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with communication, disrupted concentration, and most importantly, varying degrees of hearing
loss. Exposure to high noise levels also adversely affects job performance and increases accident
rates. One of the major problems with attempting to protect workers' hearing acuity is the tendency
of many workers to ignore the dangers of noise. Because hearing loss, like cancer, is insidious, it's
easy to ignore. It sneaks up slowly and is not apparent (in many cases) until after the damage is done.
Alarmingly, hearing loss from occupational noise exposure has been well documented since the
eighteenth century, yet since the advent of the industrial revolution, the number of exposed workers
has greatly increased (Mansdorf, 1993). However, today the picture of hearing loss is not as bleak
as it has been in the past, as a direct result of OSHA's requirements. Now that noise exposure must
be controlled in all industrial environments, that well-written and well-managed hearing conserva-
tion programs must be put in place, and that employee awareness must be raised to the dangers of
exposure to excessive levels of noise, job-related hearing loss is coming under control.
15.7.3 d etermining W orKplaCe n oise l evels
The unit of measurement for sound is the decibel. Decibels are the preferred unit for measuring
sound, derived from the bel, a unit of measure in electrical communications engineering. The deci-
bel is a dimensionless unit used to express the logarithm of the ratio of a measured quantity to a
reference quantity. In regard to noise control in the workplace, the safety engineer's primary con-
cern is first to determine if any “noise-makers” in the facility exceed the OSHA limits for worker
exposure—exactly which machines or processes produce noise at unacceptable levels. Making this
determination is accomplished by conducting a noise level survey of the plant or facility. Sound
measuring instruments are used to make this determination. These include noise dosimeters, sound
level meters, and octave-band analyzers. The uses and limitations of each kind of instrument are
discussed below.
15.7.3.1 Noise Dosimeter
The noise dosimeters used by OSHA meet the American National Standards Institute (ANSI)
Standard S1.25-1978, “Specifications for Personal Noise Dosimeter,” which set performance and
accuracy tolerances. For OSHA use, the dosimeter must have a 5-dB exchange rate, use a 90-dBA
criterion level, be set at slow response, and use either an 80-dBA or 90-dBA threshold gate, or a
dosimeter that has both capabilities, whichever is appropriate for evaluation.
15.7.3.2 Sound Level Meter (SLM)
When conducting the noise level survey, the industrial hygienist or survey technician should use an
ANSI-approved sound level meter (SLM)—a device used most commonly to measure sound pres-
sure. The SLM measures in decibels. One decibel is one-tenth of a bel and is the minimum differ-
ence in loudness that is usually perceptible. The SLM consists of a microphone, an amplifier, and
an indicating meter that responds to noise in the audible frequency range of about 20 to 20,000 Hz.
Sound level meters usually contain weighting networks designated “A”, “B”, or “C”. Some meters
have only one weighting network; others are equipped with all three. The A-network approximates
the equal loudness curves at low sound pressure levels, the B-network is used for medium sound
pressure levels, and the C-network is used for high levels.
In conducting a routine workplace sound level survey, using the A-weighted network (referenced
dBA) in the assessment of the overall noise hazard has become common practice. The A-weighted
network is the preferred choice because it is thought to provide a rating of industrial noise that indi-
cates the injurious effects such noise has on the human ear (gives a frequency response similar to
that of the human ear at relatively low sound pressure levels).
With an approved and freshly calibrated (always calibrate test equipment prior to use) sound level
meter in hand, the environmental professional is ready to begin the sound level survey. In doing so,
the industrial hygienist is primarily interested in answering the following questions:
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