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

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 :