Casella CEL CEL-181 User Manual
Page 4
Casella USA
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exposure duration must be reduced in order to
contain the total noise immission or ‘noise dose'.
One of the main variables found between
different noise control regulations is the
relationship between noise level and exposure
time. The International Standards Organization
followed by most European countries favor an
'equal energy' concept where the exposure
duration is halved for each 3dB increment in
noise level (q = 3) whilst in North America the
'exchange rate' calls for a halving of exposure
for each 5dB increment in noise level (q = 5). At
other times regulations calling for q = 4 and q =
6 have been noted.
The problem that arises in actual industrial
situations is that the noise level is never a
constant level to compare against the criteria but
it is continually changing as machines come on
and off load, pressures are vented and
processes move through various phases. It is
necessary, therefore, to continually monitor the
noise climate and process the results such that
the total noise is accumulated in the correct
manner in order to provide a noise dose figure
for comparison against the damage risk criteria.
Therefore, noise dose is proportional to the time
integral of the instantaneous level that is
amplitude weighted and referred to a criterion
level. A constant factor is added to scale the
results in terms of exposure and to provide an
answer of 100 for one daily dose enabling
results to be expressed as a percentage of
permitted exposure.
The basic criteria discussed above are accepted
in most countries of the world; however, as an
added complication some have advanced
further than others with their noise control
programmes and have been able to reduce the
criterion in order to give a better safety margin in
respect of the more noise susceptible subjects.
In order to accommodate these requirements all
CEL personal noise dose meters are available
having their criterion levels set accordingly;
these calibration settings are also manufacturer
preset. The various settings are indicated by a
letter code on the serial number plate and a
complete schedule is set out below:
Figure 2