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2.4.2 Physiological and subjective acoustics
The sound pressure level, in decibels, of a sound or noise is 20
log10 (p1/p0) where
p1 is the root-mean-square (r.m.s.) sound pressure and
p0 is the reference sound pressure of 20 μPa (see ISO
The subjective loudness of a sound as a function
of frequency, as perceived by the human ear/brain system, varies with the level
of the sound. The figure overleaf is taken from ISO 226:1987 and shows
equal-loudness level contours measured for young listeners with normal hearing
for sinusoidal plane sound waves coming from directly in front of the listener;
the sound pressure level is the level in the undisturbed wave.
Sound at high levels is capable of causing temporary and permanent damage to hearing. Susceptibility to damage differs between individuals, but it can be noted that the European Community Directive (1986) on workplace noise requires that preventative action be taken if a worker is exposed to an A-weighted sound pressure level (averaged over an eight-hour day) of more than 85 dB(A).
European Community (1986) Directive 86/188/EEC, OJ No L 137/28-34 of
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