Читать книгу Engineering Acoustics - Malcolm J. Crocker - Страница 162

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 the lowest level for which no increase in loudness occurs.

(Source: From Ref. [40]; used with permission.)

Figure 4.17 Dependence of loudness level L_N (left ordinate) on duration T_i of 1‐kHz tone impulses of constant sound pressure level compared with measurements of A‐weighted sound pressure level L_A (right ordinate) using the time constants “impulse,” “fast,” or “slow.” [17].

Another problem with A‐weighting is that it does not allow for the fact that loudness increases with the bandwidth of the noise and also with the duration of the noise event for very short impulsive‐type sounds of duration less than about 200 ms. The concept of the critical band is of fundamental importance in psychoacoustics. It is of concern in studies of loudness, pitch, hearing thresholds, annoyance, speech intelligibility, masking, and fatigue caused by noise, phase perception, and even the pleasantness of music.

Figure 4.16 shows the loudness level of bands of filtered white noise centered at 1000 Hz as a function of bandwidth for the different constant sound pressure levels shown on each curve. The curves were obtained by a matching procedure in which listeners equated the loudness of a 1000‐Hz pure tone with bands of noise of increasing bandwidth. The level at which the pure tone was judged to be equal in loudness to the band of noise is shown as the ordinate. Thus the curves do not represent equal loudness contours, but rather they show how the loudness of the band of noise centered at 1000 Hz changes as a function of bandwidth. The loudness of a sound does not change until its bandwidth exceeds the so‐called critical bandwidth. The critical bandwidth at 1000 Hz is about 160 Hz. (Notice that, except for sounds of very low level of about 20 phons, for which loudness is almost independent of bandwidth, the critical bandwidth is almost independent of level and that the slopes of the loudness curves are very similar for sounds of different levels.) Critical bands are discussed further in Section 4.3.6 of this chapter.

The solid line in Figure 4.17 shows that sounds of very short duration are judged to be very quiet and to become louder as their duration is increased. However, once the duration has reached about 100–200 ms, then the loudness level reaches an asymptotic value. Also shown by broken lines in Figure 4.17 are A‐weighted sound pressure levels recorded by a sound level meter using the “impulse,” “fast,” and “slow” settings. It is observed that the A‐weighted sound pressure level measured by the fast setting on the sound level meter is closest of the three settings to the loudness level of the sounds.

Further methods of rating loudness, noisiness, and annoyance of noise are discussed in Chapter 6.