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14 CHAPTER 1. FUNDAMENTAL CONCEPTS OF ACOUSTICSp 2 over a short time period, negligible with respect to the fly over time.In this case the time interval for integration is important.3. Physiological experiments have allowed to see that for the human earthesoundintensityimpressioncansometimenoticeablychangein0.1sec.Consequently this is a maximum value of time integration for variablesound.4. Technical application (the measuring equipment is based on this) : theintegrationinterval istakenrelatively short (e.g. 125ms), such thattheneedleofthemeasuring equipment iscapableofdetecting relative quickfluctuations of the sound pressure; a sound whose variating intensity intime is perceived by the human ear. The measurement system deliversa result that variates in time approximately in the same way. Themeasured values for p eff are more or less located between followinglimits :2×10 −5 Pa of the largest limit to the smallest is 10 7 , which is very large.All the common sounds are situated between those limits. For this reason,andalso because earlier onebelieved thatahuman hearsaccordingto the law of Weber–Fechner, i.e. logarithmic, one have introduced thedB-scale (response proportional with the logarithm of the stimulus).1.5.2 The dB-scaleThe dB-scale allows us to describe sound, like a person perceives, in a correctway. The sound pressure level (SPL) is defined as :L p = 10log p2p 2 0= 20log p p 0(1.26)with p 0 = 20 × 10 −6 Pa the reference pressure (it equals the threshold ofhuman hearing). An example of the dB-scale and a comparison with thelinear scale is given in Figure 1.2.The hearing threshold equals 0 dB, the pain threshold is 120 dB. Calculatingwith dB causes often problems because of the fact that we are used towork with the linear scale. Summation and subtraction are best performedby first transforming to the linear scale, then by executing the operation andfinally by transforming back to the dB-scale. For the sum of two values L p1en L p1 :L p = 10log(10 Lp 1 /10 +10 Lp 2 /10 ) (1.27)
1.5. SOUND LEVELS 15Figure 1.2: Some sound values in the linear and dB scale. Source:Bruël&Kjær.Note that when on a given position a certain source produces a sound pressurelevel 10 dB smaller than the sound pressure level of another source, thisfirst source has no big contribution (less than 1 dB) to the sound pressurelevel in that point. This will enable us to perform measurement of soundsources without the need of shutting down all other sources.When multiplying a sound pressure in the linear scale with a certainfactor this results in the addition of a value in the dB scale. Examples (seealso Figure 1.3 :Multiplication with a factor 2 equals an addition of around 6 dB.Multiplication with a factor 3 equals an addition of around 10 dB.Multiplication with a factor 10 equals an addition of around 20 dB.Attention: these rules are only valid for sound pressures p. Later theconcepts of sound intensity and sound power will be introduced. These parametersare proportional to the energy of sound (i.e. ∼ p 2 ) and this is forexample the reason that a multiplication of the sound power with 10 equalsan addition of 10 dB.
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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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6.4. 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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144 APPENDIX A. MATERIAL PROPERTIES
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152 BIBLIOGRAPHY[12] Bruel and Kjae
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