Читать книгу Engineering Acoustics - Malcolm J. Crocker - Страница 160

Figure 4.6 in this chapter shows that the ear is most sensitive to sounds in the mid‐frequency range around 1000–4000 Hz. It has a particularly poor response to sound at low frequency. It became apparent to scientists in the 1930s that electrical filters could be designed and constructed with a frequency response approximately equal to the inverse of these equal loudness curves. Thus A‐, B‐, and C‐weighting filters were constructed to approximate the inverse of the 40‐, 70‐, and 90‐phon contours (i.e. for low‐level, moderate, and intense sounds), respectively (see Figure 4.6). In principle, then, these filters, if placed between the microphone and the meter display of an instrument such as a sound level meter, should give some indication of the loudness of a sound (but for pure tones only).

The 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].)