Environmental Engineering Reference
In-Depth Information
One of the major problems with attempting to protect an inhabitant's hearing acu-
ity is the tendency of many people to ignore the dangers of noise. Because hearing
loss, like cancer, is insidious (you do not always know that you have cancer and you
do not always know that you have lost hearing), it's easy to ignore. It sort of sneaks
up on you slowly and is not apparent (in many cases) until after the damage is done.
Alarmingly, hearing loss from noise exposure has been well documented since the
18th century; yet, since the advent of the industrial revolution, the number of exposed
people has greatly increased.
Determining Noise Levels
The unit of measurement for sound is the decibel. Decibels are the preferred unit
for measuring sound; the name is derived from the word
bel
, a unit of measure in
electrical communications engineering. The decibel is a dimensionless unit used
to express the logarithm of the ratio of a measured quantity to a reference quan-
tity. Determination of noise exposure is accomplished by conducting a noise-level
survey of the site or area of concern. 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.
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
sets 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 a 90-dBA threshold gate, or a dosimeter can be used that has
both capabilities, whichever is appropriate for evaluation.
Sound Level Meter
When conducting a noise-level survey, the operator 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 1/10 of a bel and is the minimum
difference in loudness that is usually perceptible. The sound level meter consists
of a microphone, an amplifier, and an indicating meter, which responds to noise
in the audible frequency range of about 20 to 20,000 Hz. Sound level meters usu-
ally 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.
When conducting a routine sound level survey, using the A-weighted network
(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 noises that indicates the injurious effects such noise has on the
human ear (i.e., gives a frequency response similar to that of the human ear at rela-
tively low sound pressure levels). With an approved and freshly calibrated (always
calibrate test equipment prior to use) sound level meter in hand, the user is ready to
