auditorium acoustics pesentation (3)

8/6/2019 Auditorium Acoustics Pesentation (3)

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Acoustics is the interdisciplinary science that deals with thestudy of Sound, Ultrasound and infrasound (all mechanical wavesin gases, liquids, and solids). A scientist who works in the field ofacoustics is an acoustician. The application of acoustics intechnology is called acoustical engineering. There is often much

overlap and interaction between the interests of acousticians andacoustical engineers.

The word acoustic is derived from the Greek word

(akoustikos), meaning "of or for hearing, ready to hear and thatfrom (akoustos), "heard, audiblewhich in turn derivesfrom the verb (akouo), "I hear.


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Physical Acoustics Biological Acoustics Acoustical Engineering

Aeroacoustics Bioacoustics Acoustics measurementsand instrumentation

General Linear acoustics Musical acoustics Acoustics signal

processingNonlinear acoustics Physiological acoustics Architectural acoustics

Structural acoustics andvibration

Psychoacoustics Environmental acoustics

Underwater sound Speech Communication(production, Perception,Processing andCommunication)

Transduction

Ultrasonics

Room acoustics

The table belowshowsseventeen majorsubfieldsofacoustics. These havebeen groupedintothree domains:physicalacoustics,biologicalacoustics

andacoustical engineering.


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When working with an acoustician in the design or renovationof a hall it is helpful for all to have an understanding of the basicconcepts in auditorium acoustic design. You cant really design ahall just by knowing the basics of auditorium design. Theacoustician maintains an arsenal of trade secrets and insidertechniques, reserved to managing sound once its been launched

from the loudspeaker. Finally understanding auditorium acousticswill help give you second thoughts when you find there is a problemwith understanding sound from a loudspeaker. Your speaker wasprobably installed by perfectly competent sound contractor and isstill working just fine. If its loud enough to hear and you still cantunderstand it, then your problem isnt loud speakers, its acoustics.


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Auditorium design begins with the loudspeaker and how itplays sound into the hall. It ends with how the hall returnsreflections of the sound back to the audience. The speakers shouldproduce a sound level at about 65 dB,A everywhere in the seatingarea. It should have at least 20 dB of head room so that short

lived bursts of sound up to 85 dB,A can be replicated without anyhint of speaker or electronic distortion. Electronic distortion mustbe avoided. Distortion of the signal is one of the fastest ways tocause people to lose their understanding of the sound. In addition,the loudspeaker system should sound similar no matter where a

person is seated. This is achieved when the speaker system istested and confirmed to provide a fairly flat frequency responsecurve for every seat in the house.


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The natural noise floor of the hall itself should be at least 20to 30 dB quieter than the speaker level heard at the seats

throughout the entire frequency range for speech. Generally theambient noise levels for an empty good auditorium would be about25 to 30 dB,A. This noise is the sound of the hall when everything isturned on, lights, air conditioning and even the sound system.

The only thing that isnt happening is that someone isnttalking into the mic. Only a fairly good sound meter can measuresound levels this low. Then we open the doors and the audiencemoves into the hall. The background noise levels rise up to about35 dB,A because of the breathing and other rustling that peoplenaturally do. It is a well-established fact of human behavior that in a

group, we collectively manage to make just a little more noise thanthe background noise level. Thats why we act quietly in a libraryand noisily in a packed diner. (See Figure A ).


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Early reflections are very helpful but not necessary for

achieving good intelligibility in a quiet hall. But many halls are quiteperfect and thats when early reflections become a good little helper,specially for those of us who have begun to lose some hearingefficiency. A bright and clear sounding auditorium will be providingsomething like 30 separate early reflections, each arriving wellwithin the first 1/40th second following the initial impact of thedirect signal.

Some of these will be strong and some will be weak but the overall

summed power of all the early reflections should be in the range of60 dB,A to provide good speech reinforcement. (see Figure B & C).


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Fig C (a) (b)

a) Direct signal leave the speakers and impact the audience.

b) Indirect signals are those bounce off nearby surfaces into the audience.


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The next group of reflections is to be avoided. They are thosethat arrive at the listeners ear within a time delay following thedirect signal between 1/40t h second and second. These reflectionsshould be relatively indistinct and quiet, about 15 dB below thespeech signal range. The late reflections and echoes are the most

problematic part on auditorium acoustics. Some designers simpleeliminate all late reflections by absorbing them.

Other more tricky designers like to cover the surfaces of thehall with sound scattering devices to disburse late reflections,

breaking them up into a plethora of fairly quiet reflections that donot obscure the perception and understanding of speech.


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The Classic Auditorium is tall, a little longer than wide andhas balcony seats running on the two sidewalls and the back wall.The ceiling has a deeply coffered design and the sidewalls are linedwith pillars or rounded pilasters. This hall uses a central speakercluster elevated high over the proscenium of the raised stage.

This hall is a classic example of minimalist design. It employsan elevated, central speaker position to provide fairly uniform soundlevels to every seat in the audience. The room is a high volumehall, with ceilings 40 to 50 feet high over the main floor. From theviewpoint of the speaker cluster, the main floor, two sidewalls and

rear wall is nearly completely absorptive when the hall is fullyoccupied. The only remaining surface that the speakers can see isthe ceiling and it appears to be a very diffusive surface acting toscatter sound in all directions. (See Figure D).


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It is instructive to run through the basic acoustic calculationsfor the large classic auditorium design. Here are some basic ratios.Each seated person occupies about 7 square feet of floor space and

provides about 3 square feet of sound absorptive surface. A hall thatis 200 feet wide and 300 feet long will provide about 7,000 seats onthe main floor. The rear balcony section will be about 50 feet deepand provide about 1250 seats. The sidewall balconies will be about30 deep and provide 1000 seats each. The total seating of the hall is9250 seats. Occupied hall calculations are based on the hall being2/3rds full, just over 6000 people.

The audience provides about 18,500 square feet of absorptiondistributed over the floor, side and back walls. The hall has a volumeof 3 million cubic feet. Empty it will have a reverb time of about 7

seconds. This means in all its complexity, it has a physical acousticsurface equivalent of about 21,400 square feet. The hall has a floorand ceiling surface area of 60,000 sq.ft. surface area each. Thewalls have a surface area of 50,000 sq.ft. The average absorptioncoefficient of the surface of the hall is already about 12.5%, includingatmospheric absorption effects.


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The floor, sidewalls and rear wall present 100,000 sqft ofsurface area that intercept the sound from the speakers. With thesound absorption of the people added to the natural absorption of

the hall surfaces, these 3 surfaces present about 31,000 sqft ofabsorption facing the loudspeaker. This means that 31% of theincident sound is absorbed during the first reflection. Soundreflecting off these 3 surfaces is reduced in strength by about 2 dB.Ifwe assume that the sound reflects back across the room, travelingan average of 225 feet and taking about 1/5 second. Sound is

further attenuated by the natural surface absorption of about 10%upon this reflection. As a result, by the time sound makes one roundtrip in the hall from the proceneum wall into the audience and back,it has been attenuated down to 62% of its original strength, about -2.5 dB all in one round trip time period of 1/10th second. After one

second, there will be 20 such round trips and the overall sound willhave dropped by about 25 dB in strength. The hall will have a reverbtime of about 2.4 seconds. Adding stage curtains that show evenwhen pulled back will drop the reverb time to about 1.8 seconds.This is a reasonable reverb time for a large hall.


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But there is more than reverb time to hall acoustics.There are the good early reflections and the bad latereflections. The early reflections need to be cultivated. The latereflections need to be weeded out. The balcony facing issculpted to provide early reflections back down to the mainfloor. The back wall of the balcony and ceiling is sculpted toprovide early reflections into the balcony seats. (See Figure E) .


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The late reflections are mainly dealt with by combining twofeatures. It begins with having a heavily coffered ceiling. Anysound that is heading upwards eventually hits the cofferedceiling, only to be splintered into a cascade of tiny and off angledreflections. The second factor is the height of the ceiling. A 70ceiling with a loudspeaker mounted some 35 off the floor startssplashing sound back onto the main floor at about 40 ms afterthe direct signal has passed through the audience. The cofferingof the ceiling breaks this reflection up into dozens of low-level

reflections with a variety of additional time delays. The highcoffered ceiling acts to diffuse and randomize the only possiblelate reflections in the hall. Other sounds that are travelingupward that went over the heads of the balcony seating continuesupwards after the wall bounce and are also intercepted by thedeeply coffered ceiling scattering grid ceiling. The low level of

time-delayed backfill continues until it is overwhelmed with therise and decay of the reverberant part of the hall sound. (SeeFigure F) The classic auditorium of the early 1900s was almost,nearly a perfectly balanced system of people, space, speakers andsurfaces. It was a symphony in sound and architecture.


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CofferedCofferedCeilingCeiling

Controls roomreflectionswhen manypeople aretrying to talk,yet maintains

brightness.Increasesclarity andlowers theoverall volume.Can becustomized to

include wiringfor a projector,speakers,lights, etc.


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SoundSoundPanelsPanels

Controls wallreflections &flutter echoeswith a voicedblend ofabsorption &diffusion. EachPanel comeswith a bevelcut face andframe


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SoundSoundPlanksPlanks

Absorptionand diffusionhelp toacousticallycalm yourclassroom

whilemaintainingbrightness,enhance detailand improveintelligibility.Think of these

as "speedbumps" forsound.


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PicturePicturePanelsPanels

A room canboth soundgreat and lookgreat. It iscombinationof the science

of soundabsorption /diffusion withthe art ofprinting andproduced thePicture Panel.


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WallWallPanelsPanels

Large spacessuch ascafeterias,gymnasiumsbecome hugeecho chambers

when full ofstudents.Adding WallPanels helps toabsorb excessnoise andreduces the

echo toacceptablelevels.


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DECIBELS

The mostcommon measure ofasoundslevelis(Sound Pressure Level,orSPL,)expressedin decibels,abbreviateddB. decibelsare basedon the logarithmicratioofthe soundpowerorintensitytoareference powerorintensity.Soundpowerandintensityare not easyto measure. However,soundpressure is easily measuredwith

asoundlevel meter.S

oundpressure mayalsobe expressedin dBsince soundpressure squaredisproportionaltosoundpowerorintensity.

DBA (A-weighteddecibel)Asingle-number measurementbasedon the decibelbutweightedtoapproximate the response ofthe human earwith respecttofrequencies.

Absorption In acoustics, the energy of sound waves being taken in and trapped within a

material rather than being bounced off or reflected. Materials are rated in terms of theirability to absorb sounds.

REVERBERATIONTIMEReverberation time isdefinedasthe length oftime requiredforsoundtodecay 60dBfrom itsinitiallevel.

where:

RT(60)=reverberation time (sec)V=room volume (ft3 )S=surface area(ft 2 )?=absorption coefficientofmaterial(s)atgiven frequency

auditorium acoustics pesentation (3)

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