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

52 3. Sound Wave Propagation and Characteristicsmodified by dividing the sum by n, the number of levels taken into consideration,in order to obtain the average value of L p :( )1n∑L p = 10 log 10 L pi10(3.26)nExample Problem 5Determine the average sound pressure level L p for a series of measurements takenat different times: 96, 88, 94, 102, and 90 dB.SolutionUsing Equation (3.26) we write[ ]1L p = 10 log5 (1096/10 + 10 88/10 + 10 94/10 + 10 102/10 + 10 90/10 )= 97 dBn=13.13 Weighting Curves and Associated Sound LevelsHuman perception of loudness depends on the frequency of a sound. A noisehaving most of its energy concentrated in the middle of the audio spectrum (e.g.,in the region of 1 kHz) is perceived as being louder than noise of equal energybut concentrated either in the low-frequency region (say, 40 Hz) or in the highfrequencyregion (near 15 kHz). This frequency effect becomes more apparent withsoft sounds than is the case with loud sounds, which provides the raison d’êtrefor the presence of a loudness control on some audio amplifiers. This controlsupplies a loudness contour at low volumes, which applies greater amplificationto the high- and low-frequency contents of the program material relative to themiddle-frequency components.Frequency weighting takes typical human hearing response into account whenthe loudness generated by all of the audible frequency components present isto be represented by a single value. Rather than describing the sound level ineach frequency band, we can use the A-weighted sound level to report the overallloudness. The A-weighted sound level is obtained from the conversion chartof Table 3.3, which also lists the B and C weightings in 1/3-octave bands.A-weighting is almost exclusively used in measurements that entail human responseto noise. Sound level that is measured with A-weighting is reported interms of dB(A) or simply dBA rather than the generic decibel dB. Similarly,B-weighted and C-weighted measurements are designated as dB(B) (rarely used)and dB(C), respectively. In the conversion table, it will be noted that all of the