Subjective Responses

Except when it is a heeded warning of danger, a noise which excites a fear reflex is psychologically harmful; noises which prevent rest or sleep are a detriment to health and well-being. These reactions are psychological, yet physiological damage can result from them. Annoyance and irritation—less specific reactions—impair the quality of life.

Noise levels above the threshold of discomfort—louder than a thunderclap—are disturbing; so are any loud sounds which are not expected; so is any increase in sound level which is more rapid than a critical rate, even if it is expected. Some noises have a higher annoyance factor than others; among the most objectionable is the jet aircraft. Sound pressure data do not adequately describe these noises.

The phon represents loudness rather than sound pressure. It is an empirical unit; the loudness level of a noise in phons is numerically equal to the sound pressure level in decibels of a 1000 Hz tone which sounds equally loud. Thus, the phon is a subjective unit.

The sone is another unit of loudness using a different scale. A loudness level of 40 phons represents 1 sone, and each 10-phons increase doubles the number of sones. The change in sensation of loudness is better represented by

FIG. 6.3.5 Comparison of objective and subjective noise scales.

sones than by phons.

For the combined high frequencies and high noise levels produced by jet aircraft, other criteria have been developed; one is the unit of noisiness called the noy. This unit is used to express the perceived noisiness (PN) or annoyance in dBs, as PNdB. Such PNdB values can be conveniently approximated with a standard sound-level meter.

In Figure 6.3.5 the relative values for sound levels in objective and subjective units are compared; the chart is for comparison, not conversion.

These units deal with continuous noise, but fluctuating or intermittent noise is more annoying. To deal with these characteristics (for transportation noise), a procedure can calculate a "noise pollution level." A composite noise rating also describes the noise environment of a community over twenty-four hours of the day. Like the others, it accounts for loudness and frequency characteristics, as well as fluctuations and frequency and impulsive noises, in its calculations.

One of the most disturbing elements of noise within buildings is its impairment of privacy; voices or other noises penetrating a wall, door, or window are especially annoying. This reaction is psychological; the annoyance is in response to an intrusion, which seems impertinent.

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