This document discusses key concepts in auditorium acoustics, including how sound propagates both in open and enclosed spaces. It explains the effects of different types of reflections - from flat, concave, and convex surfaces - on sound propagation. Key acoustic factors that determine the listener's experience are identified as direct sound, early reflections, and reverberant sound. The precedence effect and how it relates to localization is also summarized. The document provides an overview of how reverberation time is determined and measured, and the role of absorption properties in influencing reverberation time. It concludes with criteria for achieving good acoustics in an auditorium setting.

1. Auditorium Acoustics
Chapter 23

2. Sound Propagation
 Free field
 sound pressure proportional to 1/r
 SPL drops 6 dB with every doubling of
distance.
 Indoors
 Reflections

3. Reflections (Review)
 Flat surface
 Mirror
 Concave
 Concentrates/focuses
 Convex
 (Scatters)/spreads
 Rough/Irregular
 diffuses

4. 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

5. 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)

6. 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

7. 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

8. 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.

9. 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 = K
volume
area

10. 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.

11. 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.

12. 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.

13. Absorption (3)
 Calculate Absorption:
 A = S1a1 + S2a2 + S3a3 . . .
 Table 23.1 on p. 531

14. Air Absorption
 Large auditorium—air absorbs sound,
especially at high frequencies
 People and seats also absorb sound.
 Table 23.2, p. 533.
RT = 0.161
V
A + mV

15. 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