Auditorium Acoustics

Chapter 23

Sound Propagation Free field

sound pressure proportional to 1/r SPL drops 6 dB with every doubling of

distance. Indoors

Reflections

Reflections (Review) Flat surface

Mirror Concave

Concentrates/focuses Convex

(Scatters)/spreads Rough/Irregular

diffuses

Direct, Early, Reverberant Sound Direct: Sound travels straight from

source to listener (not reflected) Early: initial reflections, reaching

listener within 50 to 80 ms of direct sound.

Reverberant: reflections build up and become merged into continuous sound

Precedence Effect (1) Localization mechanisms Early reflections complicate time-based

localization Early reflections are usually not heard as

separate sounds (within 50 - 80 ms, same envelope)

Precedence Effect (2) Precedence effect: first sound is heard

as the direct sound Successive sounds arrive within 35 ms. Successive sounds share similar spectra and

env. Successive sounds are not too much louder

than the first sound

Early Sound and Concert Hall Acoustics “Intimate”: 20 ms delay between

direct and first reflected sound Rectangular shape: first reflections

usually come from the side walls. Listener preference for first

reflections coming from side rather than ceiling. Spatial responsiveness or impression

Reverberant Sound Simplification Alert:

Reverberation time at mid-frequency (500 - 2000 Hz) good indicator of “liveness.”

For steady sounds, reverberant sound builds to a steady energy level, then decays upon release.

Too much reverberant sound leads to a loss of clarity.

Determining Reverberation Time Factors

Power of the source Volume of the room Area of all surfaces in room Absorption coefficients for all surfaces

Simple (Bare Room, all surfaces same)

€

RT =Kvolume

area

More on Reverb time RT or T60

Equal to time it takes to for sound level to decrease by 60 dB

Different decay curves for initial decays and final decays can cause problems.

Decay curves can exhibit peaks due to standing waves.

Absorption (1) Consider volume and area

relationship. Since surface area reduces reverb

time, it acts as an absorptive element. Absorption is like an open window.

It completely absorbs sound. Ratio of room volume to area of

absorbing window.

Absorption (2) Window absorbs all sound

Absorption coefficient of a = 1 Total absorption for the room:

Add up absorption for each surface exposed to sound.

A equals absorption, instead of Area.

Absorption if frequency dependent.

Absorption (3) Calculate Absorption:

A = S1a1 + S2a2 + S3a3 . . .

Table 23.1 on p. 531

Air Absorption Large auditorium—air absorbs

sound, especially at high frequencies

People and seats also absorb sound.

Table 23.2, p. 533.

€

RT = 0.161V

A + mV

Criteria for Good Acoustics (p.534) Adequate loudness.

Issues: size and absorption (not too much of either) Uniformity

Issues: blending of stage sound, diffusion of hall sound (no dead spots)

Clarity Issue: needs sufficient absorption

Liveness (Reverb) Issue: feel that sound comes from all around

Freedom from Echoes Issue: too much separation in time of reflected sound