59706240 Acoustic Design Ppt

7/26/2019 59706240 Acoustic Design Ppt

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12-Nov-01 Intro to Acoustics: Reverberation 2

Learning Outcomes• Explain Soun be!aviour incluing

re"ection# absorption# energ$ ensit$#

soun eca$ an reverberation%• Learn !o& to esign an acoustic room an

its esign consierations%

• Learn &!at is Electroacoustics%



'istor$ o( Acoustical)esignThe recorded history of the acoustic design of

buildings seems to begin with the construction of

amphitheatres by the ancient Greeks . (oratories and

perform plays)

These were open air amphitheatres that housed upto 2000 people, all listening to a single orator or

small group of actors.

There is a limit to the audibility of the human voice.

an you think of some of the techni!ues used by the ancient Greeks in the

construction of their amphitheatres"

#id they work and, if so, how did they know how to make them work"



Soun be!avior

$ound waves propagate away from the source until theyencounter one of the room%s boundaries & some of the energy will

be absorbed, some transmitted and the rest reflected back into

the room.

$ound arriving at a particular receiving point within a room can be

considered in two distinct parts.'. $ound that travels directly from the sound source to the

receiving point itself. This is known as the direct sound field and is

independent of room shape and materials, but dependant upon

the distance between source and receiver.

2. fter the arrival of the direct sound, reflections from roomsurfaces begin to arrive. These form the indirect sound field that

is independent of the sourcereceiver distance but greatly

dependant on room properties



The Growth and Decay of SoundThe sound intensity measured at a particular point

increases suddenly with the arrival of the direct soundand will continue to increase in a series of small

increments as indirect reflections begin to contribute to

the total sound level.

*ventually an e!uilibrium will be reached where the

sound energy absorbed by the room surfaces is e!ual

to the energy being radiated by the source.

This is because the absorption of most building

materials is proportional to sound intensity, as the

sound level increases, so too does the absorption.



The Growth and Decay of SoundIf the sound source is abruptly switched off, the

sound intensity at any point will not suddenly

disappear, but will fade away gradually as the

indirect sound field begins to die off and reflections

get weaker.

The rate of this decay is a function of room shape

and the amount/position of absorbent material .

The decay in absorbent rooms will not take very long at all,whilst in large reflective rooms, this can take quite a long

time.



Reverberant Decay of sound in a

small absorbent enclosure.

*!is graual eca$ o( soun energ$ is +no&n as reverberationan# as a result o( t!is proportional relations!ip bet&eenabsorption an soun intensit$# it is exponential as a (unction o(time% I( t!e soun pressure level ,in . o( a eca$ingreverberant /el is grap!e against time# one obtains areverberation curve &!ic! is usuall$ (airl$ straig!t# alt!oug! t!eexact (orm epens upon man$ (actors incluing t!e (reuenc$spectrum o( t!e soun an t!e s!ape o( t!e room



OptimumReverberation Times



Geometric Acoustics

Wave Theory and Normal Modes *!e concept o( a soun ra$ an t!e geometrical

stu$ o( soun ra$ pat!s pla$ an important role int!e esign o( large rooms an auitorium#

enabling troublesome ec!oes an "utter eects tobe etecte an ealt &it! at t!e esign stage%

A limitation o( t!e geometrical approac! is t!atusuall$ onl$ primar$ an possible seconar$re"ections can be stuie be(ore t!e soun ra$being (ollo&e becomes lost in t!e reverberantsoun /el an# in most enclosures# it is restricteto (reuencies o( 300 '4 an above%



sin! Geometric Acoustics

Statistical met!os are use(ul at t!e earlieststages o( esign# !o&ever# as more an moregeometric in(ormation becomes available#&!$ not use it%

As enginneers# &e nee to be able to

etermine not onl$ !o& muc! absorber touse# but &!at t$pe o( absorber an &!ere toput it% *!is is &!ere t!e consieration o(re"ecte soun ra$s can be uite use(ul%



"aults Attributable to GeometryPicket fence echoes - results from evenly

spaced reflection paths, such as the rowsof raised seating in amphitheatres and the

evenly spaced curves of compressed fiber

fencing.#epending on the number of steps and the path difference, such

surfaces can produce a definite pinging sound when stuck by an

impulsive sound source.

+f d is the distance between successive steps, then the fre!uency of

this ping is given by pfe - (c (2d)) where c is the speed of sound in

air.



"aults Attributable to GeometrySpurious echoes - ccur when a strong

reflection of the original signal can beclearly discerned by the listener.

This is simply a matter of looking at the

internal envelope of the enclosure and

checking for possible sound paths, which

reflect off a se!uence of large, highlyreflective surfaces.



"aults Attributable to GeometryFlutter echo - ccurs when both the source and receiver are

between a pair of parallel, hard, surfaces. $ome portion of thesound emitted by the source will be %trapped% between the two

reflective surfaces and will oscillate back and forth, being !uite

slow to decay. The listener will perceive this as a %fluttering%

noise. +f the walls are a distance d apart, then the fre!uency of

this flutter can be found in the same way as picket fence echo.

Dead spots- These can occur at positions, which are far from

reflecting surfaces, and which receive sound only after it has

passed over an absorbent surface. or e/ample, at the rear of

a gently raked theatre or cinema where the sound must pass

over the audience and ceiling reflections are blocked by a

balcony.



The #lacement of Re$ectors andAbsorbersy analy1ing the paths of sound rays, it is easy to

determine which areas re!uire reinforcement (in theform of a reflector) and which re!uire damping (in

the form of absorber).

Consider someone speaking at the rate of up to 8

syllables per second. ach syllable takes about !"# ms.

Therefore, if clear reflections of the first syllable arrive

mid$way through the second %or even the third& the

speech may not be easily discernible by the listener.



Ob%ective Measures

5or man$ $ears t!e reverberation time &as t!eonl$ real ob6ective measure o( t!e acousticper(ormance o( an auitorium% 5or man$arc!itects# even toa$# it still is% 'o&ever# t!ere

are man$ more aspects to soun be!avior inrooms%

&arly Decay Time'larity and De(nition

Spatial )mpression

Speech )ntelli!ibility



Ob%ective Measures&arly Decay Times

*!e reverberation time# as iscusse earlier# re(ers to t!e timeta+en (or t!e reverberant component o( an enclosure to (all b$70 a(ter t!e source is abruptl$ s&itc!e o% In an iealenclosure t!is eca$ is exponential# resulting in a straig!t line

&!en grap!e against Soun Level% Stuies o( actualauitoria# !o&ever# s!o& t!at t!is is not al&a$s t!e case%

Researc! ,8uttru 19;. !as s!o&n t!at it is t!e initial portiono( t!e soun eca$ curve process# &!ic! is responsible (or oursub6ective impression o( reverberation as t!e later portion is

usuall$ mas+e b$ ne& souns% *o account (or t!is# t!e Earl$)eca$ *ime ,E)*. is use% *!is is measure in t!e same &a$as t!e normal reverberation time but over onl$ t!e /rst 10 - 13 o( eca$# epening on t!e &or+ being re(erence%



'larity and De(nitionlarity and #efinition & refer to the ease with which

individual sounds can be distinguished from within a

general audible stream.

This stream of sound may take many forms aconversation, a passage of music, a shouted

warning, the whirring of machinery, whatever.

The degree of clarity is, of course, greatly dependant

on the particular sounds involved , however, from anarchitectural point of view, it refers to the ratio

between the amount of early to late arriving sound

energy.



Spatial )mpression$patial +mpression refers to a feeling of being enveloped

within the music, surrounded by it not 3ust %looking in at it%.

This impression is primarily a function of interaural cross

correlation, or the relative contribution of lateral reflections.

$imply put, spatial impression is determined by the subtledifferences in signal received by each ear. +f all of the

sound energy comes from straight in front of or behind you,

the signal at each ear will be the same.

+f the sound bounces around the auditorium andapproaches from the sides, the signals at each ear will be

!uite different due to diffraction around the head and slight

time delays.



Speech )ntelli!ibility+n terms of individual communication, speech is

probably the most important and efficient means, even

in today%s multi&media society.

The intelligibility of speech refers to the accuracy with

which a normal listener can understand a spoken word

or phrase.

Given the fact that some of the information

communicated through speech is contained within

conte/tual, visual and gestural cues, it is still possible

to understand meaning even if only a fraction of the

discrete speech units are heard correctly.



Desi!nin! Auditoria These are serious requirements and it

must be remembered that, when an audienceenters an auditorium, they have every right toexpect comfort, safety, pleasant surroundings,good illumination, proper viewing and good

sound." L.L. Doelle, nvironmental !coustics



Outline of Acoustic Requirements for Good SoundThere should be ade!uate loudness in every part of the

auditorium, especially in remote seats.

The sound energy should be uniformly distributed within

the room.

ptimum reverberation characteristics should be

provided in the auditorium to facilitate whatever functionis re!uired.

The room should be free from acoustical defects

(distinct echoes, flutter echoes, picket fence echo,

sound shadowing, room resonance, sound

concentrations and e/cessive reverberation).

ackground noise and vibration should be sufficiently

e/cluded in order not to interfere in any way with the

function of the enclosure.



Ade*uate +oudness

The auditorium should be shaped so that the audience is as close tothe sound source as possible. +n larger auditoria the use of a balcony

brings more seats closer to the sound source.

The sound source should be raised as much as is feasible in order to

secure a free flow of direct sound to every listener .

The floor on which the audience sits should be properly raked as

sound is more readily absorbed when it travels at gra1ing incidence

over the audience.

s a general rule, however, the gradient along aisles of sloped

auditoria should not be more than '45 in the interests of safety. The

audience floor of theatres for live performance, especially open or

arena stages should be stepped.



Ade*uate +oudnessThe sound source should be closely and abundantly

surrounded by large sound&reflective surfaces in order toincrease the sound energy received by the audience.

+t must be remembered that the dimensions of the

reflecting surfaces must be comparable with the sound

waves to be reflected.+n addition, the reflectors should be positioned in such a

way that the time&delay between the direct and reflected

sound is as short as possible, preferably not e/ceeding 60

msec and definitely not more that 50 msec.The floor area and volume of the auditorium should be

kept at a reasonable minimum, thus shortening the sound

paths.



Recommene <olume-per-seat values (or variousauitoria

Type of

AuditoriumMinimum Optimum Maximum

'ooms for

(peech".) ).! *.)

Concert +alls ." -.8 !.8

pera +ouses *.# #.- -.*

Catholic

Churches#.- 8.# !".

ther

Churches#.! -." 0.!

1ultipurpose

+alls#.! -.! 8.#

Cinemas ".8 ).# #.



&limination of Defects

*!e basic e(ects attributable to room geometr$ !ave beentouc!e in a previous lecture an consist o( ec!oes# sounconcentrations# soun s!ao&ing# istortions# couple spacesan room resonance%

1. E!OES

These are probably the most serious and most common defect. They occur

when sound is reflected off a boundary with sufficient magnitude and delay to

be perceived as another sound, distinct from the direct sound. s a rule, ifthe delay is greater than '27 sec ('8m) for speech and ''2 sec (68m) for

music then that reflection will be a problem.

Solution" Either alter the #eometry of the offendin# surface or apply

a$sor$er or diffusion.

%. SO&'D O'E'T(AT)O'

$ometime referred to as %hot&spots%, these are caused by focused reflections

off concave surfaces. The intensity of the sound at the focus point is

unnaturally high and always occurs at the e/pense of other listening areas.

(olution2 Treat with absorber or diffusers, better still, redesign it to focus the

sound outside or above the enclosure.

Elimination of Defects



*. SO&'D S!ADO+)',

9ost noticeable under a balcony, it is basically the situation where a

significant portion of the reflected sound is blocked by a protrusionthat itself doesn%t contribute to the reflected component. +n general,

avoid balconies with a depth e/ceeding twice their height as they will

cause problems for the rear&most seats beneath them.

(olution2 'edesign the protruding surface to provide reflected

sound to the affected seats or get rid of the protrusion.. D)STO(T)O'S

These occur as a result of wildly varying absorption coefficients at

different fre!uencies. This applies an undesirable change in the

!uality and tone coloration (of fre!uency distortions) to sound within

the enclosure.(olution2 3alance the absorption coefficients of acoustical

finishes over the whole audible range.





. O&P/ED SPAES

:hen an auditorium is connected to an ad3acent space, which has a

substantially different ;T, the two rooms will form a coupled space. slong as the airflow is unrestricted between the two spaces, the decay

of the most reverberant space will be noticeable within the least

reverberant. This will be particularly disturbing to those closest to the

interconnection.

(olution2 4dd some form of acoustic separation %a screen or a door& ormatch the 'T of both rooms.

0. (OOM (ESO'A'E

;oom resonance is similar to distortions in that it causes an undesirable

tone coloration, however, room resonance results from particularly

emphasi1ed standing waves, usually within smaller rooms. This is asignificant concern when designing control rooms and recording

studios.

(olution2 4pply subtle changes in overall shape of the room or find out

which surfaces are contributing and use large sound diffusers.




Conflicting Requirements for Speech and Music

Speech

The acoustics of a space designed for speech must primarily ensure

definition and intelligibility, remembering, of course, that understanding in the

speech communication process depends as much upon gesture and facial

movements as it does on vocal projection.

The audience's expectations regarding the actual quality of the speech

signal is not too critical, as long as the speaker's voice and accent arerecognizable and the vocal information is understandable.

Music

Music audiences, on the other hand, have inherited quite a developed

expectation of particular sound qualities for various styles and eras of music.

hilst definition is a prerequisite for speech, excessive clarity in music gives

the subjective impression of brittleness or dryness. !n addition, it accentuates

un"anted bo"ing or fret noise, making the musicians job even more difficult.



Electro-acoustics

*!e reasons (or using soun ampli/cation euipment&it!in an arc!itectural contextTo increase the sound level when a sound source is too

weak to be heard.

To provide additional sound to audiences beyond the

intended range of the source.To pro3ect sound back to the stage for the benefit of the

performers.

To alter the ;everberation Time or other impression of an

auditoria.To reduce the relative effects of background noise.

To provide paging, information or warning facilities.

To reproduce electronic or recorded material.



Spea,er #lacement

*!ere are essentiall$ t!ree t$pes o( louspea+ers$stem=

1. A centrally located system.

Also +no&n as a !ig! level s$stem# t!is isessentiall$ a single cluster o( louspea+erslocate near t!e source% Suc! a s$stem givesmaximum realism as t!e ampli/e soun#

&!ilst increasing louness an clarit$# is stillassociate &it! t!e original source%



A centrally located system.



2. A distributed system.asicall$ a number o( louspea+ers space t!roug!out t!e

auitorium% *!is is also +no&n as a lo& level s$stem as eac!iniviual spea+er operates at a lo& ampli/cation level toservice onl$ a small part o( t!e &!ole auience



:hilst it is preferable to use a centrally located system, thereare many situations in which it must be used, for e/ample

:here the ceiling height is too low for the installation of acentral system.

:here not all of the audience have a direct sightline with thecentral loudspeaker.

:hen the amplified sound is used to overcome high

background noise levels.:here the serviced space may be divided into several

smaller spaces.

+n large halls where the source position may vary

significantly.:hilst realism cannot be e/pected from a distributed

loudspeaker system, it does provide high intelligibility wherethe room is not too reverberant.



3. A stereophonic system

*&o or more louspea+er clusters at strategic

positions &it!in t!e auitorium% Suc! s$stems areuse &!en t!ere are a number o( ierent sources tobe ampli/e or t!e source is uite mobile% $ usingt&o or more microp!ones# eac! connecte to t!eiro&n cluster o( spea+ers# t!e spatial relations!ip

bet&een t!e sources is preserve in t!e ampli/esoun% *!is is ac!ieve because t!e soun isampli/e at intensities proportional to t!e istancebet&een t!e source an t!e microp!one an t!e ear

perceives t!e resultant irectional cues%



A stereophonic system



ne of the main points to consider when placing speakers is the factthat their directionality is fre!uency dependant.

s discussed in previous lectures, low fre!uency sounds are prettymuch omni&directional, being able to diffract around obstacles(including the speaker cabinet) !uite readily.

<igh fre!uencies, however, are highly directional with only limiteddiffraction capacity.

The speech band (the fre!uencies in which we are most ofteninterested) occupies the mid&fre!uencies.

This means that they only partially diffract around the speakercabinet.

s a result, no matter where the speakers are placed, somemembers of the audience will receive significant low fre!uency

energy but little higher fre!uency energy. This can make the speechsound muddy and even more difficult to understand.



s a result of these problems, line or columnspeakers are often preferred over conventional

radial or multicellular horn speakers.These consist of =&'0 loudspeakers mounted

ne/t to each other to form a column. $uchloudspeakers act to concentrate the sound

energy into a beam, which has a wide angularspread in the hori1ontal plane and a narrowspread in the vertical plane .

This minimi1es the amount of sound energyradiated away from the audience, which oftencauses further reflection problems.



T-& -AAS &""&'T

The +aas effect refers to a phenomenon where the sound thatarrives at a listener first determines the perceived direction of thesource.

This is pretty reasonable if we consider the normal physical situationwherein the direct sound travels in a straight line between sourceand receiver whilst reflected sound must take a more comple/ route.

To accommodate this, we need to place a delay on loudspeakers close to theaudience. This has to be such that the direct sound arrives first, very closelyfollowed by the loudspeaker output.



'ase Study ./ +ecture

RoomThe follo1in# case study is $ased on an actual consultin# 2o$. )t

is intended solely to inform students of the processes in3ol3ed

in acoustic desi#n and the interaction that can occur $et1een

the acoustic consultant and the architect.

This study presents results from an acoustic analysis carried out on

the proposed design for a 200 seat lecture facility. The proposed

building is an earth&covered structure designed to take ma/imum

advantage of thermal mass, passive design and *$# principles. +t

features a natural ventilation tower, which draw air through an intake

plenum beneath the seating to take advantage of ground temperature

cooling in summer and heating in winter.The preference of the architect is for a slightly %live% facility that is

suitable for both unassisted speech and music production. The

re!uirements for speech and music are slightly different so some

compromises will be re!uired.



'ase Study ./ +ecture

Room

*D 3ie1 of internal en3elope model

'ase St d 0



The follo1in# case study is $ased on an actual consultin# 2o$. )tis intended solely to inform students of the processes in3ol3ed

in acoustic desi#n and the interaction that can occur $et1een

the acoustic consultant and the architect.

+ncreasingly, modern auditorium are multi&functional re!uiring that

the acoustics be suitable for a range of purposes. The budget rarelyallows for variable acoustic treatments so the design process

becomes one of compromise.

This study presents the acoustic design and performance analysis of

proposed additions to a gymnasium building. The redeveloped

facility is to serve as both a gymnasium as well as the mainceremonial assembly hall for a medium si1e educational institution. +t

will also be used to house a new pipe organ.

'ase Study 0/Assembly -all



'ase Study/ Assembly

-all



>!at are $ourrecommenations (or

eac! case?



Iniviual @ase Anal$sis stu$

to be submitte next meeting

*'AN8 OB 5OR OBR 8IN) A**EN*ION

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59706240 Acoustic Design Ppt