Casella CEL CEL-181 User Manual
Page 3
Casella USA
17 Old Nashua Road #15
Amherst, NH 03031-2839
1
The Risk of Industrial Deafness
It has been known for many years that people
employed in noisy occupations invariably
suffered from deafness. Only in recent years,
however, has the full extent of the hazard been
appreciated with the realization that the effects
of excessive noise exposure are non-reversible
but preventable.
The efficiency of the human ear, along with the
other body functions, naturally decreases with
advancing years. However, this effect known as
presbycusis would not, in general, leave an
individual with a 'social handicap' in later life. It
would not cause any problems with everyday
conversation or limit their ability to work or enjoy
a full social life. In addition to this natural
deterioration, however, will be added other
losses in hearing acuity resulting from disease,
accidents or excessive noise exposure, which
could well result in a total degree of deafness
that would present a considerable social
handicap quite early in life.
Within the ear is a very clever, yet delicate,
system that enables us to hear. A diaphragm
(the tympanic membrane) converts the air
pressure fluctuations that constitute sound into a
mechanical motion. This motion is then
transmitted through the middle ear, with some
mechanical advantage by a system of small
bones (the ossicles), to the inner ear, Here, the
mechanical movement is converted into nerve
impulses by the cochlea which are then
transmitted to the brain, for interpretation, by the
eighth nerve. Sudden very high intensity sounds,
such as those associated with explosions, can
cause a rupture of the tympanic membrane and,
in severe cases, a disruption of the auditory
ossicles. Such dramatic effects are not,
however, as serious as would first appear. Many
eardrums wiII, in fact, heal themselves and,
thanks to modern medical and surgical methods,
many types of malfunction of the outer and
middle ear can be rectified. Far more serious
damage will result from prolonged exposure to
much lower levels. Continued exposure to even
only moderately intense sounds will result in a
permanent reduction in the ability of the cochlea
to produce nerve impulses for transmission to
the brain and, because of its nature and
location, treatment for a noise-damaged cochlea
is just not possible. As nerve signals to the brain
are not being generated, amplification of the
sounds by a hearing aid would be of little benefit
in these cases. The only 'cure' for industrial
deafness is, therefore, prevention-Hearing
Conservation.
In order to fully understand the subject an
extensive research campaign was directed
towards establishing exactly how much noise
caused a given degree of deafness. Firstly, it
was established that certain frequencies were
more dangerous than others. To accommodate
this fact a special frequency filter has been
derived for use when the hazard potential of
sound is being assessed, The response of this
filter gives added weight to the more hazardous
frequencies in the range 1-5kHz and
proportionally derates those outside this band
according to a carefully defined formula. This 'A
weighted' response, as it is known is specified in
all the international and national standards
relating to the assessment of deafness risk that
are known to date. 'A weighting' is, therefore,
built into all CEL personal noise dose meters
and it is not necessary to make any further
separate measurements of frequency when
using these instruments to measure auditory
hazards.
Figure 1
The other parameter of risk was established as
the total noise immission which is proportional to
the measured noise level in dB (A) and the
duration of exposure, Results of large scale
investigation of noise exposed workers that
compared their total noise immission with the
degree of deafness acquired has shown that if
noise immission is contained to around 90dB(A)
for an eight hour working day they should not
acquire a degree of deafness that would
constitute a social handicap after a working
lifetime. As the noise level is increased so the
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