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76 CHAPTER 4. SOUND ABSORPTIONFigure 4.8: Reverberation room.P max the maximum amplitude of the sound pressureThese parameters are used to calculate the so-called Standing Wave Ratio ofSWR :SWR = P maxP min(4.30)One can show that the reflection factor R (given by R = √ r, with r thereflection coefficient) can be calculated, using following equation :r = SWR−1SWR+1(4.31)From this, the absorption coefficient a can be determined : a = 1−R 2 .The method with the Kundt tube is a very simple method, which cangive fairly accurate results. There are however two major limitations in theapplication of the method :The method only gives the absorption coefficient at perpendicular incidence(in contrast to the method in the reverberation room from whicha is obtained at random incidence).
4.4. THE DIRECT AND DIFFUSE SOUND FIELD 77The measurements are only valid in a fairly limited frequency range.On the one hand there is a lower limit for the frequency f L which isdetermined by the length l of the tube : f L = 3c (at lower frequencies4Lthe minimum and maximum can not be observed within the length ofthe tube). On the other hand, there is an upper limit f U that is givenby f U = 170 with d the thickness of the tube. This limit has to bedtaken into account in order to avoid acoustic resonances in the lateraldirection of the tube. One could reduce the thickness of the tube, butthen the disturbance of the microphone on the sound field would be toolarge and the wave would be damped too much throughout the tube.Figure 4.9: The Kundt tube, used to measure the absorption coefficient of amaterial.4.4 The direct and diffuse sound fieldSuppose W is the total transmitted acoustic power of a sound source (machine)which is put in a room. In what follows, we calculate the soundpressure at a certain distance from the source.The acoustic power produced by the sound source is fully absorbed bythe walls of the room. Therefore :W = ∑ iI i a i S i = Ī ∑ ia i S i = ĪA (4.32)with Ī the spatially averaged sound intensity. For a diffuse field W = p2 effA 4ρcand thus :p 2 effp 2 0= ρcp 2 04AW 0W 0(4.33)
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VRIJE UNIVERSITEITBRUSSELAcousticsB
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iiWhen sound becomes noise, it will
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ContentsI Introduction to acoustics
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CONTENTSvii4.3 Measuring the acoust
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Part IIntroduction to acoustics1
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4 CHAPTER 1. FUNDAMENTAL CONCEPTS O
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10 CHAPTER 1. FUNDAMENTAL CONCEPTS
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24 CHAPTER 2. THE HUMAN HEARING SYS
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128 CHAPTER 8. NOISE ON THE WORK FL
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140 CHAPTER 9. COMMUNITY NOISE2. No
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142 CHAPTER 9. COMMUNITY NOISE
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144 APPENDIX A. MATERIAL PROPERTIES
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152 BIBLIOGRAPHY[12] Bruel and Kjae
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