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 

K

volume area 

More on Reverb time   RT or

T

60 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

=

S 1 a 1

+

S 2 a 2

+

S 3 a 3

. . .

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  0.161

A



V 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