There are many terms common to hearing loss prevention that are used in this document,
books, and journal articles. The reader should find the definitions in this section to be
helpful. These definitions, while accurate, were written in as non-technical a fashion as
||A single incident which produces an abrupt hearing loss. Welding
sparks (to the eardrum), blows to the head, and blast noise are examples of events capable
of producing acoustic trauma. (See also ototoxic and ototraumatic.)
||The sound level which when reached or exceeded necessitates
implementation of activities to reduce the risk of noise-induced hearing loss. OSHA
currently uses an 8-hour time weighted average of 85 dBA as the criterion for implementing
an effective hearing conservation program.
|ATTENUATION: Real Ear Attenuation at Threshold (REAT)
||A standardized procedure for conducting psychoacoustic tests on
human subjects designed to measure sound protection features of hearing protective
devices. Typically, these measures are obtained in a calibrated sound field, and represent
the difference between subjects' hearing thresholds when wearing a hearing protector vs
when not wearing the protector.
||Estimated sound protection provided by hearing protective devices
as worn in "real-world" environments.
||A valid audiogram against which subsequent audiograms are compared
to determine if hearing thresholds have changed. The baseline audiogram is preceded by a
quiet period so as to obtain the best estimate of the person's hearing at that time.
||Noise of a constant level as measured over at least one second
using the "slow" setting on a sound level meter. Note, that a noise which is
intermittent, e.g., on for over a second and then off for a period would be both variable and
||Efforts, usually by management, to limit workers' noise exposure by
modifying workers' schedule or location, or by modifying the operating schedule of noisy
||Any use of engineering methods to reduce or control the sound level
of a noise source by modifying or replacing equipment, making any physical changes at the
noise source or along the transmission path (with the exception of hearing protectors).
||The unit used to express the intensity of sound. The decibel was
named after Alexander Graham Bell. The decibel scale is a logarithmic scale in which 0 dB
approximates the threshold of hearing in the mid frequencies for young adults and in which
the threshold of discomfort is between 85 and 95 dB SPL and the threshold for pain is
between 120 and 140 dB SPL.
||When applied to noise, refers to an instrument that measures sound
levels over a specified interval, stores the measures, and 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 parameters such as peak level, equivalent sound
level, sound exposure level, etc.
||The relationship between sound level and sound duration based upon
a 3 dB exchange rate, i.e., 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,
||The study or measurement of how work is done as it relates to
worker fatigue, discomfort or injury.
||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.
Sometimes, 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. NIOSH recommends a
3-dB exchange rate. Note that the equal-energy rule is based on a 3 dB exchange rate.
||Any sound for which any combination of frequency, intensity, or
duration is capable of causing permanent hearing loss in a specified population.
|HAZARDOUS TASK INVENTORY
||A concept based on using work tasks as the central organizing
principle for collecting descriptive information on a given work hazard. It consists of a
list(s) of specific tasks linked to a database containing the prominent characteristics
relevant to the hazard(s) of interest which are associated with each task.
|HEARING DAMAGE RISK CRITERIA
||A standard which defines the percentage of a given population
expected to incur a specified hearing loss as a function of exposure to a given noise
||A specified amount of permanent hearing loss usually averaged
across several frequencies which 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 impaired hearing.
||Hearing loss is often characterized by the area of the auditory
system responsible 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 - which includes the ossicles) the resulting hearing loss
is referred to as a conductive loss. When an injury or medical condition 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, a welder's spark which damaged
the ear drum would cause a conductive hearing loss. Because noise can damage the tiny hair
cells located in the cochlea, it causes a sensorineural hearing loss.
|HEARING LOSS PREVENTION PROGRAM AUDIT
||An assessment performed prior to putting a hearing loss prevention
program into place or before changing an existing program. The audit should be a top-down
analysis of the strengths and weaknesses of each aspect of the program.
|HTL (HEARING THRESHOLD LEVEL)
||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
||The unit 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 4,000 Hz.
||Used to generally characterize impact or impulse noise which is
typified by a sound which 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 be repetitive, or may be a
single event (as with a sonic boom). Note: if impulses occur in very rapid
succession (such as with some jack hammers), the noise would not be described as
||The subjective attribute of a sound by which it would be
characterized along a continuum from 'soft' to 'loud'. Although this as a subjective
attribute, it depends primarily upon sound pressure level, and to a lessor extent, the
frequency characteristics and duration of the sound.
|MATERIAL HEARING IMPAIRMENT
||As defined by OSHA, a material hearing impairment is an average
hearing threshold level of 25 dB HTL at the frequencies of 1000, 2000, and 3000 Hz.
||Any unwanted sound.
||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-INDUCED HEARING LOSS
||A sensorineural hearing loss that is attributed to noise and for
which no other etiology can be determined.
|NRR (NOISE REDUCTION RATING)
||The NRR is a single-number rating method which attempts to describe
a hearing protector based on how much the overall noise level is reduced by the hearing
protector. When estimating A-weighted noise exposures, it is important to remember to first
subtract 7 dB from the NRR and then subtract the remainder from the A-weighted noise
level. The NRR theoretically provides an estimate of the protection that should be met or
exceeded by 98% of the wearers of a given device. In practice, this does not prove to be
the case, so a variety of methods for "de-rating " the NRR have been discussed.
||A term typically associated with the sensorineural hearing loss
resulting from therapeutic administration of certain prescription drugs.
||A broader term than ototoxic. As used in hearing loss prevention,
refers to any agent (e.g., noise, drugs, or industrial chemicals) which has the potential
to cause permanent hearing loss subsequent to acute or prolonged exposure. (See also
|PERMISSIBLE EXPOSURE LIMIT
||OSHA-permissible limits; presently 90 dBA. A time-weighted average
exposure that must not be exceeded during any 8-hour work shift of a 40-hour work week
||The gradual increase in hearing loss that is attributable to the
effects of aging, and not related to medical causes or noise exposure.
|SENSORI-NEURAL HEARING LOSS
||A hearing loss resulting from damage to the inner ear (from any
||A hearing loss related to non-occupational noise exposure.
|SOUND LEVEL METER (SLM)
||A device which measures sound and provides a readout of the
resulting measurement. Some provide only A-weighted measurements, others provide A- and
C-weighted measurements, and some can provide weighted, linear, and octave (or narrower)
band measurements. Some SLMs are also capable of providing time-integrated measurements.
|SPL (SOUND PRESSURE LEVEL)
||A measure of the ratio of the pressure of a sound wave relative to
a reference sound pressure. Sound pressure level in decibels is typically referenced to 20
ÁPa. When used alone, (e.g., 90 dB SPL) a given decibel level implies an unweighted sound
||Standard Threshold Shift: OSHA uses the term to describe a
change in hearing threshold relative to the baseline audiogram of an average of 10 dB or
more at 2000, 3000 and 4000 Hz in either ear. Used by OSHA to trigger additional
audiometric testing and related follow up.OR
Threshold Shift: NIOSH uses this term to describe a change of 15 dB or more at any
frequency, 500 through 6000 Hz, from baseline levels that is present on an immediate
retest in the same ear and at the same frequency. NIOSH recommends a confirmation
audiogram within 30 days with the confirmation audiogram preceded by a quiet period of at
least 14 hours.
|TLV (THRESHOLD LIMIT VALUE)
||A guideline provided by the American Conference of Governmental
Industrial Hygienists to denote the exposure, which when reached or exceeded, may be
hazardous. For noise the TLV is 85 dBA and the exchange rate is 3 dB.
|TWA (TIME WEIGHTED AVERAGE)
||A value, expressed in dBA, which is computed so that the resulting
average would be equivalent to an exposure resulting from a constant noise level over an
||Audiometric monitoring programs will encounter two types of changes
in hearing sensitivity, i.e. threshold shifts: permanent threshold shift (PTS) and
temporary threshold shift (TTS). As the names imply, any change in hearing
sensitivity which is persistent is considered a PTS. Persistence may be assumed if the
change is observed on a 30-day follow-up exam. Exposure to loud noise may cause a
temporary worsening in hearing sensitivity (i.e., a TTS) that may persist for 14 hours (or
even longer in cases where the exposure duration exceeded 12 to 16 hours). Hearing health
professionals need to recognize that not all threshold shifts represent decreased
sensitivity, and not all temporary or permanent threshold shifts are due to noise
exposure. When a permanent threshold shift can be attributable to noise exposure, it may
be referred to as a noise-induced permanent threshold shift (NIPTS).
||Two weighting curves are commonly applied to measures of sound
levels to account for the way the ear perceives the "loudness" of sounds.
A-weighting: A measurement scale that approximates the
"loudness" of tones relative to a 40 dB SPL 1000 Hz reference tone. A-weighting
has the added advantage of being correlated with annoyance measures and is most responsive
to the mid frequencies, 500 to 4000 Hz..
C-weighting: A measurement scale that approximates the
"loudness" of tones relative to a 90 dB SPL 1000 Hz reference tone. C-weighting
has the added advantage of providing a relatively "flat" measurement scale which
includes very low frequencies.