Room AcousticsRoom Acoustics

ReverberationReverberation

ReverberationReverberation

direct direct soundsound

reflected reflected soundssounds

Sustained soundSustained sound

Reverberation time = time it takes for loudness Reverberation time = time it takes for loudness decrease by 60 dBdecrease by 60 dB

Acoustics characteristics of auditoriumsAcoustics characteristics of auditoriums

““liveness” : liveness” : reverberation timereverberation time

““intimacy”: intimacy”: time to the first reflected sound to arrivetime to the first reflected sound to arrive

““fullness/clarity”: fullness/clarity”: direct sound versus reflected sound direct sound versus reflected sound volumevolume

““warmth/brilliance”:warmth/brilliance”: reverberation time for low reverberation time for low frequencies larger/smaller than for high frequenciesfrequencies larger/smaller than for high frequencies

““texture”: texture”: temporal pattern of reflectionstemporal pattern of reflections

goodgood badbad

““blend”: blend”: all seats hear all instrumentsall seats hear all instruments

““ensemble”:ensemble”: musicians can hear themselves musicians can hear themselves

Problems in acoustic designProblems in acoustic design

FocusingFocusing

Problems in acoustic designProblems in acoustic design

FocusingFocusing

EchoesEchoes

reflecting reflecting wallwall

stagestage audienceaudience

““pessimal” designpessimal” design

ShadowsShadows

only low only low frequencies frequencies behind the behind the overhangoverhang

ResonancesResonances

analogyanalogy

vocal chords instrumentsvocal chords instruments

formants room resonancesformants room resonances

We want lots of resonances, evenly spread in the We want lots of resonances, evenly spread in the frequency (no favored frequencies)frequency (no favored frequencies)

Dimension

Design #1

Design #2

Design #3

Width

1.14 x Height

1.28 x Height

1.60 x Height

Length

1.39 x Height

1.54 x Height

2.33 x Height

Dimension

Design #1

Design #2

Design #3

Width

1.14 x Height

1.28 x Height

1.60 x Height

Length

1.39 x Height

1.54 x Height

2.33 x Height

8-ft high, 16-ft wide, 8-ft high, 16-ft wide, and 16 ft long and 16 ft long

10.89-ft high, 12.4-ft 10.89-ft high, 12.4-ft wide, and 15.14 ft long wide, and 15.14 ft long

Walt Disney Hall (LA Philharmonic)Walt Disney Hall (LA Philharmonic)

Precedence effectPrecedence effect

ALLALL the sound seems to come from the direction the sound seems to come from the direction of the earliest soundof the earliest sound

Electronic enhancement has to be delayed by 20-Electronic enhancement has to be delayed by 20-50 ms50 ms

External noiseExternal noise

Solution: shielding, shielding, shielding !Solution: shielding, shielding, shielding !

Acceptable noiseAcceptable noise

Recording studio 25dBRecording studio 25dBAuditoriums, Auditoriums, classrooms 30dBclassrooms 30dB

Homes 40dBHomes 40dB

Restaurants 50db (!)Restaurants 50db (!)

Calculation of reverberation time (engineer style)Calculation of reverberation time (engineer style)

55.2 0.050Rs

V VT

v A A

volume (ftvolume (ft33))

absorption (ftabsorption (ft22))

A is the sum over all absorbing surfacesA is the sum over all absorbing surfaces

reverberation time (s)reverberation time (s)

Example:Example:

13ft by 20ft by 8ft room13ft by 20ft by 8ft room

4 walls of plaster (absorption coefficient 0.1)4 walls of plaster (absorption coefficient 0.1)

carpet floor (absorption coefficient 0.3)carpet floor (absorption coefficient 0.3)

absorptive tile ceiling (absorption coefficient 0.6)absorptive tile ceiling (absorption coefficient 0.6)

2 (20 8) 2 (13 8) 0.1 (13 20) 0.3 (13 20) 0.6

286 sabins

A

wallswalls floorfloor ceilingceiling

13 20 80.050 0.36 seconds

286.8RT

volumevolume

Warning:Warning:

• The aborption coefficient depends on the The aborption coefficient depends on the frequencyfrequency

• people absorb sound: 1 person = 2.5 sabinspeople absorb sound: 1 person = 2.5 sabins

• person + upholstered seat = 3 sabinsperson + upholstered seat = 3 sabins

Trial and error leads to rules of thumbTrial and error leads to rules of thumb(RPG Diffusor Systems INC)(RPG Diffusor Systems INC)

““The ceiling of an Auditorium should be primarily sound reflected/diffusive (not 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 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 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 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 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 (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, audience should also be reflective/diffusive (not absorptive) to promote loudness, intelligibility, envelopment, and to prevent echoes off the rear wall.intelligibility, envelopment, and to prevent echoes off the rear wall.Overall reverberance and loudness is controlled by placing absorption and diffsorption Overall reverberance and loudness is controlled by placing absorption and diffsorption (mid frequency absorption and high frequency diffusion) on the underside of the roof (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 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 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 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 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 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 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 rooms. Very little absorption is required in these rooms. Problems such as flutter and echoes are prevented with diffusion and diffsorption. “echoes are prevented with diffusion and diffsorption. “

Avery Fisher Hall (Lincoln Center)Avery Fisher Hall (Lincoln Center)

cloudsclouds