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