Environmental Engineering Reference
In-Depth Information
Hertz (Hz) —The unit of measurement for audio frequencies. The frequency
range for human hearing lies between 20 Hz and approximately 20,000 Hz.
The sensitivity of the human ear drops off sharply below about 500 Hz and
above 4000 Hz.
Impulsive noise —A term used to generally characterize impact or impulse
noise, which is typified by a sound that rapidly rises to a sharp peak and
then quickly fades. The sound may or may not have a “ringing” quality
(such as a striking a hammer on a metal plate or a gunshot in a reverberant
room). Impulsive noise may be repetitive, or it may be a single event (as with
a sonic boom). Note: If impulses occurring in very rapid succession (such as
with some jack hammers), the noise would not be described as impulsive.
Loudness —The subjective attribute of a sound by which it would be character-
ized along a continuum from “soft” to “loud.” Although this is a subjective
attribute, it depends primarily upon sound pressure level and, to a lesser
extent, the frequency characteristics and duration of the sound.
Material hearing impairment —As defined by OSHA, an average hearing
threshold level of 25 dB HTL at the frequencies of 1000, 2000, and 3000
Hz.
Mechanical noise —Noise generated as a result of moving parts or components
such as gears, shafts, or reciprocating parts.
Medical pathology— A disorder or disease; with regard to noise, a condition or
disease affecting the ear, which a physician specialist should treat.
Noise —Any unwanted sound.
Noise dose —The noise exposure expressed as a percentage of the allowable
daily exposure. For OSHA, a 100% dose would equal an 8-hour exposure
to a continuous 90-dBA noise. A 50% dose would equal an 8-hour exposure
to an 85-dBA noise or a 4-hour exposure to a 90-dBA noise. If 85 dBA is
the maximum permissible level, then an 8-hour exposure to a continuous
85-dBA noise would equal a 100% dose. If a 3-dB exchange rate is used
in conjunction with an 85-dBA maximum permissible level, a 50% dose
would equal a 2-hour exposure to 88 dBA or an 8-hour exposure to 82 dBA.
Noise dosimeter— An instrument that integrates a function of sound pressure
over a period of time to directly indicate a noise dose.
Noise hazard area— Any area where noise levels are equal to or exceed 85
dBA. OSHA requires employers to designate work areas as a “noise hazard
area,” post warning signs, and warn employees when work practices exceed
90 dBA. Hearing protection must be worn whenever 90 dBA is reached or
exceeded.
Noise hazard work practice— Performing or observing work where 90 dBA is
equaled or exceeded. Some work practices, however, will be specified as a
“rule of thumb.” Whenever attempting to hold a normal conversation with
someone who is 1 foot away and shouting must be employed to be heard,
one can assume that a 90-dBA noise level or greater exists and hearing
protection is required. Typical examples of work practices where hearing
protection is required are jackhammering, heavy grinding, heavy equip-
ment operations, and similar activities.
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