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

Sound In Enclosed Spaces

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

Sound Absorption Absorption coefficient α=Iα/Ii Iα=sound power intensity absorbed (w/cm2) Ii=sound power impinging on material (w/cm2) 1.0 is total absorption

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

Sound Absorption Absorption A=Sα S=surface area (ft2 or m2) A=total absorption (sabins) S=surface area (ft2 or m2) α=absorption coefficient sabins (m2)= 10.76 sabins (sf)

Sound Absorption Total Absorption Σα=S1α1 + S2α2 + S3α3 +…+Snαn or ΣA=A1 + A2 + A3 +…+An

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

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

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

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

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

Reverberation

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

Reverberation Time Period of time required for a 60 db drop after sound source stops TR= K x V/ΣA TR: reverberation time (seconds) K: 0.05 (English) (0.049 in SR-6) or 0.16 (metric) V: volume (ft3 or m3) ΣA: total room absorption, sabins (ft2 or m2)

Reverberation Time Application Volume S: p. 782, F.18.13 ft3x1000 3.5 35.0 350 S: p. 782, F.18.13

Reverberation Time Calculated for “center band frequencies” 125 250 500 1000 2000 4000 Pay specific attention to 125 hz and 500 hz

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

Reverberation Example Compare to requirements and adjust ft3x1000 3.5 35.0 350 S: p. 782, F.27.13

Auditorium Design

Noise Control Sound System Auditorium Design Room Acoustics Noise Control Sound System

Auditorium Design Factors Audience Size Range of Performance Activities Audience Sophistication

Auditorium Design Parameters 1. Audience Size (floor area): 6-8sf/seat 2. Reverberation (volume): Motion Picture Theatre 100 cf/seat Lecture Hall 100 cf/seat Music Room/Auditorium 200-250 cf/seat Symphony Hall 200-300 cf/seat Choral Rehearsal 350 cf/seat Band Rehearsal 500 cf/seat

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

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

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

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’

Auditorium Design Parameters 7. Check Echoes

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

Reverberation Time Symphonic 492,480 ft3 TR=1.5 seconds ft3x1000 3.5 35.0 350 S: p. 782, F.18.13

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

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

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

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

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

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

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

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

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

Auditorium Design Example Determine acoustical tile area

Auditorium Design Example Determine acoustical tile area Sabins design 16,088 - Sabins given 12,538 Sabins short 3,550 Translate to area of acoustical tile Sabins/Δα=3,550/0.67= 5,299 sf

Auditorium Design Example Add 5,299 sf of acoustical tile on walls Recompute TR500

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

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