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Auditorium Acoustics

The room in which we listen to sounds has an important influence on what we hear.This section will identify some of the principal means currently available for judging

the quality of an auditorium. However, the design of such spaces is still considered an

inexact science.

Sound Propagation in an Auditorium

y As sound waves travel at about 345 meters/second, the sound coming directlyfrom a source within an auditorium will generally reach a listener after a time

of anywhere from 0.01 to 0.2 seconds.

y Shortly after the arrival of the direct sound, a series of semi-distinct reflectionsfrom various reflecting surfaces (walls and ceiling) will reach the listener.

These early reflections typically will occur within about 50 milliseconds.

y The reflections which reach the listener after the early reflections are typicallyof lower amplitude and very closely spaced in time. These reflections merge

into what is called the reverberant soundorlate reflections.

y If the source emits a continuous sound, the reverberant sound builds up until itreaches an equilibrium level. When the sound stops, the sound level decreases

at amore or less constant rate until it reaches inaudibility.

y For impulsive sounds, the reverberant sound begins to decay immediately.Direct Sound and Early Reflections

y Direct sound will decrease by 6 dB for each doubling of distance propagated.y Our auditory system will determine the direction of a sound source from the

direct sounds reaching the ear.

y Early reflections which arrive within about 35 milliseconds are not heard asseparate from the direct sounds. Rather, they tend to reinforce the direct sound.

y The source is perceived to be in the direction from which the first sound arrivesprovided that (1) successive sounds arrive within about 35 milliseconds, (2) the

successive sounds have spectra and time envelopes reasonably similar to thefirst sound, and (3) the successive sounds are not too much louder than the first.

This is referred to as the precedence effect.y From a study by Leo Beranek (1962), a concert hall is considered ``intimate'' if

the delay time between the direct and first reflected sound is less than 20

milliseconds.

y First reflections usually arrive from the nearest side wall or from the ceiling forthose seated in the center.

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y Reflections from the ceiling or overhead reflectors are not as perceptuallydesireable as those from side walls.

Late Reflections

y During a continuous sound, the reverberant sound level is reached when therate at which energy is supplied by the source is equal to the rate at which

sound is absorbed by the room and its contents.

y Too much reverberant sound will result in loss of clarity.y In a bare room, where all surfaces absorb the same fraction of the sound that

reaches them, the theoretical reverberation time is proportional to the ratio of

volume to surface area.

y Reverberation time is typically defined as the time required for the sound levelto decrease by 60 dB (or ).

Calculating Reverberation Time

y When expressed in units of cubic and square meters, the reverberation time isgiven by RT = , where is the volume of the room and is the

effective ``total absorption'' area.

y The ``total absorption'' area is calculated as the sum of all surface areas in theroom, each multiplied by its respective absorption coefficient.

Air Absorption

y Air contributes a substantial amount to the absorption of high frequency sound.y Taking account of air absorption, RT = 0.161 , where is a

constant which varies with air temperature, humidity, and frequency.

Criteria for Good Acoustics

y Optimum reverberation time is a compromise between clarity (requiring shortreverberation time), sound intensity (requiring a high reverberant level), and

liveness (requiring a long reverberation time).y The optimum reverberation time of an auditorium is dependent on the use for

which it is designed.y Reflected sound arriving from the sides seems to be important to the overall

reverberance of the room.

y Important subjective attributes of concert hall acoustics include intimacy,liveness, warmth, loudness of direct sound, reverberant sound level,

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definition orclarity, diffusion oruniformity, balance and blend, ensemble,

andfreedom from noise.y In addition to the attributes above,spatial impression and early decay time are

important. The spatial impression is dependent on contributions to the early

reflections from above and especially from the sides. The initial rate of decay

of reverberation is apparently more perceptually important than the totalreverberation time.

y Echoes, flutter echoes, sound focusing, sound shadows, and background noiseshould be avoided in an auditorium design.

y The greater the early decay time (up to two seconds), the greater the preferencefor the concert hall. Above two seconds, the trend it reversed.

y Narrow halls are generally preferred to wide ones.y Preference is shown for halls having a high ``binaural dissimilarity''.y Less ``definition'' is preferred. Definition represents the ratio of energy in the

first 50 milliseconds to the total energy.

Background Noise

y Concert halls should meet at least the NC-20 curve and preferably the NC-15curve (see Figure 23.9 on pg. 471 of Rossing).