of the sound source;
• the reverberation time in a room is the same wherever the listener happens to be;
• the lack of intelligibility in a room is almost always due to a long reverberation time;
• reverberation time is determined by the room volume, and total amount of sound absorption in it.
The reverberation time according to Sabine:
T = 0,161 Volume / Absorption
α S = ∑ (Si αi)
= α S + 4mV + nAP
The absorption :
A
Thus:
T = 0,161 V/( α S + 4mV + nAP )
where: V
A
S
Si
m
=
=
=
=
=
total volume of the room (m3 )
α = average absorption coefficient
n = number of persons
total absorption (m2 or Sabine)
AP = absorption per person (m2 or Sabine)
total surface area (m2 )
surface area (m2 )
atmospheric absorption (attenuation constant) see chapter 1,4 for details.
The effects of the number of people in the room should normally be taken into consideration. In many theatres
and cinemas however, the effect of the variation in audience numbers is minimised by the use of plush soundabsorbing seating, having the same absorption as a person actually in the seat.
56
In other situations like airports where the reverberation time of the empty hall is known, the influence of the
audience can be calculated by:
New reverberation time =
VT
_____________
V + ( 6 T n Ap)
where:
V
T
n
Ap
=
=
=
=
volume of room (m3 )
reverberation time of empty room (s)
number of persons
absorption per person (e.g. 0.5 Sabine)
5
T
4
li
er
p
up
t
mi
h
urc
h
c
3
concert hall
2
1
it theatre
upper lim
ema
icture cin
p
n
o
ti
o
m
r speech
studio fo
opera
1 000
10 000
100 000
V (m3)
Preferable reverberation times, dependent upon the room’s volume
Calculation example for determining the Reverberation Time (T)
Room with dimensions of 30 x20 x 10m
Total Volume = 6000m3 T= 0.161 x 6000 / A
A = Absorption is the sum of all surfaces multiplied with the corresponding absorption coefficients.
SURFACE
Floor
Side wall
Side wall
Front wall
End wall
Ceiling
MATERIAL
(carpet)
(bricks)
(bricks)
(woodpanel)
(woodpanel)
(hardboard)
α
= 30
= 30
= 30
= 20
= 20
= 30
Total
x 20 x 0.37
x 10 x 0.1
x 10 x 0.1
x 10 x 0.1
x 10 x 0.1
x 20 x 0.15
Absorption
Sabine
= 222
= 30
= 30
= 20
= 20
= 90
= 412m2
T = 0.161 x 6000 / 412 = 2.34 s (neglecting the atmospheric absorption & audience occupation).
57
18.1.4 Calculation of Direct and Indirect Sound Fields
Zero order reflections
Second order reflections
First order reflections
It is important to have a good understanding of the different sound fields in a room. Early sound carries the
intelligibility, late sound gives the disturbance. Early sound is experienced by our ears as the sum of all
speech related sounds arriving in a time window of 20-30 ms. This is the direct sound coming straight from
the source(s) plus the indirect sound due to reflections as long as they are within the time window (splittime).
First arrived sound
Early reflections
0
Late reflections
Reverberation
Reverberation
Reverberation
10
20
U s e fu l
30
40
50
60
70m s
D is tu r b in g
The level of this early useful sound (Ldir) can be calculated according the approach as explained in ch.17.
= Ls + 10 Log Pel - LQ1 - 20 Log r1
Direct Sound:
Ldir
= Ls + 10 Log Pel - LQ2 + 10 Log(1-α1) - 20 Log r2
via ceiling
Indirect Sound:
Lindir
Lindir
= Ls + 10 Log Pel - LQ3 + 10 Log(1-α2) - 20 Log r3
via floor
Pel = power rating of loudspeakers (W)
Ls = SPL1.1 = SPL value for 1W at 1m on axis.
LQ = off axis level difference
α = absorption coefficient
r = distance in meters (m)
58
18.1.5 Calculation of Reverberant Sound Fields
All the speech related sound which arrives later than 20-30ms is regarded as useless and disturbing and
consists of a chaos of reflections and is called reverberation.
The level of this reverberant disturbing sound (Lrev) depends of the source(s) , the volume and the
reverberation time of the room. The following formulas can be used to calculate the reverberant sound field:
25T (1- α)
Lrev
= 120 + 10 Log
α
Volume
1
x
=
6T
Surface
Pac
= Electrical Power x efficiency = Pel x η
Volume
Pac
T
= reverberation time (s) = RT60
α
= average absorption coefficient
η
= loudspeaker efficiency as fraction
η [%] = loudspeaker efficiency as percentage
Lrev = 120 + 10Log25/100 - 10LogV + 10LogT(1-α)η[%] Pel
Ldir = SPL1.1 + 10 Log Pel - 20 Log r
(on axis only)
Ldir = SPL1.1 + 10 Log Pel - LQ - 20 Log r
(off axis)
Lindir = SPL1.1 + 10 Log Pel - LQ + 10 Log (1- α) - 20 Log r
Lrev = 114 - 10 Log V + 10 Log T + 10 Log (1- α) + 10 Log ∑ η[%] Pel
18.1.6 Calculation of the early / late ratio
The level difference between useful Direct sound and disturbing Reverberant sound is called early/late ratio and
is a good measure for Speech Intelligibility. The useful sound level will vary at different positions in the room,
depending on distance, angle and useful reflections. The reverberant sound level is only caused by the total
radiated sound energy and the room properties like V, T. (1- α). The early/late ratio is the input for the speech
intelligibility graph on the next pages and gives a quick insight in either STI or Alc.
After calculating on a particular position in the room this level difference we enter the chart at the bottom and go
up to the intersection with the actual reverberation time (e.g. T = 3s) and read the Speech Transmission Index
(STI) value at the right edge of the chart. ( Example: 4dB > STI = 0.585 )
Calculation procedure for the different sound pressure levels.
Example: In a rectangular room 20 x 20 x 10m we recommend loudspeakers mounted at the
left and right hand side of the stage and both aiming to the centre(16m) of the auditorium.
We calculate with the 1000Hz data of the room and the loudspeakers.
Volume = 20 x 20 x 10 = 4000m3
Loudspeaker: SPL 1.1 = 96dB(SPL)
RT60 = 2.5s
η= 0.36 %
α = 0.2
PHC = 50W
Distance = 16m
SPLdir = 96 + 10Log50 - 20Log16 = 96 + 17 - 24
= 89 dB(SPL)
Due to sound from 2 loudspeakers 89 + 3
= 92 dB(SPL)
On half distance (+6 dB) and - 6 dB off axis we expect the same level = 92 dB(SPL)
SPLrev = 114 - 10Log4000 +10Log2.5 +10Log0.8 +10Log2x0.36x50
= 96.5 dB(SPL)
Difference between Lrev and Ldir is 4.5 dB >>
T = 2.5s >> RASTI = 0.60
59
SPEECH INTELLIGIBILITY GRAPH STI & RASTI
T[s]
STI
.2
.25
.3
.4
.5
.6
.8
1
1.25
1.6
2
2.5
3
4
5
6
8
10
10
9
8
7
6
5
4
3
2
1
0
.82
.78
.74
.70
.65
.60
.57
.52
.48
.44
.40
.35
.31
.27
.23
.18
.14
.10
-1 -2 -3 -4 -5
Lr - Ld [dB]
This diagram helps to make a direct translation from the level difference between useful Direct sound and
disturbing Reverberant sound into Speech Intelligibility. The useful sound level will vary at different
positions in the room, depending on distance, angle and useful reflections.
The corresponding formulas are used in the example below, which is a sound reinforcement
system application with two loudspeakers (1) & (2).
Lrev = 114 - 10 Log V + 10 Log T + 10 Log (1- α ) + 10 Log ∑η(%) Pel =
100
dB(SPL)
Ldir (1) = SPL1.1 + 10 Log Pel - 20 Log r1 - LQ1 =
93
Ldir (2) = SPL1.1 + 10 Log Pel - 20 Log r2 - LQ2 =
90
Lref (1) = SPL1.1 + 10 Log Pel - 20 Log r3 - LQ4 =
87
Lref (2) = SPL1.1 + 10 Log Pel - 20 Log r4 - LQ5 =
87
Total useful sound (within 25 ms) added acc. 2.2.1
Difference: Reverberant minus Useful
96
4
dB(SPL)
dB
dB(SPL)
dB(SPL)
dB(SPL)
dB(SPL)
After calculating on a particular position in the room this level difference we enter the chart at the bottom
and go up to the intersection with the actual reverberation time (e.g. T = 3s) and read the Speech
Transmission Index (STI) value at the right edge of the chart. ( Example: 4dB > STI = 0.585 )
60
18.1.7 Speech Transmission Index (STI & RASTI)
Introduction
In any location where verbal communication takes place, the quality of the speech transmission is of interest,
and in many cases such as auditoria, theatres, conference rooms etc., it is of paramount importance.
Many different approaches have been taken in the past to try and assess 'intelligibility', using both subjective
and objective methods. The subjective methods used have been based upon word scores which are arrived at
using teams of trained speakers and listeners. This method is time consuming and not always practicable due
to the human factors involved. Efforts have therefore been directed to devise methods which yield an objective
measure of the speech intelligibility based upon the important acoustic factors background noise and
reverberation. The method of Articulation Index (AI), for example, is based primarily on the measured signal to
noise ratio with corrections being applied to allow for the effects of reverberation.
STI
A method is now available, called STI, which allows objective measurement of the quality of speech
transmission with respect to intelligibility. STI stands for Speech Transmission Index.
The STI method is being standardised by the IEC and published in IEC 268, Part 16.
The STI method is in some respects similar to the method of Articulation Index but offers several advantages,
some of these being:
-The effects of both background noise and reverberation are automatically taken into account and no
corrections need be applied to the results.
-The measurement is made with the signal and background noise present. The signal and noise need
not be measured separately.
-A STI measurement can be made in less than ten seconds.
The approach used in the STI method is based on the theory that the ability to understand speech is mainly
determined by the correct reception of the low-frequency modulations of the speech carrier signal, which are
associated with the fluctuation rhythms encountered in speech. STI is an index which varies between 0 and 1
and serves as a measure of the speech intelligibility. The index is derived from the measured reduction in signal
modulation between the speaker and listener positions.
Measuring RASTI
Special software has been developed to enable measurements of STI values and to provide additional
information of diagnostic value. The STI measurement system consists of a Laptop PC which sends out the
special STI test signal, and analyses the received signal, and, on the basis of the measured change in signal
modulation, calculates the STI value. Since the STI value is derived from the measured degradation of the
signal, it automatically includes the effects of room reverberation and background noise, and no corrections
need be applied to the results. STI values measured in different buildings can therefore be directly compared.
STI values can be converted to an expected PB-word score (phonetically balanced cvc nonsense words) on the
basis of comparative studies which have been made, and published.
The system can be used for the objective measurement of speech intelligibility in auditoria, churches, theatres,
schools. It will also find application in the testing of Public Address systems in airports, railway stations,
industry, and emergency PA-systems. It is the ideal instrumentation for rapid and unambiguous assessment of
the effectiveness of measures intended to improve the speech intelligibility in different situations.
The STI Method
STI is a method of quantifying the intelligibility of transmitted speech and is based upon the method of the
Speech Transmission Index STI. Perfect transmission of speech implies that the temporal speech envelope at
the listener's position replicates the speech envelope at the speaker's mouth. Speech intelligibility can be
quantified in terms of the changes brought about in the modulation of the speech envelope as a result of noise
and reverberation in the room. The reduction in modulation can be described by a modulation reduction factor.
The modulation reduction factor expressed as a function of modulation frequency is called the Modulation
Transfer Function, MTF. This function provides an objective means of assessing the quality of the speech
transmission, and from it, the STI value is derived.
61
Speech
STI -Signal
The Test Signal
The STI method involves measurement of the reduction in modulation of a transmitted test signal. The test
signal used has certain characteristics which are representative of a human voice. An example of a human
voice signal is shown in Fig. 2. The signal used in the RASTI method consists of an intensity modulated noise
carrier signal and the modulation envelope is shown in Fig. 3. The characteristics of the human voice which the
STI signal is designed to simulate are: the carrier signal, and the low frequency intensity modulations. The STI
carrier signal consists of two octaves of band limited pink noise centred at 500 Hz and 2 kHz.
The levels in these octave bands are chosen to equal the average levels found in normal speech, ie. 59 dB in
the 500 Hz octave and 50dB in the 2 kHz octave, at 1 m.
The low-frequency modulations present in human speech are simulated in the STI test signal by 9 discrete
modulation frequencies between 1 Hz and 11,2 Hz. These frequencies span the range found in human speech.
Measurement
A STI measurement is made by transmitting the special test signal and analysing it at the listener's position with
a view to calculating the reduction in modulation index for each of the nine modulation frequencies. The 9
modulation reduction indices obtained are interpreted as though they were brought about by background noise
alone. The signal to noise ratios which alone would have resulted in the measured reduction in modulation are
calculated. The STI value is the arithmetic average of these "apparent" signal to noise ratios, normalized so that
the index lies between 0 and 1.
Background Noise and Reverberation
The reduction in modulation of a speech signal is determined by two factors: the signal to noise ratio, S/N, at the
listener's position, and the reverberation (which is a function of the speaker and listener positions). A given STI
value could be produced by any combination of these two factors. In practice, the relative contribution of S/N
ratio and room reverberation will not be known exactly, but useful information can be obtained from a study of
the Modulation Transfer Function, MTF. If the predominant factor is background noise, then the MTF will be flat,
as the background noise affects the modulation to the same extent at all modulation frequencies. If
reverberation is the major factor, the MTF will have a negative slope. This is because rapid fluctuations in the
sound intensity envelope become more blurred as a result of reverberation, compared to the slower
fluctuations. The reduction in modulation is therefore greater at higher modulation frequencies.
Theoretical Models
62
STI values can be quite readily calculated from a knowledge of the acoustic characteristics of the building in
question. At the design stage it is therefore possible to specify the desired speech intelligibility in terms of STI
values; actual measurement of STI values in the finished building can then serve as a check on specifications
and as a valuable feedback into the theoretical models.
The Modulation Transfer Function, MTF, upon which STI is based, can be calculated from a knowledge of the
reverberation time, T, and the signal to noise ratio.
Limitations
There are certain limitations on the measurement conditions within which the RASTI method can produce valid
absolute results:
-Linear transmission is assumed in the STI method. Non-linear distortion and clipping are not taken fully
into account.
-Intense pure tones in the background spectrum outside the 500 Hz and 2 kHz bands are not allowed
-The background noise should also be reasonably stationary during the measurement.
-The reverberation time of the room should not be strongly dependent upon frequency.
If these conditions are not met, the results obtained can not be interpreted as absolute measurements but can
nevertheless be used for comparison purposes for measurements made under the same conditions.
18.1.8 Subjective %ALcons and RASTI requirements.
%ALcons = 1 - 10%
RASTI
≥ 0.50
Speech intelligibility adequate for complicated messages and lectures and for
untrained speakers & listeners.
%ALcons = 10 - 15% Speech intelligibility adequate for less complicated messages by untrained speakers,
RASTI = 0.50 - 0.45 but still adequate for complicated messages in a clear and well articulated voice.
%ALcons = 15 - 30% Speech intelligibility adequate only for simple messages and announcements.
RASTI = 0.45 - 0.32 Complicated messages require trained speakers & listeners.
%ALcons = 30%
RASTI
= 0.32
Limit of acceptable intelligibility for simple messages, for trained speakers & listeners.
18.1.9 Converting RASTI to %ALcons
RASTI
0.20
0.22
0.24
0.26
0.28
0.30
0.32
0.34
0.36
0.38
0.40
0.42
RASTI
%ALcons
58
52
47
42
37
34
30
27
24
22
20
18
%ALcons
%ALcons = 170.5405 e
0.44
0.46
0.48
0.50
0.52
0.54
0.56
0.58
0.60
0.62
0.64
0.66
RASTI
0.68
16
14
13
11
10
9.1
8.2
7.4
6.6
5.9
5.3
4.8
%Alcons
4.3
- 5.419(STI)
0.70
0.72
0.74
0.76
0.78
0.80
0.82
0.84
0.86
0.88
0.90
3.8
3.4
3.1
2.8
2.5
2.2
2.0
1.8
1.6
1.4
1.3
STI = - 0.1845 Ln (%ALcons) + 0.9482
Source: Farrel Becker
63
19.0 Designing For The Acoustic Environment
19.1 LOUDSPEAKER PLACEMENT AND COVERAGE
A few practical considerations must be taken into account when selecting, placing and aiming
a loudspeaker in a sound system design.
1.
The loudspeakers must be positioned in such a way that they are able to produce an
even spread of sound, reaching all audience areas of the room with adequate loudness
and clarity. If this is not so, some listeners could be exposed to an uncomfortably high
SPL, while others may have difficulty in actually hearing the audio signal sufficiently.
2.
Speech requires generally a good transmission and reproduction of the 500 Hz to 5 kHz
frequency band, while music requires at least 100 Hz to 10 kHz to give satisfactorily
results. This should be taken into consideration in selecting a loudspeaker type.
3.
For speech applications, upto the 4 kHz octave band is essential for the annunciation of
consonants, and therefore intelligibility. Therefore we use the loudspeaker opening angle
data at 4 kHz for the calculations for equal coverage.
4.
For ceiling systems the spacing of the loudspeakers should be determined by looking at
the covered areas (-6dB) at 4 kHz. The audience area divided by this coverage area
gives the number of speakers. It means that the audience will hear the announcements
at about the same level for the required spectrum.
5.
In installations with multiple loudspeakers, spacing the loudspeakers less than 15 meters
apart will help minimise echo otherwise proper delayed signals should be applied. (See
chapter 10 for a description of time delay)
6.
Given the specifications of the loudspeakers we intend to use, it is possible to calculate
the SPL at any point in a room or area, either by using the formulas provided in chapters
17 or 18 in this manual or using "EASE", the software package described in the
Simulating and Measuring Appendix at the end of this book.
7.
Depending on the application, a good general rule would be to calculate the level (SPL)
at 1.20 meters from the floor, which is the average ear height of a person sitting.
A popular speech peak level, known as the Comfortable Listening Level (CLL) is
generally agreed upon as 80 dB(SPL), which is the peak level in average conversation
measured on a distance of 1m. This assumes that the ambient noise level is low in the
room, which is not always the case.
8.
Background noise, or ambient noise, can make a great deal of difference to the level
required for an adequate intelligibility, especially in noisy environments such as factories or airports. To keep the level more than 15 dB louder than the ambient noise, the
use of proper callstations with build-in compressor/limiter is required.
64
19.2 SUMMARY OF THE LOUDSPEAKER-DESIGN
One of the vital requirements of any sound system is its ability to produce an even spread of
sound, reaching all parts of an area or room with equal intensity and clarity. In doing this, the
complete speech (and/or music) spectrum should reach the listener's ears as unchanged and
true to the original as possible.
The performance of a sound system can be predicted before it is installed or purchased.
The level of the direct sound as received from the loudspeakers, including beneficial early
reflections from side walls and/or ceiling, are calculated for the important octave bands.
With these calculations we optimise the coverage for the audience at 4000 Hz.
The level of the reverberant sound caused by the selected solution can be calculated if the
Volume / Reverberation time / Absorption is known.
This can be done per octave band (125 - 250 - 500 - 1000 - 2000 - 4000 - 8000 Hz).
After that the intelligibility is calculated with the values for 1000 Hz, to verify that the
listeners can hear the reproduced sound clearly.
SUMMARISING THE DESIGN PROCEDURE
1.
2.
3.
4.
5.
6.
7.
8.
9.
Select the correct loudspeaker type(s).
Select the optimum loudspeaker position(s).
Select the best aiming points.
Check the coverage at 4000 Hz.
Calculate the SPLdir on the aiming point(s).
Calculate the SPLdir on the - 6dB points.
Select the Powertapping(s).
In reverberant rooms calculate SPLrev.
Check the intelligibility in STI or Alcons(%).
Repeat(?)1-7/9 for other loudspeaker/place/aiming.
(on axis only)
Ldir = SPL1.1 + 10 Log Pel - 20 Log r
Ldir = SPL1.1 + 10 Log Pel - LQ - 20 Log r
Lindir = SPL1.1 + 10 Log Pel - LQ + 10 Log (1- α) - 20 Log r
Lrev = 114 - 10 Log V + 10 Log T + 10 Log (1- α) + 10 Log ∑ η[%] Pel
65
19.3 SPEECH INTELLIGIBILITY IN CHURCHES & COMMUNITY HALLS
The impact of a good sermon or message is greatly affected by the degree to which
speech is intelligible at the ears of the listener. Because of this, large amounts of money are
often spent on a sophisticated sound installation intended to get the word into every corner of
the listener area. Microphones - mixing desk - amplifiers - equalisers and loudspeakers are
the things wrestled with to achieve the desired effect. Unfortunately the final result is often
disappointing and at times even worse than having no sound system at all. If we look around
in many churches, the acoustic difficulties are obvious. Ancient churches are often high and
monumental, having awesome acoustics for traditional music - organ - choir and community
singing. Unfortunately this is per definition always in conflict with speech requirements.
Often the reverberation is so high that a sound system adds to the problem instead of
solving it. For good speech intelligibility we need a sound system that avoids an increase of
the reverberation but still amplifies articulation.
Let's start at the beginning - with the human voice: Speech consists of words and
pauses. Words contain both vowels and consonants. There is loudness variation. There is
pitch variation. There is a frequency spectrum (which is the lowest bass sound through to the
highest treble one). Let's first look at loudness. The vowels in a sentence have a frequency
spectrum below 1000Hz (two octaves above middle C for the musicians), and they create the
impression of loudness. The human mouth producing these sounds does so with a wide
opening angle and because the sound hits everything within range, this causes lots of
reverberation.
The consonants of the words in a sentence, having a frequency spectrum above 1000
Hz, provide the articulation. The human mouth produces these sounds with a narrow opening
angle and, because of this, is very directional. See the graph showing the loudness of
speech and the contribution of each octave band to speech intelligibility.
The complete speech spectrum should arrive at the listeners ears as unchanged as
possible. Unfortunately, because the listener is often seated some distance from the source
of the sound, the higher notes are absorbed by the air, while the lower notes activate
reverberation as they bounce off hard walls and ceilings. At the listeners ears we therefore
encounter two problems :
1. A decreasing original direct speech spectrum.
2. An increasing reverberant low toned indirect/reflected speech spectrum.
This means that the loudness might be okay but the consonants in the speech are
hidden or masked by the reverberation, causing low speech intelligibility or in technical terms
the so called "Speech Transmission Index" (STI or RASTI) is low.
19.3.1 Small reverberant traditional church building.
The sound system is normally needed for enhancing the speech intelligibility, and because
the reverberation is mainly low toned, the only thing we need to do is reproduce the mid and
high frequency spectrum of speech (above 1000 Hz). The loudspeaker system suitable for
this venue is a few columns with a multiple loudspeaker-array. These radiate in a nice wide
horizontal pattern while almost no sound is radiated vertically. This avoids sound hitting
reflective ceilings etc. avoiding unwanted reverberation. The columns should be placed
vertically and mounted close to, and somewhat at each side of the person speaking, taking
care that every seat is within the audible reach of a column.
66
19.3.2 Large reverberant monumental cathedral.
We can use the previous solution but due to larger distances we now need a more widely
distributed loudspeaker column solution. Mounting the long cardioid loudspeaker columns on
every pillar, to improve on the decreasing direct speech spectrum, will help. Unfortunately the
loudspeakers produce the sound at the same time, which means that the congregation hears
the preacher's voice from the direction of the closest loudspeaker. To overcome this disturbing effect, the sound from each (group of) loudspeaker(s) must be delayed (5 metres = 15
milliseconds) using time delay equipment.
19.3.3 Small low ceiling auditorium.
This type of venue does not cause too much trouble with respect to reverberation, but due
to the room's absorbent character, the loudness is normally low and in need of pure
amplifica-tion. We can achieve this with columns and/or with a nice pattern of (delayed)
ceiling loud-speakers in order to cover the complete audience area. If music must be
reproduced too, full range two-way loudspeakers should be used in front. In this case several
sound signals (speech & music) are amplified by the system at the same time and a mixing
desk, controlled by an operator, is needed. As an added bonus, a cassette recording can be
made of the music and sermon, and the mixer could also feed a separate induction loop
amplifier for the hard of hearing. These two functions should be considered in all
applications.
19.3.4 Large high ceiling auditorium.
We can use the previous solution but, due to larger areas to be covered, we now need
more and/or more powerful full range loudspeakers. To ensure that every seat receives
roughly the same direct sound (<5dB), we can group several loudspeaker cabinets in a
cluster high above the stage. Cabinets with "constant directivity" (the same opening angle for
all the relevant frequencies) are ideal for this purpose. Cabinets located throughout the
audience area can provide an alternative solution, but then we need proper time delays to
avoid sound coming from the wrong direction. If a TV-monitor system is used, we need to lip
synchronise our audio to the video image, and in this case no time delays should be applied.
19.3.5 Wide low ceiling auditorium.
Because a high mounted cluster might be impracticable, we must now consider a ceiling
mounted system. The advantage here is that an almost ideal coverage can be achieved, with
direct sound from the loudspeakers providing good intelligibility. Unfortunately, the fact that
the sound is coming from above may appear very spiritual, but it isn't audibly correct. Ceiling
loudspeaker groups with electronic time delay will solve this.
If music amplification is also needed, full range loud-speakers, placed left & right at the front
of the room, will give good low tone reproduction and solve the orientation problem. The
delayed ceiling system gives the extra mid and high tones needed for clarity and intelligibility.
67
19.3.6 The total (church) sound system chain
Microphones are the receivers of sound generated by such sources as preacher,
singer, musical instruments and congregation. These microphones are either directional and
mounted on stands, or omni-directional and tie clipped or lavalier. To get more freedom of
movement, wireless microphones with transmitters can be used, with radio receivers connected directly to the mixing desk. Accessories like floor stands; table stands; screened
(balanced!) cables (with XLR plugs) etc. are also needed.
The Mixing Desk is the heart of the audio system and is the control console where all
the microphones, guitars, electronic organ, cassette player, etc. come together. This is where
the final, mixed sound is sent to the amplifiers, tape recorder and/or monitor loudspeaker(s).
In order to give the sound engineer an undistorted judgement of the total sound, the favourite
place for this desk is in the middle of the auditorium.
An Equaliser gives extensive control over the whole frequency spectrum and can be
used for optimising the frequency response of the loudspeakers. It can even equalise the
complete audio chain, from microphone to ear. Used with care, this would guarantee maximum amplification for the frequency spectrum required, at the same time compensating for
the menace of acoustic feedback.
The Amplifier is used to amplify the power of the mixing desk to a level that will feed
the loudspeakers properly. This is either a 100V or 70V line level amplifier (favourable if long
loudspeaker distances are involved) or a direct low impedance 4 or 8 ohm amplifier. If using
the latter, make sure that the amplifier wattage is always lower than the loudspeaker wattage
so that the amplifier is not able to overload the loudspeakers.
Loudspeakers used solely for speech are mostly column types, having small vertical
opening-angles to reduce reverberation. For speech and music, use good full range two-way
cabinets, which generate the full speech and music spectrum.
For small churches needing only speech amplification, we can reduce the equipment
to a few microphones, one mixing amplifier and a few loudspeaker columns. The individual
microphone volume levels are controlled on the amplifier, which also allows tone-control of
the loudspeakers. Once carefully set up, such a system should work without intervention,
every time you switch on the amplifier.
19.3.7 Predicting & calculating the performance of the church system.
The performance of a sound system can be predicted before it is installed or purchased. Per seat, the amount of direct sound (received from the loudspeakers), and the amount
of reverberant sound (via the reflections) can be calculated for every relevant octave band
(125 -250 - 500 - 1000 - 2000 - 4000 - 8000 Hz). After that the timing of the sound from
individual loudspeakers is checked and the speech intelligibility is calculated.
Nowadays this time-consuming calculation work and the corresponding presentations
can all be done with PC software like Catt Acoustics or E A S E. (Electro Acoustic Simulator
for Engineers). The program provides pictures of the audience areas with plotted calculation
results regarding: coverage(SPLdir), loudness(SPLtot) clarity(dB), intelligibility(STI), arrival
times(ms), etc.
68
19.4 SYSTEM CALCULATION
19.4.1 High ceiling area e.g. Large exhibition hall
-3dB
-6dB at 4kHz
Dimensions : 172 x 72 x 15.25 m
T: 2.5s
Ambient Noise : 70 dB(SPL)
Mounting height loudspeakers : 15 m
Total number: 3 rows of 7 = 21
Loudspeaker data for 1000 Hz:
PHC = 50 Watt
Sensitivity = 89 dB(SPL)
Efficiency = 0.32%
Dispersion = 110o
What is the expected intelligibility in ALcons and/or RASTI ?
CALCULATION PROCEDURE
Ldir
= 89 + 10 Log50 - 20 Log(15 -1.5) =
89 + 17 - 22.6
Benefit of the closest four neighbour speakers (estimated) 4 x 71 dB = 77
Ldir(tot) = 83.4 dB(SPL) & 77 dB(SPL)
=
83.4 + 0.9 dB
Roomdata:
Lrev
Volume = 172 x 72 x 15.25 m = 188820 m3
α = V/6TS = 188820 / (6 x 2.5 x 32840) = 0.38
= 83.4 dB(SPL)
dB(SPL)
= 84.3 dB(SPL)
Surface : 32840 m2
1- α = 0.62
= 114 - 10 Log V + 10 Log T + 10 Log(1-α) + 10 Log{Σ η(%) x PHC(W)}
= 114 - 52.76 + 3.98
2.08
+ 10 Log (21 x 0.32 x 50)
= 88.4 dB(SPL)
Lrev - Ldir = 88.4 - 84.3 = 4.1 dB
>>>>>>>
ALcons = 5.7% (see Chapter 18.1.5)
Signal to Noise ratio = (88.4 & 84.3) - 70 = 20 dB
Intelligibility : ALcons = 20%
This is not good enough, therefore call stations with limiters are needed to minimise the dynamic behaviour of
the announcements. Bosch installations with Plena Voice Alarm or Praesideo Call Stations have those limiters
and therefore ideal for Public Address installations. The intelligibility is then not further affected by a bad signal
to ambient noise ratio as long as it is better than 15 dB.
69
19.5.2 Sound Reinforcement System Calculation & EASE
In this rectangular room we recommend loudspeakers mounted to the left and to the right
hand side of the stage and both aiming to the centre(16m) of the auditorium.
In the pictures the beams (3 & 6 dB) of the speakers and the covered areas are shown.
We calculate with the 1000Hz data of the room and the loudspeakers.
Volume = 20 x 20 x 10 = 4000m3
RT60 = 2.5s
α = 0.2
Distance = 16m
η= 0.36 %
PHC = 50W
Loudspeaker: SPL 1.1 = 96dB(SPL)
70
Calculation procedure for the different sound pressure levels.
SPLdir = 96 + 10Log50 - 20Log16 = 96 + 17 - 24
= 89 dB(SPL)
Due to sound from 2 loudspeakers 89 + 3
= 92 dB(SPL)
On half distance (+6 dB) and - 6 dB off axis we expect the same level = 92 dB(SPL)
SPLrev = 114 - 10Log4000 +10Log2.5 +10Log0.8 +10Log2x0.36x50
= 96.5 dB(SPL)
Difference between Lrev and Ldir is 4.5 dB >>
T = 2.5s >> RASTI = 0.60
SPEECH INTELLIGIBILITY GRAPH STI & RASTI
T[s]
STI
.2
.25
.3
.4
.5
.6
.8
1
1.25
1.6
2
2.5
3
4
5
6
8
10
10
9
8
7
6
5
4
3
2
1
0
.82
.78
.74
.70
.65
.60
.57
.52
.48
.44
.40
.35
.31
.27
.23
.18
.14
.10
-1 -2 -3 -4 -5
Lr - Ld [dB]
This quick calculation is only valid for two positions in the room (on the aiming point
and close to the stage centre).
To repeat these calculations for more or even every position in the room an EASE
simulation can be of great help.
71
Simulations for 1000Hz from the software package EASE
SPLdir + SPLrev
SPLdir
Speech Intelligibility acc. STI
72
20.0 Appendix
20.1 Definitions
Anechoic Room
A room whose boundaries effectively absorb all incident sound over the frequency range of interest, thereby
creating essentially free-field conditions.
Decibel Scale
A linear numbering scale used to define a logarithmic amplitude scale, thereby compressing a wide range of
amplitude values to a small set of numbers.
Directivity Factor
The ratio of (1.) the mean square sound pressure at a specified distance and direction from the sound source to
(2.) the mean square sound pressure at the same distance from a nondirectional source which radiates the
same acoustic power.
Free Field : An environment in which there are no reflective surfaces within the frequency range of interest.
Isolation : Resistance to the transmission of sound by materials and structures.
Pink Noise
Broadband noise whose energy content per constant bandwidth (Hz) is inversely proportional to frequency, but
constant per octave or 1/3 octave of bandwidth.
Sound Energy
Energy that is transmitted by pressure waves in air or other materials per unit area and is the objective cause of
the sensation of hearing. Unwanted sound is commonly called noise.
Sound Intensity (= Sound Power)
The sound energy transmission per unit area and per unit time in a specified direction.
Sound Level
The level of sound pressure measured with a sound level meter and one of its weighting networks.
When A-weighting is used, the sound level is given in dB(A).
Sound Pressure
2
A dynamic variation in atmospheric pressure (in N/m = Pascal). The pressure at a point in space minus the
static pressure at that point.
Sound Pressure Level
p
The fundamental measure of sound pressure defined as: Lp = 20 Log p dB
o
The reference pressure po is 20 µPa for measurements in air.
Standing Wave
A periodic wave having a fixed distribution in space which is the result of interference of progressive waves of
the same frequency and kind. Characterised by the existence of maxima and minima amplitudes.
Wavelength
The distance along a periodic wavefront between points of comparable amplitude with a phase difference of
one period. Equals the ratio of the speed of sound in the medium to the fundamental frequency.
White Noise
A broadband noise having constant energy per constant bandwidth (Hz) but increasing 3dB with each doubling
of relative bandwidth (octave, 1/3 octave, decade).
73
20.2 Symbols and Units
Symbol / Unit
Quantity
Remarks
B
V
S
A
T
α
m
f
ψ
Tr
c
ρ
Hz
m3
m2
m2
s
m-1
Hz
%
K
m/s
kg/m3
Bandwidth
Room Volume
Room Surface
Absorption
Reverberation Time
Absorption Coefficient
Air absorption constant
Frequency or Centre frequency of band
Relative Humidity
Room Temperature
Sound Velocity
Specific Density
e.g. Decade - Octave
w
I
p
pdir
pdif
pref
ps
J/m3
W/m2
Pa
Pa
Pa
Pa
Pa
Energy Density
Sound Intensity
Sound Pressure
Direct Sound Pressure
Diffuse Sound Pressure
Reference Pressure
Characteristic Sensitivity
Lp
Ldir
Ldif
Ls
dB(SPL)
dB(SPL)
dB(SPL)
dB(SPL)
Sound Pressure Level
Direct Sound Level
Diffuse Sound Level
Characteristic Sensitivity Level
Ls ≡ 20 Log(ps/pref)
LQ
Γ(θ,ϕ)
θ,ϕ
dB
-
Off axis level difference
Directivity Function
Angles with reference axis
See Polar diagram of LS.
Γ(θ,ϕ)
= p(r,θ,ϕ)/p(r)
Γ(θ,ϕ =0) = Γ(max)
=1
Pel
Pac
η
Q
D
r
Alcons
STI
W
W
dB
m
%!
Consumption
Radiation
η = Pac /Pel
Q = Imax /Iav; Iav = Pac /(4πr2)
D = 10 Log(Q)
ts
s
Electrical power
Acoustical power
Efficiency
Directivity Factor
Directivity Index
Distance to source
Loss of Consonants
Speech Transmission Index
RASTI = 0.9482 - 0.1845Ln(Alcons)
Splittime
T ≈ 0.161 V/A
m = m(f,ψ,Tr)
273.13K = 00C
c ≈ 344 m/s (air, 200C)
ρ ≈ 1.21 kg/m3 (air, 200C)
1 Pa = 1N/m2 ≅ 94 dB(SPL)
pref ≡ 20 µPa
74
Lp = 20 Log(p/pref)
Articulation Loss
IEC Norm
(Farrell Becker)
Time window useful sound
20.3 Equations
Sound velocity
c = 20.1 √Tr ≈ 344 m/s (air, 293 K = 200C)
Reverberation Time
T = 24 Ln10 V/cA ≈ 0.161 V/A
Absorption
Sabine: A = 4mV + αS
Eyring: A = 4mV - S Ln(1-α) ≈ 4mV + S[α + α2/2 + α3/3 + ...]
Air Absorption Constant
Can be approx. by: m ≈ 1.7 10-8 f2/ψ
for: Tr = 20 0C and the Relative Humidity ψ in % !
Characteristic Sensitivity
ps = pdir for Pel = 1W, r = 1m, Γ = 1(on axis);
Characteristic Sensitivity Level (Sensitivity)
Ls ≡ 20 Log(ps/pref)
Energy Density
w = p2/ρc2
Sound Intensity
I = pv;
I = wc = p2/ρc
Direct Sound Pressure and Level
pdir2 = ρc QP/4πr2
pdir2 = ρcPacΓ2/(4πr2) = ps2PelΓ2/r2
Ldir = Ls + 10 Log(Pel) - 20 Log(r) + 20 Log(Γ)
75
ps = √(ρcη /4π)
Diffuse Sound Pressure and Level
pdif2 = 4ρcPac(1-α)/A = 25ρcPacT(1-α)/V = 25ρcηPelT(1-α)/V
Ldif = 10 Log(pdir2/pref2)
Ldif = 10 Log(25ρc/pref2) + 10 Log(Pac(1-α)T/V)
= 134 + 10 Log(ηPel(1-α)T/V)
= 120 + 10 Log(25Pac(1-α)T/V)
10 Log(25ρc/pref2) ≈ 134;
10 Log(ρc/pref2) ≈ 120
Total Sound
Ltot = 10 Log(ptot2/pref2)
≈ LP + 10 Log[Γ2/(4πr2) + 4(1-α)/A]
pref2/ρc ≈ Pref;
LP = 10 Log(Pac/Pref);
Pref ≡ 1.10-12 (W)
Reverberation Radius
wdir(rg) ≡ wdif
rg2 = QA/(16π(1-α)) = (6 Ln10/4πc) QV/(1-α)T ≈ 3.20 10-3 QV/(1-α)T
rg ≈ 0.057 √(QV/(1-α)T)
Direct-to-Reverberant-Ratio
D/R = wdir/wdif = pdir2/pdif2 = rg2/r2
Loss of Consonants
ALcons = 200 r2T2/V ≤ 9T
(Peutz)
ALcons = 0.90 Tpdif2/pdir2 ≤ 9T (% !)
Average frequency spacing of adjacing maxima
∆fmax ≈ 4/T
(Kuttruff, p.70)
Level difference (between most probable maximum and mean value)
∆L = 10 Log(Ln(BT))
(Kuttruff, p. 71)
76
20.4 Surface material list with absorption coefficients
MATERIAL
125
250
500 1000 2000
ABS BLOCK .19
.64
.73
.62
.20
AC PLASTER .32
.32
.52
.81
.88
ACDECK 2.50.54
.97 1.00
.91
.55
ACOUSTILE .26
.57
.63
.96
.44
BAFFLE1
.19
.32
.64
.80
.75
BAFFLE2
.23
.60 1.00 1.00 1.00
BREEZE B-R .15
.40
.60
.60
.60
BREEZE B-S .14
.20
.24
.30
.41
BRICKS C
.16
.13
.15
.11
.13
BRICKS CC .02
.03
.03
.04
.05
BRICKS CCP .01
.01
.02
.02
.02
CARPT COMM .03
.05
.09
.23
.38
CARPT HVY .02
.06
.14
.37
.60
CARPT INOT .01
.05
.10
.20
.45
CARPT LPAD .08
.27
.39
.34
.48
CARPT PAD .08
.24
.57
.69
.71
CELOTEX
.07
.15
.15
.15
.15
CHAIRS LUX .19
.37
.56
.67
.61
CHAIRS LTH .44
.54
.60
.62
.58
CHAIRS WD .15
.19
.22
.39
.38
CHPBRD 8MM .25
.30
.04
.04
.04
CHPBRD16MM .25
.07
.02
.02
.02
CHPBRD25MM .20
.05
.02
.02
.02
CINDBLK R .15
.40
.60
.60
.60
CINDBLK S .14
.20
.24
.30
.41
CIRRUS
.35
.29
.52
.79
.92
CIRRUS 75 .36
.38
.67
.89
.97
COLORSON1 .11
.30
.77 1.00 1.00
COLORSON2 .46 1.00 1.00 1.00 1.00
CONCRETE R .02
.03
.03
.03
.04
CONCRETE S .01
.01
.02
.02
.02
CORTEGA
.31
.32
.51
.72
.74
DOOR
.15
.10
.06
.08
.10
DOOR 2
.15
.11
.10
.07
.06
DRAPE MED .05
.07
.13
.22
.32
DRAPE THIN .04
.05
.11
.18
.30
DRAPE THK .05
.12
.35
.48
.38
4000
.14
.84
.31
.56
.56
1.00
.60
.40
.14
.07
.03
.54
.65
.65
.63
.73
.11
.59
.50
.30
.04
.02
.02
.60
.40
.97
1.00
1.00
1.00
.07
.05
.77
.05
.07
.35
.35
.36
8000Hz
DESCRIPTION IN DETAIL
.10
GLZD TILE RAND PERF 8"X16" TILES/GLS
.84
1/2" Thick Zonolite
.31
2"THK ACOUS DECK 16GA PERF STL DECK
.56
SUFACE TILE GLAZED/PERFORATED(AIR)
.56
1.5"X1.5#W/2MIL PLAS COV PER SIDE
1.00
1"X1.5# W/FABRIC COVER
.60
Breeze blocks rough
.50
Breeze blocks smooth
.15
Bricks clay
.07
Bricks compressed clay
.03
Unglazed bricks painted
.71
Commercial grade carpet
.70
Heavy carpet on concrete
.80
Indoor-outdoor carpet
.63
LATEX BACKING ON 40 OZ HAIRFELT
.75
ON 40 OZ HAIRFELT OR FOAM RUBBER
.10
1"THK AP INSULATION W/FOIL SURFACE
.59
FAB.WELL UPOLST. SEATS UNOCCUP
.45
LEATHER UPOLST SEATS UNOCCUP
.30
CHAIR METAL OR WOOD UNOCCUP
.04
Chip board 8mm on 3cm air
.02
Chip board 16mm on 3cm air
.02
Chip board 25mm on 3cm air
.60
CINDER OR CONCRETE BLOCKS ROUGH
.50
CINDER OR CONCRETE BLOCKS SMOOTH
.96
CIRRUS ceiling material armstong
1.00
CIRRUS 75 ceiling material armstong
1.00
1"THK #6 core fabric covered panel
1.00
2"THK #6 core fabric covered panel
.08
Concrete wall or floor (Rough)
.06
Concrete wall or floor (Smooth)
.79
CORTEGA ceiling material armstong
.02
Door
.07
Door 1 3/4"SOLID CORE WOOD
.39
Draperies medium
.38
Draperies thin
.39
Draperies
ELASCON
.37
.73
.99
.95
.67
.6
.5
FIB/BD AIR
FIB/BD PTD
FIB/BD UP
FIB/GLS 1"
FIB/GLS 2"
FIB/GLS1"A
FIB/GLS2"A
FIB/GLS4"A
FLEXBOARD
FLOOR CONC
FLOOR TILE
FLOORS 1
FLOORS 2
FLOORS 3
FLOORS 4
FOAM 2"
FOAM 3"
FOAM 4"
.30
.05
.05
.07
.19
.09
.20
.39
.18
.04
.02
.01
.02
.15
.04
.08
.14
.20
.0
.10
.10
.23
.51
.25
.56
.91
.11
.04
.03
.01
.03
.11
.04
.25
.43
.70
.15
.10
.15
.42
.79
.60
.89
.99
.09
.15
.03
.01
.03
.10
.07
.61
.98
1.00
.0
.10
.25
.77
.92
.81
.93
.98
.07
.30
.03
.02
.03
.07
.06
.92
1.00
1.00
.10
.10
.30
.73
.82
.75
.84
.93
.03
.50
.03
.02
.03
.06
.06
.95
1.00
1.00
.0
.10
.30
.70
.78
.74
.80
.88
.03
.60
.02
.02
.02
.07
.07
.92
1.00
1.00
.10
.15
.30
.70
.78
.74
.80
.88
.03
.60
.02
.02
.02
.07
.07
.92
1.00
1.00
1/2" MOUNTED OVER 1" AIR SPACE
1/2" MNT SOLID BCK SOME PAINTED
1/2" MOUNTED/SOLID BACKING UNPAINTED
AF 100 1" MOUNTING 4
AF 100 2" MOUNTING 4
AF 530 1" MOUNTING 4
AF 530 2" MOUNTING 4
AF 530 4" MOUNTING 4
3/16" ASBESTOS MNT OVER 2" AIR SPACE
Concrete floor with thin carpet
LINOLEUM ASPHALT RUBBER CORK TILE
CONCRETE OR TERRAZZO
LINOLEUM ASPHALT RUBBER CORK TILE
WOOD
WOOD PARQUET IN ASPHALT ON CONCRETE
SONEX 2" THICK ON FLOOR
SONEX 3" THICK ON FLOOR
SONEX 4" ON FLOOR
GLASS 1
GLASS 2
GLS/WL 1"
GLS/WL 2"
.18
.35
.15
.35
.06
.25
.35
.70
.04
.18
.70
.90
.03
.12
.85
.90
.02
.07
.90
.95
.02
.09
.90
.90
.02
.09
.90
.90
LARGE PANES HEAVY GLASS
ORDINARY WINDOW GLASS
1" MNT/SOLID BACKING COVERED/FABRIC
MNT 1" AIR SPACE OPEN WEAVE
77
Elascon
MATERIAL
GLASS-ROOF
GRAVEL
GRASS 1"
GRASS SPRT
GYP 12.5MM
GYP 2X 5/8
GYP 9.5MM
GYP12.5MMB
GYPBRD 1/2
GYPBRD 5/8
GYPSUM BRD
HARDBOARD
125
.24
.25
.10
.10
.30
.28
.32
.05
.25
.55
.29
.09
250
.18
.60
.25
.20
.20
.12
.07
.05
.10
.14
.10
.45
500
.11
.65
.60
.40
.05
.10
.05
.05
.05
.08
.05
.25
1000
.11
.70
.65
.40
.02
.07
.05
.03
.04
.04
.04
.15
2000
.11
.75
.80
.40
.02
.13
.02
.02
.07
.12
.07
.10
4000
.11
.80
.90
.50
.02
.09
.02
.02
.07
.11
.09
.10
8000Hz
.11
.70
.60
.40
.02
.10
.02
.02
.07
.10
.09
.10
DESCRIPTION IN DETAIL
Movable glass roof
GRAVEL LOOSE MOIST 4" THK
GRASS MARION BLUEGRASS 2" HIGH
GRASS FOR SPORTFIELDS (SOCCER)
Plaster board 12.5mm on 3cm air
Contruction #8 2 layer 5/8"
Plaster board 9.5mm on 5cm air
Plaster board 12.5mm on >30cm air
1/2' DRYWALL
5/8" THK MOUNTED 16" CNTR WITH GLS
1/2" NAILED TO 2X4 16" O.C.
1/8" MOUNTED OVER 2" AIR SPACE
LAKE/POND .05
LAPENDARY11.00
LINEAR
.20
.05
.97
.30
.05
.92
.40
.05
1.00
.50
.05
1.00
.60
.05
1.00
.70
.05
1.00
.80
LAKE MODERATLY SMOOTH
2"THK LAPW/PERF PLAS COVER
linear Frequency Dependence
MARBLE
.01
MASONITE
.12
MASONRY PT .10
.01
.28
.05
.01
.19
.06
.01
.18
.07
.02
.19
.09
.02
.15
.08
.02
.16
.08
NUB CLG 1" .77
NUB WALL1" .08
OMEGA
.10
.94
.29
.26
.75
.68
.72
.98
.92
.82
1.00
.98
.78
1.00
.93
.71
1.00
.88
.65
1" NUBBY ceiling material armstong
1" NUBBY wall materials armstrong
Carpet like wall cover for acous tmt
PARQUET FL
PILLOWBAFL
PLAST/LTHR
PLAST/LTHS
PLAST/TILE
PLASTERBD1
PLASTERBD2
PLASTERBD3
PLATGLS1/4
PLYWD 1/2
PLYWD 1/4
PLYWD 2"
PLYWD 3/8
PLYWD 6MM
POOL/SWIM
PUB IN WDP
PUBLIC TKC
PUBLIC TNC
PUBLIC WC
.02
.20
.03
.02
.01
.32
.30
.05
.18
.30
.60
.05
.28
.20
.01
.57
.50
.38
.31
.15
.78
.03
.02
.01
.07
.20
.05
.06
.25
.30
.0
.22
.30
.01
.61
.70
.60
.51
.10
1.00
.04
.03
.02
.05
.05
.05
.04
.20
.10
.05
.17
.12
.01
.75
.85
.80
.73
.08
1.00
.05
.04
.03
.05
.02
.03
.03
.17
.09
.0
.09
.07
.01
.86
.95
.90
.80
.05
1.00
.04
.04
.04
.02
.02
.02
.02
.15
.09
.02
.10
.04
.02
.91
.95
.90
.82
.05
1.00
.03
.03
.05
.02
.02
.02
.02
.10
.09
.0
.11
.04
.02
.86
.90
.90
.82
.05
1.00
.03
.03
.05
.02
.02
.02
.02
.10
.09
.02
.11
.04
.02
.85
.90
.90
.82
Wooden parquet floor
Pillow-baffle 1" thk mounted hanging
ROUGH FINISH ON LATH
SMOOTH FINISH ON LATH
GYPSUM OR LIME SMOOTH FINISH ON TILE
Plaster board 9.5mm on 5cm air
Plaster board 12.5mm on 3cm air
Plaster board 12.5mm on >30cm air
1/4" PLATE GLASS
1/2" THICK OVR/2"/4" AIR SP.
1/4" MNT/OVR 3" AIR SPACE
2"GLUED TO 2 1/2" PLASTER ON LATH
3/8"
Plywood panelling 6mm on 5cm air
SWIMMING POOL
Congregation in wooden pews
Public on thick upholstered chairs
Public on thin upholstered chairs
Public on wooden chairs
ROCKWOOL 1
ROCKWOOL 2
ROCKWOOL 3
ROOF AS2
RPG DIFSR1
RPG DIFSR2
.34
.36
.31
.05
.98
.75
.52
.62
.70
.55
.90
.51
.94
.99
.99
.83
.93
.57
.83
.92
.98
.95
.77
.34
.81
.92
.92
.85
.80
.24
.69
.86
.84
.55
.77
.26
.69
.86
.84
.3
.70
.20
SANSERRA T
SBV CIRRUS
SNDLKCORTG
SNOW
SOIL
SONEX 2"
SONEX 3"
SONEX 4"
SPRAY ACOU
SS60FR701A
SS85CLASSC
SS85FR701A
.30
.30
.34
.45
.15
.08
.14
.20
.08
.06
.09
.07
.32
.31
.32
.75
.25
.25
.43
.70
.29
.24
.61
.30
.51
.42
.48
.90
.40
.61
.98
1.00
.75
.60
.86
.76
.77
.54
.64
.95
.55
.92
1.00
1.00
.98
.87
.93
1.00
.90
.64
.71
.95
.60
.95
1.00
1.00
.93
.88
.83
1.00
.99
.66
.76
.95
.60
.92
1.00
1.00
.76
.70
.69
.99
1.00
.68
.83
.90
.45
.92
1.00
1.00
.75
.52
.55
.99
78
GLAZED TILE
1/2" MNT/OVER 1" AIR SPACE
MASONRY PAINTED
2" THICK MNT/SOLID BCK
MOUNTED OVER 1" AIR SPACE
MOUNTED OVER 2" AIR SPACE
Delta sorb AS2 on 25 cm space
RPG DIFFUSER UNPAINTED ABSORPTION
RPG DIFFUSER PAINTED ABSORPTION
SANSERRA TRAVERTONE
SCORED BEVELED TEGULAR CIRRUS
SECOND LOOK CORTEGA CEILING MATERIAL
SNOW FRESHLY FALLEN 4" THK
SOIL ROUGH
SONEX 2" THICK ON FLOOR
SONEX 3" THICK ON FLOOR
SONEX 4" ON FLOOR
Sprayed cellulos fiber 1"on concrete
SS60 FR701A Wall materials armstrong
SS85 Classic vinyl wall materials
SS85 FR701A Wall materials armstrong
MATERIAL
STDNTS WC
STEEL
125
.30
.05
250
.41
.10
500
.49
.10
1000
.84
.10
2000
.87
.07
4000
.84
.02
8000Hz
DESCRIPTION IN DETAIL
.81
Students in wooden seats
.10
Steel panel or wall or surface
TBLE TP WD
TEG CIRRUS
TERRAZZO
TILE FLOOR
TILE GLAZD
TREES
TRIBUNE ES
TUNDRA 1"
.15
.33
.01
.02
.01
.03
.06
.70
.11
.33
.01
.03
.01
.06
.06
.92
.10
.55
.01
.03
.01
.11
.06
.75
.07
.74
.02
.03
.01
.17
.12
.99
.06
.86
.02
.03
.02
.27
.12
1.00
.07
.92
.02
.02
.02
.31
.09
.97
.07
.94
.02
.02
.02
.15
.08
.89
Table top wood
Tegular cirrus ceiling
Concrete or terrazzo
Linoleum asphalt rubber cork tile
Glazed tile
Trees firs 20 sq ft grd area pertree
Tribune with empty seats (Stadium)
1"open plan tundra ceiling material
ULTIMA
.32
UNGL BRICK .01
URETH PANE .07
.34
.01
.11
.71
.02
.20
.87
.02
.32
.87
.02
.60
.85
.03
.85
.79
.03
.85
Ultima ceiling material armstong
Unglazed brick
1"thk urethane foam panel
VELOUR HVY .14
VELOUR LT .03
VELOUR MED .07
.35
.04
.31
.55
.11
.49
.72
.17
.75
.70
.24
.70
.65
.35
.65
.65
.35
.65
Heavy 18oz draped to 1/2 area
Light (10 oz) hung touching wall
Med. 14oz draped to 1/2 area
WATER RVR
WATER POOL
WINDOW DP
WINDOW SP
WOOD FLR 1
WOOD FLR 2
WOOD FLR 3
WOOD FLR 4
WOOD FLR 5
WOOD FLR 6
WOOD GRID0
WOOD GRID1
WOOD GRID2
WOOD GRID3
WOOD GRID4
WOOD GRID5
WOOD GRID6
WOOD GRID7
WOOD GRID8
WOOD GRID9
WOOD PANEL
WOODPANEL1
WOODPANEL2
.05
.01
.25
.33
.20
.20
.20
.02
.15
.04
.10
.01
.02
.10
.60
.60
.60
.44
.07
.10
.30
.20
.10
.05
.01
.10
.25
.15
.15
.15
.03
.11
.04
.36
.05
.28
.10
.80
.80
.70
.90
.46
.14
.25
.12
.10
.05
.01
.07
.10
.10
.08
.15
.04
.10
.07
.99
.08
.80
.15
.85
.80
.70
.80
.37
.56
.20
.10
.10
.05
.01
.06
.07
.08
.05
.30
.05
.07
.06
.99
.09
.90
.20
.99
.80
.60
.85
.47
.72
.17
.10
.08
.05
.02
.04
.06
.08
.03
.50
.05
.06
.06
.50
.09
.50
.20
.80
.50
.40
.62
.26
.37
.15
.08
.08
.05
.02
.02
.04
.05
.02
.60
.05
.07
.07
.35
.09
.50
.20
.60
.50
.40
.60
.30
.23
.10
.07
.07
.05
.02
.02
.02
.05
.02
.60
.05
.07
.07
.30
.09
.40
.20
.50
.50
.40
.60
.25
.20
.10
.07
.07
Water
Swimming pool
Double pane glass
Single pane glass
Wooden floor on beams
Wooden floor covered with linoleum
Wooden floor with thin carpet
Wooden floor or linoleum on concrete
Hardwood floor on beams
Wood parquet in asphalt on concrete
Wooden grid 90/15mm on 6cm air + gw
Wooden grid 35/15mm on 2cm air
Wooden grid 35/15mm on 2cm glaswool
Wooden grid 35/15mm on 40cm air
Wooden grid 35/15mm on 40cm glaswool
Wooden grid 60/15mm on 40cm air + gw
Wooden grid120/15mm on 40cm glaswool
Wooden grid 90/15mm on 40cm air + gw
Wooden grid 90/15mm on 40cm air only
Wooden grid 90/15mm on 6cm air only
3/8" to 1/2" thick ovr/2"/4" air sp.
Wood panelled wall 16mm on 4cm air
Wood panelled wall 18mm on 4cm air
MIRROR
α = 10%
α = 20%
α = 30%
α = 40%
α = 50%
α = 60%
α = 70%
α = 80%
α = 90%
α =100%
.10
.10
.20
.30
.40
.50
.60
.70
.80
.90
1.00
.05
.10
.20
.30
.40
.50
.60
.70
.80
.90
1.00
.0
.10
.20
.30
.40
.50
.60
.70
.80
.90
1.00
.0
.10
.20
.30
.40
.50
.60
.70
.80
.90
1.00
.0
.10
.20
.30
.40
.50
.60
.70
.80
.90
1.00
.0
.10
.20
.30
.40
.50
.60
.70
.80
.90
1.00
.0
.10
.20
.30
.40
.50
.60
.70
.80
.90
1.00
Ideal Sound Reflector
10% Sound absorbing
20% Sound absorbing
30% Sound absorbing
40% Sound absorbing
50% Sound absorbing
60% Sound absorbing
70% Sound absorbing
80% Sound absorbing
90% Sound absorbing
100% Sound absorbing
Atmospheric absorption (m/km) versus Rel.Humidity (acc.Cyril Harris)
Label
125
250
500
1000 2000 4000 8000
Description (T=200C)
RH=20%
0.10 0.23 0.56 1.39 4.28 14.5 47.1 Air with 20% Rel.Humidity
RH=40%
0.08 0.18 0.43 1.06 2.59
7.2 23.7 Air with 40% Rel.Humidity
RH=50%
0.07 0.16 0.40 0.97 2.40
6.1 19.2 Air with 50% Rel.Humidity
RH=60%
0.07 0.15 0.37 0.91 2.25
5.6 16.2 Air with 60% Rel.Humidity
RH=80%
0.06 0.14 0.34 0.82 2.07
5.1 13.3 Air with 80% Rel.Humidity
79
21.0 E A S E Software
Advanced Acoustical Design and Analysis Tool for Contractors,
Engineers and Acoustical Consultants.
An acoustic simulation program called "E A S E" (Electro Acoustic Simulator for
Engineers), distributed by Renkus Heinz, simplifies the design of a sound system
and allows its performance to be accurately predicted.
This programs takes the guesswork out of system design, helps eliminate costly mistakes, reduces
installation time and makes it easier to share design results with clients.
EASE, the Electro Acoustic Simulator for Engineers, have gained worldwide acceptance and
recognition as the most advanced and accurate of all design programs. Now available in entirely new
versions, they have taken a quantum leap forward in performance and versatility.
Every Licence receives
An ongoing license with no fixed termination or renewal dates and no periodic license review or renewal
fees. The license covers the complete software package, including loudspeaker performance data and
physical data files form over 25 major loudspeaker manufacturers.
Plus... Software Support
EASE and EASE JR licenses also include an extensive operation manual and comprehensive software
support. EASE workshops are held on a continuing basis.
Ongoing program improvements
Dr. Ahnert of ADA and Dr. Feistel, the scientists behind EASE, are dedicated to continually improving
the program. As one of Europe’s leading acoustical consultants, Dr. Ahnert uses the program in his
own work.
User Friendly
EASE and EASE JR operate with a Mouse in a familiar “windows” like environment. Pop-up menus,
caution messages and sub-menus add further to the ease of operation and prevent operation errors.
Outstanding Graphic Capabilities
Full color screens & color prints, eye catching cut-away views, multiple viewing angles with spin and
rotate options, and spectacular “Speaker” and “Spectator” views all enhance visual presentations.
Screen processing capabilities allow the easy assembly of slide shows, permit color changes and the
gathering of slides from numerous projects into a single show. BitMap and TIFF files allow screens to
be exported to other presentation programs.
Simplified Room Modeling + DXF File Exchange
A library of prototype rooms, create shape options “on-the-screen” graphic editing, intuitive
loudspeaker aiming, and multiple views for easy visualization, all simplify and speed up the modeling
process. DXF file exchange allows 3-D room drawings to be imported from virtually all architectural
CAD programs, including AutoCAD.
Open Loudspeaker & Surface Materials Data Bases
Extensive open databases include loudspeaker data on most major manufacturers products and on
over 100 surface materials. These databases are continually expanded and up-dated. A useful feature
is the ability to enter your own favorite loudspeaker or surface material databases.
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Proven Accuracy
The accuracy of mirror image modelling and of EASE simulations has been conclusively documented in
over three years of use in all types of venues. This has resulted in extensive verification of the
correlation between EASE simulations and actual measurements. Most of the realizations were
measured using Bruel & Kjaer sound level meters and RASTI equipment for verification.
Extensive Simulation Menu
Maps, Direct SPL, Total (direct + reflected) SPL, Alcons, Rasti, C7, C50 and C80 projections onto the
audience areas in multiple illuminating presentation modes. Also maps First Signal Arrival Times, Initial
Time Delay (delay between first two direct signals) and Coverage Overlap. Accurately shows
shadowing and the effects of phase interference. Delays of up to 999 ms allow accurate simulations in
systems with cluster delay lines. Measured RT times and those calculated with Eyring, Sabine or Ray
Tracing methods can be used in the intelligibility projections. (EASE also offers calculations based on
Schroeder’s back integration method.)
Beam Show
Accurately traces reflections allowing sound paths to be easily visualized. This enables quick
identification of surface areas that may require acoustical treatment. 3D isometric, end, side or top
views can be selected.
Unique Acoustical Probe
Allows detailed analysis of suspected problem areas; provides ETC and EFC curves, displays direct
sound arrival times and angles, magnitude and phase. Shows comb filtering and identifies the exact
frequency and level of severe peaks and notches. Individual reflections can be selected, viewed and
analyzed.
Ray Tracing
Reduces the calculation time required, especially in complex rooms or rooms with multiple speakers.
Extended time frame investigations can be used to produce the reflectograms in the EARS auralization
process.
Movie
Uses Ray Tracing to provide an animated display (movie) of the dispersion of sound into and around
the room This feature quickly shows potential reflection problems and thus provides excellent
troubleshooting assistance.
EASE Junior
EASE JR was developed specifically for the system designer who does not need the detailed acoustic
analysis features like the Acoustical Probe, Ray Tracing and the Movie module. It features the same
user friendly program, outstanding graphic capabilities, simplified room modeling techniques, open
loudspeaker and surface materials data bases, and extensive simulation menus complete with Beam
Show.
Computer Requirements
EASE and EASE JR run under MS-DOS, on IBM or 100% compatible computers with an EGA or VGA
graphics system, to take full advantage of the program’s color graphic capabilities. Epson standard and
HP Paintjet, Laserjet and Deskjet printers are supported.
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22.0 Measuring Equipment
22.1 MLSSA - Transfer Analyzer
MLSSA is a powerful system analyzer based on maximum-length sequences.
PC based transfer analysis of linear systems, but its primary application is in the area of audio
and acoustics measurements, enabling measurements in the time and frequency domain.
22.2 Neutrik Audiograph 3300
The Neutrik Audiograph 3300 is a universal audio/acoustic analysis system.
(Production stopped)
1. Compact and transportable, mains operated
2. Performs highly diverse measurement tasks quickly and precisely
3. Easy to operate due to built-in generator & plotter intelligence
MEASUREMENTS :
Reverberation time
Frequency response of loudspeakers
Frequency response of amplifiers & equalizers
Frequency response of acoustic feedback loop
22.3 B & K Speech Transmission Meter 3361
Speech Transmission Meter Type 3361 (Production stopped)
Consisting of Transmitter Type 4225 and Receiver Type 4419
This set measures RASTI values directly using two instruments:
Transmitter Type 4225 which sends out a special test signal,
Receiver Type 4419 which analyses the signal and calculates the RASTI value.
The instruments are battery powered and fully portable, enabling rapid and objective
measurement of the quality of speech intelligibility and provide further information of
diagnostic value.
22.4 Gold Line Audio Spectrum Analyzer DSP30
The Real Time Analyzer DSP30 is a portable Digital 1/3 Octave Sound Level Meter.
The instrument is handy for field measurements, due to storage capabilities, various decay
settings,various scales and an optional PC connection. This unit can be used for equalisation
using pink noise, ambient noise analysis and system performance checking.
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22.5 MLSSA - Transfer Analyzer
MLSSA is a powerful system analyzer based on maximum-length sequences, it performs transfer
analysis of linear systems, but its primary application is in the area of audio and acoustics
measurements.
MLSSA is a single channel analyzer which employs a special type signal called a maximumlength sequence (MLS) as the preferred alternative to the conventional white noise stimulus.
Unlike white noise, a maximum-length sequence is deterministic and periodic yet still retains
many desirable characteristics of white noise. The deterministic nature of the MLS means that it
can be pre-computed and need not be measured simultaneously with the system response. The
periodic nature of the MLS means zero windowing error as long as the entire period of the
sequence is used to make the measurement (which MLSSA does automatically).
The MLS technique measures the impulse response - the most fundamental description of any
linear system - from which a wide range of important functions are derived through computeraided post-processing. The transfer function, for instance, is obtained by applying an FFT to a
segment of the impulse response. From the transfer function MLSSA then derives and displays
the frequency response, phase response and group delay.
An important advantage of the MLS approach is its ability to make extended wideband impulse
response measurements containing up to 65535 points. The time-bandwidth product of a MLSSA
measurement can exceed 20,000. Therefore this MLS approach easily makes wideband (20 kHz)
transfer function measurements with true 1 Hz frequency resolution or, low frequency (1 kHz)
transfer functions with 0.065 Hz resolution.
Below is a list of important functions MLSSA can derive by post-processing the impulse response.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Step response
Energy-time-curves
Schroeder reverberant decay curves
Cumulative energy
STI & RASTI for speech intelligibility
Cumulative spectral decay waterfall
Energy-time-frequency waterfall
Wigner distribution waterfall
Polar response waterfall
Early/late ratios
Reverb/direct ratio
Sound Pressure Level (dB-SPL)
In addition, MLSSA provides a fully programmable digital bandpass filter which allows
calculation of reverberant decay curves and reverberation time for any octave or 1/3 octave
band. You can use this bandpass filter to display filtered versions of any post-processed
function.
MLSSA also supports two references which can be used as corrections for:
1. The transfer functions of filters, transducers and amplifiers in the measuring channel.
2. The microphone response data obtained from your microphone's calibration data sheet.
Thus you can easily correct for minor microphone response errors as well as other
components in the measuring chain.
The MLSSA hardware consists of a full size A2D-160 sampling digitizer board (AT-16bit or
PC-8bit) with two AFM-50 antialiasing filter modules, to be installed in a (transportable) PC.
The MLSSA software consists of dedicated MLSSA software which runs on a IBM
compatible PC, MLSSA should not run under Microsoft Windows.
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MLSSA EXAMPLE IN TIME DOMAIN
MLSSA EXAMPLE IN FRQUENCY DOMAIN
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MLSSA EXAMPLE LEVELS & REVERBERATION
IEC Octave Band Acoustical Parameters
+------------------------------------------------------------------------------+
¦
Band
¦
3
¦
4
¦
5
¦
6
¦
7
¦
8
¦
9
¦ 500- ¦
¦ Parameter ¦ 125 ¦ 250 ¦ 500 ¦ 1000 ¦ 2000 ¦ 4000 ¦ 8000 ¦ 4000 |
+-------------+-------+-------+-------+-------+-------+-------+-------+--------¦
¦S
[dB-SPL] ¦ 45.5 ¦ 66.0 ¦ 72.5 ¦ 78.2 ¦ 75.9 ¦ 78.9 ¦ 70.8 ¦SPL¦
¦N
[dB-SPL] ¦ 31.6 ¦ 42.6 ¦ 51.8 ¦ 55.9 ¦ 52.2 ¦ 51.2 ¦ 43.8 ¦weighted¦
¦SNR
[dB] ¦ 13.8 ¦ 23.4 ¦ 20.6 ¦ 22.4 ¦ 23.7 ¦ 27.6 ¦ 27.0 ¦Averages¦
¦C50
[dB] ¦ -4.21 ¦ 1.18 ¦ 0.74 ¦ 1.30 ¦ 2.28 ¦ 5.21 ¦ 6.16 ¦ 2.111 ¦
¦C80
[dB] ¦ -2.10 ¦ 2.07 ¦ 1.33 ¦ 1.87 ¦ 3.00 ¦ 6.03 ¦ 7.35 ¦ 2.777 ¦
¦D50
[%] ¦ 27.5 ¦ 56.8 ¦ 54.3 ¦ 57.4 ¦ 62.8 ¦ 76.9 ¦ 80.5 ¦60.045 ¦
¦TS
[ms] ¦ 173.9 ¦ 128.8 ¦ 150.6 ¦ 123.6 ¦ 98.1 ¦ 51.3 ¦ 35.2 ¦116.957 ¦
¦EDT-10dB [s] ¦ 2.736 ¦ 3.273 ¦ 3.716 ¦ 3.073 ¦ 2.882 ¦ 2.469 ¦ 2.056 ¦ 3.143 ¦
¦RT-20dB [s] ¦ 2.437 ¦ 3.206 ¦ 3.596 ¦ 3.426 ¦ 3.031 ¦ 2.453 ¦ 1.712 ¦ 3.296 ¦
¦(-5,-25)
r ¦-0.994 ¦-0.995 ¦-0.999 ¦-0.998 ¦-1.000 ¦-1.000 ¦-0.999 ¦-0.999 ¦
¦RT-30dB [s] ¦ 2.217 ¦ 2.886 ¦ 3.205 ¦ 3.061 ¦ 2.882 ¦ 2.443 ¦ 1.733 ¦ 2.994 ¦
¦(-5,-35)
r ¦-0.977 ¦-0.994 ¦-0.993 ¦-0.994 ¦-0.999 ¦-1.000 ¦-1.000 ¦-0.995 ¦
¦RT-USER [s] ¦ 2.200 ¦ 3.015 ¦ 3.461 ¦ 3.337 ¦ 3.030 ¦ 2.498 ¦ 1.706 ¦ 3.211 ¦
¦(-10,-25) r ¦-0.995 ¦-0.993 ¦-0.999 ¦-0.997 ¦-1.000 ¦-1.000 ¦-1.000 ¦-0.998 ¦
+------------------------------------------------------------------------------+
MLSSA EXAMPLE SPEECH TRANSMISSION INDEX
+---------------------------------------------------------------------+
¦Frequency-Hz ¦ 125 ¦ 250 ¦ 500 ¦ 1000 ¦ 2000 ¦ 4000 ¦ 8000 ¦
+-------------+-------+-------+-------+-------+-------+-------+-------¦
¦level dB-SPL ¦ 41.7 ¦ 65.8 ¦ 72.1 ¦ 78.3 ¦ 75.4 ¦ 78.9 ¦ 70.0 ¦
¦m-correction ¦ 1.000 ¦ 1.000 ¦ 1.000 ¦ 1.000 ¦ 0.999 ¦ 1.000 ¦ 0.998 ¦
¦
0.63
¦ 0.714 ¦ 0.794 ¦ 0.761 ¦ 0.808 ¦ 0.843 ¦ 0.920 ¦ 0.952 ¦
¦
0.80
¦ 0.629 ¦ 0.746 ¦ 0.707 ¦ 0.761 ¦ 0.800 ¦ 0.893 ¦ 0.932 ¦
¦
1.00
¦ 0.517 ¦ 0.681 ¦ 0.637 ¦ 0.698 ¦ 0.742 ¦ 0.855 ¦ 0.903 ¦
¦
1.25
¦ 0.417 ¦ 0.617 ¦ 0.575 ¦ 0.636 ¦ 0.685 ¦ 0.816 ¦ 0.869 ¦
¦
1.60
¦ 0.357 ¦ 0.572 ¦ 0.535 ¦ 0.582 ¦ 0.640 ¦ 0.783 ¦ 0.832 ¦
¦
2.00
¦ 0.306 ¦ 0.550 ¦ 0.495 ¦ 0.529 ¦ 0.602 ¦ 0.755 ¦ 0.798 ¦
¦
2.50
¦ 0.221 ¦ 0.527 ¦ 0.458 ¦ 0.485 ¦ 0.576 ¦ 0.729 ¦ 0.769 ¦
¦
3.15
¦ 0.214 ¦ 0.505 ¦ 0.440 ¦ 0.470 ¦ 0.557 ¦ 0.710 ¦ 0.746 ¦
¦
4.00
¦ 0.210 ¦ 0.494 ¦ 0.445 ¦ 0.486 ¦ 0.572 ¦ 0.706 ¦ 0.735 ¦
¦
5.00
¦ 0.116 ¦ 0.531 ¦ 0.504 ¦ 0.515 ¦ 0.592 ¦ 0.716 ¦ 0.740 ¦
¦
6.30
¦ 0.137 ¦ 0.492 ¦ 0.521 ¦ 0.532 ¦ 0.590 ¦ 0.734 ¦ 0.741 ¦
¦
8.00
¦ 0.154 ¦ 0.528 ¦ 0.433 ¦ 0.518 ¦ 0.586 ¦ 0.752 ¦ 0.736 ¦
¦
10.00
¦ 0.045 ¦ 0.510 ¦ 0.451 ¦ 0.519 ¦ 0.604 ¦ 0.746 ¦ 0.745 ¦
¦
12.50
¦ 0.189 ¦ 0.431 ¦ 0.460 ¦ 0.524 ¦ 0.599 ¦ 0.742 ¦ 0.757 ¦
¦ octave TI ¦ 0.353 ¦ 0.544 ¦ 0.519 ¦ 0.548 ¦ 0.589 ¦ 0.688 ¦ 0.721 ¦
+---------------------------------------------------------------------+
STI value= 0.575 (0.591 modified)
ALcons= 7.6%
Rating= FAIR
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