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 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 ( 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)

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.

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