THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

56 3. Sound Wave Propagation and CharacteristicsFigure 3.14. A histogram showing probability of exceedance for plant noise.Example Problem 7From perusal of Figure 3.14, estimate the sound levels that are exceeded 10%,50%, and 90% of the time. Also establish the percentage of the total observationtime that the sound was between 65 and 67 dB(A).SolutionThe probability-of-exceedance curve in Figure 3.14 is read to yield ≈69 dB(A)(exceeded 10% of the time), L 50 ≈ 63 dB(A) (exceeded 50% of the time), andL 90 ·≈ 58 dB(A) (exceeded 90% of the time).Equivalent sound level L eq is the sound energy averaged over a given period oftime T , i.e., it is the rms or mean level of the time-varying noise. It is defined byL eq = 10 log( ∫1 TT 0p 2p 2 ref)dt(3.27)where p 2 = p 2 (t) is the mean-square (time-varying)sound pressure and p ref =20 μPa. Equation (3.27) can be more conveniently rewritten in terms of soundlevel L = L(t) using the relationship of Equation (3.22):( ∫ 1 T)L eq = 10 log 10 L p/10 dtT 0(3.28)In order to facilitate digital processing in the measurement of L eq through the useof an integrating sound level meter, the integral form of Equation (3.28) is replaced