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Room Acoustics

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Reverberation

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Reverberation direct sound reflected sounds

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Sustained sound Reverberation time = time it takes for loudness decrease by 60 dB

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Acoustics characteristics of auditoriums “liveness” : reverberation time “intimacy”: time to the first reflected sound to arrive “fullness/clarity”: direct sound versus reflected sound volume “warmth/brilliance”: reverberation time for low frequencies larger/smaller than for high frequencies

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“texture”: temporal pattern of reflections goodbad “blend”: all seats hear all instruments “ensemble”: musicians can hear themselves

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Problems in acoustic design Focusing

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Focusing

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Echoes reflecting wall stage audience “pessimal” design

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Shadows only low frequencies behind the overhang

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Resonances analogy vocal chords instruments formants room resonances

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We want lots of resonances, evenly spread in the frequency (no favored frequencies) DimensionDimension Design #1Design #1 Design #2Design #2 Design #3Design #3 WidthWidth 1.14 x Height1.14 x Height 1.28 x Height1.28 x Height 1.60 x Height1.60 x Height LengthLength 1.39 x Height1.39 x Height 1.54 x Height1.54 x Height 2.33 x Height2.33 x Height DimensionDimension Design #1Design #1 Design #2Design #2 Design #3Design #3 WidthWidth 1.14 x Height1.14 x Height 1.28 x Height1.28 x Height 1.60 x Height1.60 x Height LengthLength 1.39 x Height1.39 x Height 1.54 x Height1.54 x Height 2.33 x Height2.33 x Height 8-ft high, 16-ft wide, and 16 ft long 10.89-ft high, 12.4-ft wide, and 15.14 ft long

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Walt Disney Hall (LA Philharmonic)

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Precedence effect ALL the sound seems to come from the direction of the earliest sound Electronic enhancement has to be delayed by 20- 50 ms

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External noise Solution: shielding, shielding, shielding ! Acceptable noise Recording studio 25dB Auditoriums, classrooms 30dB Homes 40dB Restaurants 50db (!)

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Calculation of reverberation time (engineer style) volume (ft 3 ) absorption (ft 2 ) A is the sum over all absorbing surfaces reverberation time (s)

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Example: 13ft by 20ft by 8ft room 4 walls of plaster (absorption coefficient 0.1) carpet floor (absorption coefficient 0.3) absorptive tile ceiling (absorption coefficient 0.6) wallsfloorceiling

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volume Warning: The aborption coefficient depends on the frequency The aborption coefficient depends on the frequency people absorb sound: 1 person = 2.5 sabins people absorb sound: 1 person = 2.5 sabins person + upholstered seat = 3 sabins person + upholstered seat = 3 sabins

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Trial and error leads to rules of thumb (RPG Diffusor Systems INC) “The ceiling of an Auditorium should be primarily sound reflected/diffusive (not primarily absorptive) in order to reflect early energy down to the audience promoting loudness and intelligibility. The ceiling should be 30%-50% open to the space above to vent excessive sound power and/or promote reverberance. It is important that the ceiling surfaces be sound diffusive so that gaps in the sound distribution pattern (nonuniformity) do not result from the open areas. Lower side and rear walls near the audience should also be reflective/diffusive (not absorptive) to promote loudness, intelligibility, envelopment, and to prevent echoes off the rear wall. Overall reverberance and loudness is controlled by placing absorption and diffsorption (mid frequency absorption and high frequency diffusion) on the underside of the roof deck and on the upper walls respectively. The amount of absorption and diffsorption depends on the overall volume of the auditorium. Larger auditoriums (350+ cubic feet per seat) require that approximately 50% of the upper wall area be treated with diffsorption, while smaller auditoriums (250–350 cubic feet per seat) require only about 25% of the upper wall area be treated. Typically, small auditoriums with less than 250 cubic feet per seat are not reverberant enough for music and serve only as good speech rooms. Very little absorption is required in these rooms. Problems such as flutter and echoes are prevented with diffusion and diffsorption. “

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Avery Fisher Hall (Lincoln Center) clouds

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