Objective And Subjective Values

Sound pressure and sound power are objective values; they show physical magnitudes as measured by instruments. However, while almost anyone subjected to noise exposure beyond recognized levels experiences some hearing impairment, some psychophysiological reactions (annoyance in particular) vary with the individual. They are subjective values; they must be determined in terms of human reactions.

Loudness is a subjective magnitude. Although it depends primarily on signal intensity, or sound pressure, frequency and wave form are also important. (See Figure 6.4.1) Through listening tests, a unit of loudness level has been established; it is the phon. The loudness level of a sound in phons is numerically equal to that SPL in dBs of a 1000 Hz continuous sine wave sound, which sounds equally loud. For most common sounds, values in phons do not differ much from SPLs in dBs.

To classify the loudness of noises on a numerical scale, the sone was devised as the unit of loudness. It is related to the loudness level in phons in this way: a noise of 40 phons loudness level has a loudness of 1 sone, and for each increase in level of 10 phons, the value in sones is doubled. The sone has the advantage that a loudness of 64 sones sounds about twice as loud as 32 sones; it provides a better impression of relative loudness than the dB.

Sone values are not measured directly, but they can be obtained by computation. Two general methods are accepted—one by Stevens and one by Zwicker. Their values differ slightly, but either seems satisfactory for most uses. (The Zwicker method is built into a commercial instrument; it involves separating the sound into a group of narrow-band components, then combining the magnitudes of these components mathematically.) Table 6.4.1 compares some values of sound pressure, loudness, and noisiness.

Jet airplane noise, with its broad band but predominantly high-frequency spectrum, is one of the most annoying. It is also one of the loudest continuous noises. Values in sones or phons are not adequate to describe noises of this character, and the concept of perceived noise has developed. Noisiness in this system is usually expressed in dBs, as PNdB. Perceived noise values, like values in sones, are not directly measurable, but are computed from measured data. For many uses, analysts can make acceptable approximations by taking measurements with a stan

FIG. 6.4.1 Equal loudness curves. These curves display the varying sensitivity of the normal ear with both frequency and average level. The interrupted extensions at high frequencies show the typical loss in hearing acuity with age (Reprinted partially from ISO recommendation 226).

dard sound level meter, using D weighting, and adding 7 to the observed values. For jet plane noises above 90 dBA, analysts can secure useful approximate values by taking sound-level readings using A weighting, and adding 12 to the indicated value. Since not all sound-level meters have D weighting, this latter method is often used.

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