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Technology in Architecture Lecture 17 Sound in Enclosed Spaces Reverberation Auditorium Design Lecture 17 Sound in Enclosed Spaces Reverberation Auditorium Design

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Sound In Enclosed Spaces

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Sound Absorption Amount of sound energy not reflected S: p. 771,, F.18.2

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Sound Absorption Absorption coefficient α=I α /I i α=absorption coefficient I α =sound power intensity absorbed (w/cm 2 ) I i =sound power impinging on material (w/cm 2 ) 1.0 is total absorption

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Sound Absorption Absorption coefficient S: p. 769, T.18.1

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Sound Absorption Absorption A=Sα A=total absorption (sabins) S=surface area (ft 2 or m 2 ) α=absorption coefficient sabins (m 2 )= 10.76 sabins (sf)

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Sound Absorption Total Absorption Σα=S 1 α 1 + S 2 α 2 + S 3 α 3 +…+S n α n or ΣA=A 1 + A 2 + A 3 +…+A n

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Sound Absorption Average Absorption α avg =ΣA/S α avg <0.2 “live” α avg >0.4 “dead” S: p. 774, F.18.6

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Reflection in enclosed spaces Acoustical phenomena S: p. 787, F.18.20 S: p. 788, F.18.21

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Ray diagrams Trace the reflection paths to and from adjoining surfaces angle of incidence = angle of reflection I R

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Ray diagrams Trace the reflection paths to receiver Reflected sound path ≤ Direct sound path+55 Note: check rear wall and vertical paths Note: SR-6=RR-7 SR-6: p.116, F.5-12

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Reflection in enclosed spaces Auditorium sound reinforcement S: p. 789, F.18.23

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ReverberationReverberation

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Reverberation Persistence of sound after source has ceased S: p. 771, F.18.2

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Reverberation Time Period of time required for a 60 db drop after sound source stops T R = K x V/ΣA T R : reverberation time (seconds) K: 0.05 (English) (0.049 in SR-6) or 0.16 (metric) V: volume (ft 3 or m 3 ) ΣA: total room absorption, sabins (ft 2 or m 2 )

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Reverberation Time Application Volume S: p. 782, F.18.13 ft 3 x1000 3.5 35.0 350

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Reverberation Time Calculated for “center band frequencies” 125250500100020004000 Pay specific attention to 125 hz and 500 hz

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Reverberation Example Compile data Material Absorption Coefficient Material Surface Area SR-6: p.121

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Reverberation Example Compare to requirements and adjust S: p. 782, F.27.13 ft 3 x1000 3.5 35.0 350

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Auditorium Design

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Room Acoustics Noise ControlSound System

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Auditorium Design Factors Audience Size Range of Performance Activities Audience Sophistication

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Auditorium Design Parameters 1. Audience Size (floor area): 6-8sf/seat 2. Reverberation (volume): Motion Picture Theatre100 cf/seat Lecture Hall100 cf/seat Music Room/Auditorium200-250 cf/seat Symphony Hall200-300 cf/seat Choral Rehearsal350 cf/seat Band Rehearsal500 cf/seat

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Auditorium Design Parameters 3. Ceiling Height= Volume/Floor Area generally…20 x T R

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Auditorium Design Parameters 4. Typical Dimensions L:W:H = 2H:1.5H:H Determine acceptable width (80-120’) Determine length (L = SF/Width) If Length ≥120’, use balcony

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Auditorium Design Parameters 5. Stage opening: 40-50’ wide x 25’ high

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Auditorium Design Parameters 6. Shape Walls and Ceilings a) source is 5’ from stage front, 5’ above stage floor b) trace reflectances from back wall, side walls, and ceiling @ 40-60’ from stage 40’ 60’

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Auditorium Design Parameters 7. Check Echoes

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Auditorium Design Example Given data RR-7: p.119

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Reverberation Time Symphonic 492,480 ft 3 T R =1.5 seconds S: p. 782, F.18.13 ft 3 x1000 3.5 35.0 350

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Auditorium Design Example Given data SR-6: p.115, F.5-11

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Auditorium Design Example Given data SR-6: p.116, F.5-12

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Auditorium Design Example Area takeoffs SR-6: p.120

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Auditorium Design Example Locate coefficients S: p. 769, T.18.1

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Auditorium Design Example Absorption Coefficients SR-6: p.121

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Auditorium Design Example Compute absorption Compute reverberation SR-6: p.121

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Auditorium Design Example Compute T R 500 for given data: T R =0.049 x V/ ΣA =0.049 x 492,480/12,538 =1.92 seconds

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Auditorium Design Example Compute absorption needed for volume: T R =1.5 seconds ΣA=0.049 x V/T R =0.049 x 492,480/1.5 =16,088 sabins

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Auditorium Design Example Adjust to meet absorption Add acoustical tile on walls

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Auditorium Design Example Determine acoustical tile area

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Auditorium Design Example Determine acoustical tile area Sabins design16,088 - Sabins given12,538 Sabins short 3,550 Translate to area of acoustical tile Sabins/Δα=3,550/0.67= 5,299 sf

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Auditorium Design Example Add 5,299 sf of acoustical tile on walls Recompute T R500

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Auditorium Design Example Recompute T R-500 for revised data: T R-500 =0.049 x V/ ΣA =0.049 x 492,480/16,088 =1.5 seconds Compute T R-125 for revised data: T R-125 =0.049 x V/ ΣA =0.049 x 492,480/9,593 =2.52 seconds

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Auditorium Design Example Compare T R-500 with T R-125 : T R - 125 /T R - 500 =2.52/1.5= 1.68 Ratio should be 1.6-1.85 to offset hearing deficiency

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Auditorium Design Example Check final conditions SR-6: p. 122

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Sound Systems Check final conditions S: p. 794, F.18.28

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