Approach to Sound System Design

(1)

Approach to Sound System Design

• Sound: a little bit of Physics

• SPL and sound propagation in free field

• Room Acoustic: some useful definitions

• Intelligibility

• Sound System Design: some suggestions

• Q & A

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What we call sound is simply the What we call sound is simply the sensation produced by the ear when sensation produced by the ear when

stimulated by these vibrations.

 SOUND is produced by vibrating objects. These move air, “pushing” and “pulling” from its resting state

These small fluctuations in air pressure travel away from the source at relatively high speed, gradually dying off as their

energy is absorbed by the medium.

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PROPERTIES OF SOUND

A sound wave is a series of pressure changes

moving through the air. Amplitude

(dB) is the difference between maximum and minimum

pressure: defines the loudness

Wavelength

(m) is the physical distance between two maxima( or minima): depends on the speed of sound in the medium and on the

frequency:

V =  * f

[Velocity = Wavelength * Frequency]

Frequency

(Hz) is the rate at which the pressure changes occur:

defines the pitch and timbre of the sound

(4)

In other materials, the speed of sound can vary quite

substantially.

Sound Speed

(m/s) Refers to the speed of travel of the sound wave. This varies between mediums and is also dependant on temperature.

(5)

AUDIBLE RANGE

The ear can hear sounds ranging from 20Hz to 20kHz.

It is most sensitive to frequencies between 500Hz and 4000Hz,

which corresponds almost exactly to the speech band.

(6)

MEASURABLE CHARACTERISTICS

Just how can we measure a sound? Acoustic Power (Watts)

Measures energy output by a

source, that

sound's ability to do work

Pressure (Pa)

Measures fluctuations about the local atmospheric pressure.

Use of root-mean-square (rms) rather that peak-to-peak

measures.

Intensity (W/m²)

The amount of sound energy within a specific area normal to the direction of

propagation

(7)

rms i rms rms

rms

P P P

SPL P

0 1 2

0 2

1 20log log

10 



Sound pressure level (SPL) or sound level L_p is a logarithmic measure of the rms pressure (force/area) of a particular sound relative to a reference sound source.

It is usually measured in decibel (dB(SPL), dBSPL, or dB_SPL).

It can be useful to

express sound pressure in this way when dealing with hearing, as the

perceived loudness of a sound correlates roughly logarithmically to its

sound pressure

(8)

(9)

Sound Power refers to the absolute power of a sound source (in Watts) whereas Sound Power Level

refers to the magnitude of that power relative to a reference power (in dB).

0 1

log 10

a a

W

L  W

The sound pressure ( dB) of a given speaker can be easily calculated knowing the sensivity and the driving power (W)

SPL= Sensivity + L

_W_W

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Sound propagation in free field

0

log 1

20 )

( r

r r

SPL  

2

0 1 2

0 2 0 1

1

10 log

4 /

4 log /

10



 

 



 

 r

r r

W

r L

_i^r ^r

W



(11)

Sound propagation in free field

Walking away from a sound source, the perceived level of the sound decrease

This is known as the standard inverse square law for point sources.

Practically results in 6 dB reduction in relative intensity per doubling of distance.

NOTE:

1 dB increase is barely audible

3 dB is a generally noticeable change 10 dB is considered as twice as loud

(12)

Mathematically looks like

New level= Old level + 20xlog(old distance)- 20xlog(new distance)

0 5 10 15 20 25 30 35 40 45 50 55 60 65

1 10 100 1000

r : distance from the source (m)

Attenuation of Acoustic Pressure dB L*=L+20logD - 20logD^*

(13)

Sound:

Wind &

Temperature Temperature

Gradients Gradients

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Sound and Barriers: a matter of wavelength Sound and Barriers: a matter of wavelength

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Sound waves will propagate away from the source until they

encounter one of the room's boundaries where, in general, some of the energy will be absorbed, some transmitted and the rest reflected back into the room.

Room Acoustic

Striking Sound

Reflected

Sound Absorbed Sound

Transmitted Sound

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Part of the sound emitted from the source will go directly to the listener, part will be absorbed, and reflected by walls.

S L

Direct Sound Early

Reflections

Reverberant Field

In the created field the sound does not have directivity and the inverse square low doesn’t hold anymore.

.

The indirect sound after several reflections from different surfaces becomes “ diffuse” creating a steady state field

This is called reverberation

(17)

dB

Time Background Noise Reverberant Field

LR

Early Reflections LER

Direct Sound LD

T0 T1 TN

When the sound source is turned off, direct sound will stop and only the reverberant field will remain

After some

seconds even the reverberant field decays.

The length of time taken for a sound to decay 60 dB after the source has ceased transmitting is defined as Reverberation time

(18)

Volume: defines if a sound reinforcement system is needed or not. Defines directly the reverberation time

Shape: flat, parallel walls, domes, defines echoes and reflections

These are fixed and can be hardly changed Primary: volume, shape, linear dimensions

Room Acoustic depends on:

Secondary: Walls, Ceiling, Materials, Furniture

Treatment can be suggested to improve the room Treatment can be suggested to improve the room

acoustic acoustic

(19)

T A

I a  I

Different materials reflect sound in different way:

(20)

Carpet with foam base

Marble

(21)

The reverberation time affects most of the acoustic features of the room.

Reverberation time, RT₆₀, depends on room dimensions and absorption of the walls

a S RT 0 . 161 V

60



where

RT is the reverberation time in seconds, V is the volume of the room in cubic meters,

is the average absorption coefficient of the room, and

S is the total surface area of the room in square meters

a

i n i

i i n i n

n

s a s S

a s a

s a a s

1 1 2

2

1 ..





 



 

(22)

In acoustic, rooms with smaller reverberation times are

appropriate for speech, whereas spaces designed for music require longer reverberation times.

(23)

More complex equations was developed to take care of different environment

) 1

ln(

161 .

0

60 S a

RT V



 











  



z y

x a

Z a

Y a

X S

RT 0.161V 2 2 2

60 2

(24)

In every room coexist a Direct Sound and a Reverberant Field

There is a point in which the Direct SPL and Reverberant SPL are equal. This point is at a distance , from the source, called CRITICAL DISTANCE

60

057 .

0 RT

D_C  QV Where Q is the directivity of the source

Every point farther than the Dc from the source will hear just the Reverberant field. The inverse square law is no more valid

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Room Acoustic is as important as the sound system itself.

What is a “good sound?

Fidelity. Is given by the frequency response. It depends on each item of the audio chain

Loudness: must be sufficient to achieve the required effect. Is determined by the dynamic range of the sound system

Intelligibility: is linked by the signal/noise ratio and the direct-to-

reverberant sound ratio at listener’s ear.It

depends directly on room acoustic

(26)

ability to hear a sound

ability to detect the structure of a sound

This distinction is more important in speech than in music

Audibility  Clarity

(27)

Speech

• Is made of consonant and vowels

• Vowels range from 250-500hz, carry power

• Consonants range from 1-4kHz, carry information

Lose consonants = Lose intelligibility

(28)

Intelligibility

Measure of the degree of understanding spoken language

Is not a physical quantity as Ampere, Volt, Watt

There are many index to express this degree, many way to measure, and predict it

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Factors Affecting Intelligibility

In on-to-one conversation there aren’t any problems of intelligibility

Sound System Bandwidth and Frequency Response Signal-to-Noise Ratio

Room Reverberation Geometric Factors Distortions

Non Linear Factors

(30)

Bandwidth and Frequency Response

Sound system have to guarantee a response from 100 to 10000 Hz.

Limits are fixed by worse performance

Frequency contribution to Intelligibility

0 5 10 15 20 25 30 35

125 250 500 1 2 4 8

Fequecy (Hz)

Intelligibility (%)

(31)

Signal to Noise Ratio

SPL must be adequate and heard comfortably (normal conversation 70-90 dB)

Noise masks direct sound and lowers intelligibility Increasing S/N ratio increases intelligibility

Intelligibility becomes

independent from S/N for S/N>25 dB

SPL vs Intelligibility

0 20 40 60 80 100

10 35 60 85 110

SPL (dB)

Intellegibility (%)

S/N vs Intelligibility

0 20 40 60 80 100 120

-10 -5 0 5 10 15 20

Ratio S/N (dB)

Intelligibility (%)

(32)

Reverberation and Reflections

Long RT₆₀’s decrease intelligibility

Late reflections (> 50 ms) smear and blur direct speech

Early reflections ( < 35-50 ms) are perceived as reinforce

(33)

Distortion

Clipping

Intermodulation Acoustic distortion

Are form of NOISE

Specification of various items that compose the sound system have to be carefully studied

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Measure and Predicting Intelligibility

Design for speech intelligibility is as important as design for gain, SPL and coverage

While it is quite easy to calculate SPL and RT₆₀ there aren’t models to calculate Intelligibility degree taking care of all parameters

There are more way and several index to express Intelligibility Degree

Subject Based ( AI, %AL_CONS) Quantitative ( STI, RaSTI)

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Predicting %AL

_CONS

AL_CONSis an index expressing Intelligibility

degree, in terms of lost consonants in the talker- listener path

The simplest Peutz formula take care of Directivity, RT₆₀, Room Volume, Number of Speakers, Distance Loudspeaker-Listener

The modified Peutz formula includes also Direct SPL, Reverberant SPL, and Noise SPL

(36)

%AL

_CONS

INDEX

High Q’s and Large V’s improve %AL_CONS

Long D’s, long RT₆₀’s lowers %AL_CONS

This formula fails when strong non-linear effect are present

Excell ent Good

Fair Bad

Unacce ptable

%ALCONS

0 5

10 15

20 30

VQ

N RT

AL

_CONS

200 D ( )

%

⁶⁰

2 2



2

(37)

STI and RaSTI

These methods are fully independent of human being and are fully quantitative

Take care of all factors affecting the intelligibility because measures the corruption of a speech based signal during the talker-to-listener path

Varies from 0 = no intelligibility

to 1= perfect intelligibility

(38)

STI and RaSTI main features:

Replace speech with a high frequency noise

(consonants-vowels) modulated in amplitude by a low frequency signal (phonems)

Knowing the m(f) means predict intelligibility

(39)

Al

_cons

and RaSTI are linked

STI (RASTI) 0 – 0.3 0.3 - 0.45 0.45 – 0.60 0.60 – 0.75 0.75 - 1

Unacceptable Bad Fair Good Excellent

% AL cons 100 – 33% 33-15% 15-7% 7-3% 3-0%

(40)

Common Intelligibility Scale (CIS)

There is a common scale to simplify to define the limits There is a common scale to simplify to define the limits

of acceptable intelligibility of acceptable intelligibility

CIS

0 10 20 30 40 50 60 70 80 90 100

0 0.2 0.4 0.6 0.8 1

Common Intellegibility Scale

Existing Intelligibility Scale

STI*100 100-%Alcons

Standard CEI Standard CEI

EN60849 EN60849 states that states that

CIS> 0.7 CIS> 0.7

CIS=0.7 CIS=0.7

%AL%AL_cons_cons=12=12 STI=0.5 STI=0.5

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Speech Intelligibility Optimisation:

Practical Criteria

Sound quality and intelligibility are not the same thing

 Aim the loudspeaker to the listener: keep as much sound as possible off the walls and the ceiling

Provide a direct line between loudspeaker and listener

Ensure adequate bandwidth

Avoid frequency response anomalies (corner bass increment)

Minimize D where possible

Ensure S/N ratio>10dB

Avoid delays> 50ms ( inter speaker spacing< 15m)

Use high Q in reverberant environment

Minimize SPL variations

Improve RT and acoustic environment

(42)

A sound system is basically composed of

1) Electro-Acoustic components ( speakers, microphones detectors)

2) Electronic items (mixer, amplifier, digital processors, music/message sources)

3) Environment ( Room Acoustic, RT₆₀)

Any result is a mesh of these components, and the lower

quality component will lower the performance of all the others together

(43)

A Sound Reinforcement system is a system for accurately amplifying, reproducing, and sometimes recording audio, so that persons not near the original source may experience the sound as if they were.

PA system, controls to mix the signals coming from the various microphones or other input sources (such as Tuner, CD, MP3 and so on).

(44)

How to approach a study of a sound system

1) Establish the required system functions on the basis of the user’s needs.

2) Analyse the characteristics of the environment

3) Choose the loudspeakers on the basis of the nature and

dimensions of the space, the type of message to be transmitted (speech/music), and the noise level of the

environment.

4) Choose amplifiers that are suitable for driving the speakers selected and with a sufficient number of inputs for all the sounds sources.

5) Define the sound sources (microphones, tuners, cassette, players, etc.).

6) Evaluate the connection system for the speakers and establish cable sections.

It is advisable to begin your study of a sound system with the loudspeakers, after which the amplifier power and model can be defined, and finally the sound sources and appropriate connection system can be selected.Specifically, you

need to

(45)

The The Sound Sound System System Design Design flow chart flow chart

(46)

Speaker Placement

There are essentially two types of speaker system;

A centrally located system

A distributed system/multi point diffusion

(47)

Centrally Located Centrally Located

System System

Minimize the Minimize the

Reverberant field but Reverberant field but

can result in long can result in long speaker/listener speaker/listener

distances distances

Need for Loudness Need for Loudness

Calculation Calculation

Need for Coverage Need for Coverage

Calculation Calculation

(48)

9 x H1315 9 x H1315

Central

Cluster

(49)

Distributed Distributed

system

Increase the Reverberant Increase the Reverberant field, lower the

field, lower the

speaker/listener distance speaker/listener distance

(50)

Small-medium size Small-medium size

spaces spaces

MQ60H: 180 MQ60H: 180⁰⁰ coverage, 97dB coverage, 97dB

3m away 3m away

DM61: 120 DM61: 120⁰⁰ coverage, 96dB coverage, 96dB

3m away IP55 3m away IP55

(51)

Medium-Large size Medium-Large size

spaces spaces

(52)

Speaker positioning in corridors Speaker positioning in corridors

MQ30P: 92dB after MQ30P: 92dB after

10 m10 m

(53)

(54)

 h

r l

Square compact

S

Hexagonal compact

S

Square Edge-to-Edge S

Hexagonal Edge-to-Edge S

2 r S 

3 r S 

r S 2

r S 2 tan 2

)

( 

l h r  

Several RAIN DIFFUSION Several RAIN DIFFUSION

coverage coverage

(55)

• A rule of thumb for distributed system is calculate the ratio

Coverage Spea

TotalArea

N  ker

(56)

1m 2m 4m 8m 16m 1m 2m 4m 8m 16m

90dB90dB 84dB84dB 78dB78dB 72dB72dB 66dB66dB

1m 2m 4m 8m 16m 1m 2m 4m 8m 16m

1W 1W

4 4 W W 2W 2W

SPL 1W/1m

SPL 1W/1m SPL(r)  Sens 10log P  20logr

(57)

Line Loss and Wire Section

The load is considered to be concentrated at the end of

the line. If the speakers are distributed along

the line, the section can be almost halved.

(58)

Thank you for your kind attention!

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