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50 CHAPTER 3. MEASURING SOUNDFor example, cars and airplanes that pass by repeatedly will not raise theaverage sound level L eq much but are annoying because of the repeated risingand fading away of sound. In other words, the frequent variation comparedto the background noise. Robinson proposed a measure: Noise PollutionLevel or N.P.L. : NPL = L eq +2.56σ. with L eq the energetic time averageas discussed above, and σ the standard deviation, that is to say, a statisticmeasure of the variationsof the sound. The larger these variations, the largerσ. There exists a very good correlation between the values obtained for NPLand the subjective nuisance of the fluctuating sound. For airplanes specialmodels exist. The noise loading is expressed as: L Amax +blogN +c, with Nthe number of flybys within a certain time interval, and a, b and c representconstants.3.9 The intensity meterIn Chapter 1 the notion sound intensity was introduced and it was shownthat the acoustic intensity I x in a direction x is given by : I x = v x p, where v xrepresents the particle speed of the sound wave in the x direction and p thesound pressure. The sound pressure can simply be measured with a microphone,but measuring the particle speed is far more difficult. This vectorialquantity can however be measured with the aid of a derived quantity :F x = ma⇒ ρ ∂v x∂t = −∂p ∂x⇒ v x = − 1 ∫ ∂pρ ∂xIn reality the derrivative of the pressure with respect to the distance is calculatedby discretization :I x = p.v x= − 12ρ∆r (p A +p B )∫(p A −p B )dtwhere p A and p B represent the sound pressure on two neighbouring locations.This discretized equation is used to calculate the sound intensity with theaid of the intensity meter (see Figure 3.15). This measuring device consistsof two microphones spaced out over a fixed distance with a so called spacer(a few centimetres).The use of the intensity meter offers some advantages :
3.9. THE INTENSITY METER 51Since the intensity is a vectorial quantity is one can determine thedirection in which the intensity is the largest. This means the intensitymeter can be used to localize sources of sound (One can ’scan’ wherethe sound comes from so to speak).It will be shown further that the intensity meter can also be used tomeasure the sound power of a source (see the next paragraph). For thispurpose the intensity meter has a few important advantages :– Thebackground noise canbeeliminated (under the conditionthatit is stationary).– Onecandefinearandomsurfacesurroundingthesourceofasound(instead of a simple spherical surface).– One can measure closer to the source (where the sound waves arenot necessarily in plane or spherical).There are a few issues that require attention when using an intensitymeter however :Forafixed distance between the microphones, the intensity meter hasarather limited frequency range where the measurements are valid. Theupper limit is given by f U = c (for a distance D = 0.05 this comes4Ddown to f U = 1700). At low frequencies the value derived from thetheory is still correct, but noise present in the measurement will resultin an incorrect measurement.The direction of the probe is of great importance. To measure thepower of the source, the intensity probe has to be held perpetual onthe defined surface at all times (this is not the case for an intensitymeter that is not directional).The cost of an intensity meter is much higher than that of a sonometer.The calibration of the probe is also a lot more devious (the twomicrophones must be matched perfectly.The error on the measured intensity depends on the distance r betweenthe microphones of the intensity meter and the frequency. Assume the instantaneouspressure on a certain point in time is given by :p(x) = p m sin(kx) (3.6)with p m the amplitude, k the wavenumber and x the coordinate of themeasured midpoint between the microphones. The exact expression for the
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VRIJE UNIVERSITEITBRUSSELAcousticsB
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iiWhen sound becomes noise, it will
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ContentsI Introduction to acoustics
Page 9 and 10: CONTENTSvii4.3 Measuring the acoust
Page 11: Part IIntroduction to acoustics1
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Page 69 and 70: Chapter 4Sound AbsorptionAll(constr
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6.1. ORIGIN OF NOISE 101Figure 6.2:
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6.2. REDUCING NOISE AT THE LEVEL OF
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6.3. TACKLING NOISE TRANSMISSION 11
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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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124CHAPTER 7. DIRECTIVE 2000/14/EG:
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126CHAPTER 7. DIRECTIVE 2000/14/EG:
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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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