Page 99 - Master Handbook of Acoustics

where C =
duration of exposure, hours
T =
noise exposure limit, hours

For example, when a worker is exposed to a noise level of 100 dBA for 1.5 hours and a 95-dBA

level for 0.5 hour, the noise dose is D = 1.5/2 + 0.5/4 = 0.90. Thus, the worker has been exposed to
90% of the maximum permissible noise.
The time-weighted and A-weighted sound level in dBA, sometimes referred to as TWA, may also
be computed as TWA = 105 − 16.6 log(T). A subsequent hearing conservation measure calls for a
TWA given as 100 − 16.6 log (T).

Other noise exposure regulations have been devised by the Environmental Protection Agency,
Department of Housing and Urban Development, Workman’s Compensation, and other agencies and
nongovernment groups. These regulations are subject to frequent change. Professional audio engineers
operating with high monitoring levels are risking irreparable injury to their hearing; in most cases,
their work is not subject to occupational noise-protection laws.

Dangerous noise exposure is more than an occupational problem; it is also a recreational problem.
An individual might work all day in a high-noise environment, then enjoy motorcycle or automobile
racing, listen to music playback at high level, or spend hours in a night club. As high-frequency loss
creeps in, volume controls are turned up to compensate, and the rate of deterioration is accelerated.

The audiogram is an important tool in the conservation of hearing. Comparing a current audiogram
with earlier ones establishes the trend; if downward, steps can be taken to address the causes. The
audiogram of Fig. 4-20 is that of a mixing engineer in a recording studio and shows serious hearing
loss. This hearing loss, centered on 4 kHz, may be the accumulation of many years of listening to
high-level sounds in the control room.

FIGURE 4-20 Audiograms showing serious hearing loss centered on 4 kHz, perhaps resulting from