Environmental Engineering Reference
In-Depth Information
Dosimeter —When applied to noise, refers to an instrument that measures
sound levels over a specified interval; stores the measures; calculates the
sound as a function of sound level and sound duration; and describes the
results in terms of, dose, time-weighted average, and (perhaps) other param-
eters such as peak level, equivalent sound level, or sound exposure level.
Double hearing protection— A combination of both ear-plug and ear-muff
types of hearing protection devices is required for employees who have
demonstrated temporary threshold shift during audiometric examination
and for those who have been advised to wear double protection by a medi-
cal doctor in work areas that exceed 104 dBA.
Equal-energy rule —The relationship between sound level and sound duration
based on a 3-dB exchange rate; that is, the sound energy resulting from
doubling or halving a noise exposure's duration is equivalent to increasing
or decreasing the sound level by 3 dB, respectively.
Exchange rate —The relationship between intensity and dose. OSHA uses a
5-dB exchange rate; thus, if the intensity of an exposure increases by 5 dB,
the dose doubles. This is also referred to as the doubling rate . The U.S.
Navy uses a 4-dB exchange rate; the U.S. Army and Air Force use a 3-dB
exchange rate; and NIOSH recommends a 3-dB exchange rate. Note that
the equal-energy rule is based on a 3-dB exchange rate.
Frequency— Rate at which pressure oscillations are produced; measured in
hertz (Hz).
Hazardous noise —Any sound for which any combination of frequency, inten-
sity, or duration is capable of causing permanent hearing loss in a specified
population.
Hearing handicap —A specified amount of permanent hearing loss usually
averaged across several frequencies that negatively impacts employment
and/or social activities. Handicap is often related to an impaired ability to
communicate. The degree of handicap will also be related to whether the
hearing loss is in one or both ears, and whether the better ear has normal
or impartial hearing.
Hearing loss —Often characterized by the area of the auditory system respon-
sible for the loss; for example, when injury or a medical condition affects
the outer ear or middle ear (i.e., from the pinna, ear canal, and ear drum to
the cavity behind the ear drum, including the ossicles) the resulting hearing
loss is referred to as a conductive loss. When an injury or medical condi-
tion affects the inner ear or the auditory nerve that connects the inner ear
to the brain (i.e., the cochlea and the VIIIth cranial nerve) the resulting
hearing loss is referred to as a sensorineural loss. Thus, when a welder's
spark damages the ear drum that would be considered conductive hearing
loss. Because noise can damage the tiny hair cells located in the cochlea, it
causes a sensorineural hearing loss.
Hearing threshold level (HTL) —The hearing level, above a reference value, at
which a specified sound or tone is heard by an ear in a specified fraction of
the trials. Hearing threshold levels have been established so that 0 dB HTL
reflects the best hearing of a group of persons.
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