Engineering Acoustics. Malcolm J. Crocker

levels measured with the use of the filters shown in Figure 4.14 are commonly called the A‐, B‐, and C‐weighted sound levels. The terminology A‐, B‐, and C‐weighted sound pressure levels is preferred by ISO to reduce any confusion with sound power level and will be used wherever possible throughout this book. The A‐weighting filter has been much more widely used than either the B‐ or C‐weighting filter, and the A‐weighted sound pressure level measured with it is still simply termed by ANSI as the sound level or noise level (unless the use of some other filter is specified). Several other weightings have also been proposed in the past [13]. However, because it is simple, giving a single number, and it can be measured with a low‐cost sound level meter, the A‐weighted sound pressure level has been used widely to give an estimate of the loudness of noise sources such as vehicles, even though these produce moderate to intense noise. Beranek and Ver have reviewed the use of the A‐weighted sound pressure level as an approximate measure of loudness level [36].

Figure 4.14 A‐, B‐, and C‐weighting filter characteristics used with sound level meters.

The A‐weighted sound pressure levels are often used to gain some approximate measure of the loudness levels of broadband sounds and even of the acceptability of the noise. Figure 4.15 shows that there is reasonable correlation between the subjective response of people to vehicle noise and the A‐weighted sound pressure levels measured of the vehicle noise. The A‐weighted sound pressure level forms the basis of many other descriptors for determining human response to noise described later in Chapter 6. The A‐weighted sound pressure level descriptor is also used as a limit for new vehicles (Chapter 14) and noise levels in buildings (Chapter 12) in several countries. Although the A‐weighting filter was originally intended for use with low‐level sounds of about 40 dB, it is now commonly used to rate high‐level noise such as in industry where A‐weighted sound pressure levels may exceed 90 dB. At such high levels the A‐weighted sound pressure level and the loudness level are normally in disagreement.

Figure 4.15 Relation between subjective response and A‐weighted sound pressure level for diesel engine trucks undergoing an acceleration test: ●, values measured in 1960, ○, values measured in 1968.

      (Source: Adapted from Refs. [37–39].)

      Example 4.5

      The factory noise spectrum (given in Table 4.1), was calculated to have a loudness level of 99 phon (see Example 4.2). Calculate the approximate A‐weighted sound pressure level from the octave band levels given in Table 4.1.

      Solution

Since we do not have the directly measured A‐weighted sound level, we can calculate this approximately using Figure 4.14. The A‐weighting corrections at the octave band center frequencies have been read off Figure 4.14 and entered in Column 3 of Table 4.2. The so‐called A‐weighted octave band sound pressure levels have been calculated in Column 4 and these values have been combined to give the A‐weighted sound pressure level of 89.8, i.e. 90 dB. Note this A‐weighted sound pressure level is dominated by the A‐weighted band levels in the 500, 1000, and 2000 Hz octave bands. These three band levels combine to give 89.5 dB. Similar calculations of the C‐weighted and linear (nonfiltered) sound pressure levels give 91.9 and 92.0 dB, respectively. Thus we see that the A‐weighted sound pressure level is 9 dB below the level in phons and no closer than the linear unweighted sound pressure level of 92 dB. A‐weighted levels should not be used to calculate the loudness level unless the noise is a pure tone – then a good loudness level estimate can be made using the A, B, or C filters (depending on the noise level).

Table 4.2 Combination of octave‐band sound pressure levels of factory noise to give the A‐weighted sound pressure level.

Octave band center frequency, Hz	Octave band level, dB	A‐weighting correction, dB	A‐weighted octave‐band levels, dB
31.5	75	−42	33
63	79	−28	51
125	82	−18	64
250	85	−9.0	76
500	85	−3.0	82
1000	87	0	87
2000	82	+1.5	83.5
4000	75	+0.5	75.5
8000	68	−2.0	66

Figure 4.16 Loudness level in phons of a band of filtered white noise centered at 1000 Hz as a function of its bandwidth. The overall sound pressure level of each band of noise was held constant as its bandwidth was increased, and this level is shown on each curve. The dashed line indicates that the bandwidth at which the loudness starts to increase is about the same at all of the levels tested, except for