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

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.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.

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

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.

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.

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

irms

rms

rms

rms

P

P

P

PSPL

0

12

0

21 log20log10

Sound pressure level (SPL) or sound level Lp 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 dBSPL).

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

                                                                                              

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

log10a

aW W

WL

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

SPL= Sensivity + LSPL= Sensivity + LWW

Sound propagation in free field

0

1log20)(r

rrSPL

2

0

12

0

2101 log10

4/

4/log10

r

r

rW

rWL rr

i

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

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)

Att

en

ua

tio

n o

f A

co

us

tic

Pre

ssu

re d

BL*=L+20logD - 20logD*

Sound:Sound:

Wind & Wind & Temperature Temperature

GradientsGradients

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

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

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

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

Volume: defines if a sound reinforcement system is needed or not. Defines directly the reverberation timeShape: flat, parallel walls, domes, defines echoes and reflectionsThese 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 acousticacoustic

T

A

I

Ia

Different materials reflect sound in different way:

Carpet with foam base

Marble

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

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

aS

VRT

161.060

whereRT is the reverberation time in seconds,V is the volume of the room in cubic meters, is the average absorption coefficient of the room, andS is the total surface area of the room in square meters

a

ini

iininn

s

as

S

asasasa

1

1221 ..

In acoustic, rooms with smaller reverberation times are appropriate for speech, whereas spaces designed for music require longer reverberation times.

More complex equations was developed to take care of different environment

   

)1ln(

161.060 aS

VRT

zyx a

Z

a

Y

a

X

S

VRT

222161.0260

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

057.0RT

QVDC 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

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

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

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

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

Factors Affecting IntelligibilityIn 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

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)

Inte

lligi

bilit

y (%

)

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)

Inte

lleg

ibilit

y (

%)

S/N vs Intelligibility

0

20

40

60

80

100

120

-10 -5 0 5 10 15 20

Ratio S/N (dB)

Inte

llig

ibili

ty (

%)

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

Distortion

Clipping

Intermodulation

Acoustic distortion

Are form of NOISE

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

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, %ALCONS) Quantitative ( STI, RaSTI)

Predicting %ALCONS

ALCONS is 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

%ALCONS INDEX

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

Long D’s, long RT60’s lowers %ALCONS

This formula fails when strong non-linear effect are present

Excellent

Good Fair

Bad Unacceptable

%ALCONS 0 5 10 15 20 30

VQ

NRTDALCONS

)(200%

6022

2

STI and RaSTIThese 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

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

Alcons and RaSTI are linkedSTI (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%

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 intelligibilityof 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

Exi

stin

g In

telli

gib

ility

Sca

le

STI*100

100-%Alcons

Standard CEI Standard CEI EN60849 EN60849 states that states that CIS> 0.7CIS> 0.7

CIS=0.7CIS=0.7

%AL%ALconscons=12=12

STI=0.5STI=0.5

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

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

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).

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

The The Sound Sound System System Design Design

flow chartflow chart

Speaker Placement

There are essentially two types of speaker system;

A centrally located system

A distributed system/multi point diffusion

Centrally Located Centrally Located SystemSystemMinimize the Minimize the

Reverberant field but Reverberant field but can result in long can result in long speaker/listener speaker/listener

distancesdistances

Need for Loudness Need for Loudness CalculationCalculation

Need for Coverage Need for Coverage CalculationCalculation

9 x H1315 9 x H1315 Central Central Cluster Cluster

Distributed Distributed systemsystem Increase the Reverberant Increase the Reverberant

field, lower the field, lower the speaker/listener distancespeaker/listener distance

Small-medium size Small-medium size spacesspaces

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

3m away3m away

DM61: 120DM61: 12000 coverage, 96dB coverage, 96dB 3m away IP553m away IP55

Medium-Large size Medium-Large size spacesspaces

Speaker positioning in corridorsSpeaker positioning in corridors

MQ30P: 92dB after MQ30P: 92dB after 10 m10 m

h

l r

Square compact

S

Hexagonal compact

S

Square Edge-to-Edge

S

Hexagonal Edge-to-Edge

S

2rS

3rS

rS 2

rS 2

2tan)(

lhr

Several RAIN DIFFUSION Several RAIN DIFFUSION coveragecoverage

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

CoverageSpea

TotalAreaN

ker

1m 2m 4m 8m 16m1m 2m 4m 8m 16m

90dB90dB 66dB66dB78dB78dB84dB84dB 72dB72dB

1m 2m 4m 8m 16m1m 2m 4m 8m 16m

996dB6dB 772dB2dB884dB4dB990dB0dB 778dB8dB

1m 2m 4m 8m 16m1m 2m 4m 8m 16m

93dB93dB 69dB69dB81dB81dB87dB87dB 75dB75dB

1W1W

44WW

2W2W

SPL 1W/1mSPL 1W/1mrPSensrSPL log20log10)(

Line Loss and Wire Section

The load is considered to be concentrated at the end ofthe line. If the speakers are distributed alongthe line, the section can be almost halved.

Thank you for your kind attention!

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