VRIJE UNIVERSITEIT BRUSSEL Acoustics - the Dept. of ...

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20 CHAPTER 1. FUNDAMENTAL CONCEPTS OF ACOUSTICSNote that a spherical wave behaves like a plane wave if the observer is faraway from the point source (in practice for distances larger than 3λ). For adiffuse sound field, where the waves are in all directions just as strong andindependent, we may find (no proof is given in this text) :I diffuus = p ρc = ρcv eff (1.34)1.6.2 The sound intensity levelDefinition : the sound intensity level (SIL) L I is defined as :L I = 10log I I 0(dB) (1.35)with I 0 = 10 −12 W/m 2 the standard reference sound intensity level.Notethatforatraveling planewavepropagatinginairthesoundintensitylevel is approximately equal to the sound pressure level : L I ≈ L p . Indeed :L I = 10log I I 0= 10log p2ρcI 0met ρc ≈ 412≈ L p1.7 Source power1.7.1 DefinitionDefinition : The sound power is the acoustic power (Watt) delivered by asound source : ∫W = IdS (1.36)with I the intensity in W/m 2 over the elementary surface dS. If I is constantin all directions (nondirectional source) : W = IS.In analogy with the sound pressure level and sound intensity level onecan define the sound power level.Definition : The sound power level (SWL) is defined as :SL W = 10log W W 0(1.37)
1.7. SOURCE POWER 21with W 0 = 10 −12 W the reference level.One have to pay attention to the terms we use because all those differentlevels have the same units (dB).1.7.2 The sound field produced by a point sourceFor a spherical source the source power is W = I4πr 2 , with I the intensityaccording to the radius of this sphere on a distance r from the source. In adistant sound field r ≫ λ we can write I = p2 effρcand thus :W = p2 eff 4πr2ρc(1.38)from which one can get an expression for the effective sound pressure, measuredat a distance r from the source :√Wρcp eff =(1.39)4πr 2With known source acoustic power the sound pressure can be calculatedin a given point. The effective sound pressure decreases with increasingdistance 1 . One may rewrite Equation 1.39 as a function of values in dB.r√412Wp eff = ρc = 400 for air4πr 2⇒ p 2 eff = 412W4πr 2⇒p2 eff 412W=400∗10 −12 4πr 2 400∗10 −12⇒ L p = L W −10log(4πr 2 )For r = 1 one has approximately :L p = L W −10 (1.40)
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Chapter 5Sound InsulationWhen consi
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5.1. MEASURING SOUND INSULATION 83F
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5.1. MEASURING SOUND INSULATION 85F
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5.3. THE ACOUSTICAL BARRIER 95For t
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Chapter 6Noise control6.1 Origin of
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6.1. ORIGIN OF NOISE 101Figure 6.2:
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6.3. TACKLING NOISE TRANSMISSION 11
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6.4. RADIATION NOISE 115Figure 6.17
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6.4. RADIATION NOISE 117Figure 6.20
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Part IIINoise directives119
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140 CHAPTER 9. COMMUNITY NOISE2. No
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144 APPENDIX A. MATERIAL PROPERTIES
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152 BIBLIOGRAPHY[12] Bruel and Kjae
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