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

230 10. Physiology of Hearing and PsychoacousticsTable 10.5. Calculation for the Articulation Index in the Sample Problem.Center Speech Weighting Noise SL − N = WF ×frequency (Hz) level (SL) (+12 dB) factor (WF) (N) DIFF DIFF250 72 18 47 25 450500 73 50 47 26 13001000 78 75 47 30 3 22502000 63 107 47 16 17124000 58 83 47 11 913Example Problem 1: Calculation of AILet us compute the articulation index of a male voice speaking at a normal level1 m from the listener in the presence of pink noise that contributes 47 dB in eachoctave band.SolutionTable 10.4 is used for this calculation. The 47-dB octave-band noise level is subtractedfrom the values in the second column; and the difference (up to a maximumof 30 dB) is multiplied by the weighting factors of the third column. The resultingweighted contributions are added and divided by 10,000, yielding the articulationindex. Table 10.5 below gives details of the calculations, with the values given inthe fifth column produced by subtracting 47 dB from the speech level of the secondcolumn, and each of the values in the fifth column is multiplied by the weightingfactor of the third column to yield the figures listed in the sixth column.Articulation index (AI) = (450 + 1300 + 2250 + 1712 + 913)/10000 = 0.6625The articulation index is 0.6625, or 66.25%.10.6 Speech-Interference Level (SIL)Measurements to obtain data for articulation indexes require special laboratoryequipment for determination of S/N in a number of frequency bands. A simplerprocedure for estimating the effect of noise on verbal communication makes useof octave-band levels as measured in a typical noise survey. The parameter thatis called the speech-interference level, abbreviated SIL, can be obtained by computingthe arithmetic average of octave-band levels in the three octave bands of600–1200, 1200–2400, and 2400–4800 Hz. However, the current practice usesthe arithmetic level in the “preferred” octave bands with center frequencies at 500,1000, and 2000 Hz. Speech-interference level defined thusly is referred to as PSIL.3 The value of the following expression (Speech Level + 12 dB—Noise Level) must fall between0 and 30.