Physics 1240 Thurs 4/6/06Today: Reverberation and Sabine’s formula Next time: Finish architectural acoustics, Review for Exam IIILearning goals• Measure and determine the reverberation time• Assess the quality of the reverberation of a concert hall• Estimate the reverberation time using Sabine’s formulaCT24.1 Suppose you are sitting in a center stage seat, listening to a solo soprano opera singer. The theater is symmetrical in design (looks the same on both sides of you). What acoustical quality would be compromised (negatively impacted) in such a situation?A) source widthB) acoustical intimacyC) reverberationCT24.2 The need for Dolby digital sound in movie theaters, rather than simply conventional stereo sound, is to recreate an effect that is closest to which following acoustical quality?A) reverberationB) source widthC) acoustical intimacyD) envelopmentE) volumeCT24.3 Name three important qualities (of 9 total) that we covered in lecture, that are needed for good room acoustics.A) source width, acoustical intimacy, flutter echoesB) acoustical intimacy, reverberation, envelopmentC) reverberation, source width, resonance overlapD) source width, reverberation, resonance decayClicker scores will no longer count for a grade AFTER this lecture.Reverberation – the build up and decay of sound in a roomReverberation time: How long it takes the sound to decay 60 dBReverberation time: How long it takes the sound to decay 60 dB – we call it TrTypical Reverberation timesPractice room: V = 27 m3, T = 0.6 sec ( 3m x 3m x 3m)Rehearsal room: V = 600 m3, T = 1.4 sec ( 6m x 10m x 10 m)Large concert hall: V = 20,000 m3, T = 2.2 sec (20m x 32m x 32m)Sabine’s formulaTr= (0.16 s/m) V / SeSe = α1x S1+ α2x S2+ α3x S3+ ...V – total volume of the roomS – surface areasα = (power absorbed)/(power incident) called “Absorption coefficients”Absorption coefficients - the α’s in Sabine’s formulaα = (power absorbed)/(power incident)CT24.4 If your living room seems acoustically dead, what should you do to increase the reverberation? A) remove carpet and install a hardwood floorB) texture the walls and ceiling (make rough)C) install heavy curtains over the windowsD) add a large, soft sofaCT24.5 What surface might have an absorption coefficient of 0.4?A) painted brickB) shag carpetingC) window glassCT24.6 What is a typical reverberation for a large concert hall?A) 0.5 secB) 1.0 secC) 2 secD) 4
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