| Principles
of operation
The dosemeter is an
averaging instrument
which will respond to
varying noise levels
over a period of time
and report the result
as an average or Leq
level.
The simpler meters
display a reading at
the end of the
measurement period in
terms of percentage
dose. This would normally be related to the second action level of
90 dB(A) L EP,d
that applies in the
UK. Thus if an operator has spent 8 hours in an environment where
the Leq
over that time is 90
dB(A), the read-out on
his dosemeter will be
100%.
Dosemeters are
often supplied with a
slide-rule or nomogram
to facilitate
conversions such as
the calculation of
allowable exposure
times for various
sound pressure levels
and dosages.
HSE’e Noise
Guide 3 includes an
example of such a
nomogram, based on a
dose fraction f rather
than percentage does,
were f = 1.0
corresponds to a 100%
dose.
Simple meters will
also indicate whether
a peak value (above
120dB) has been noted,
but will not display
the actual value.
More sophisticated
instruments are
available which log
noise levels
continuously and
enable whatever
parameters are
required to be read or
calculated, such as
instantaneous sound
pressure levels,
percentage dosages or
peak values.
Fundamental to the
working of a dosemeter
is the equal energy
concept, ie., the
ability to equate X
minutes of exposure to
sound level A with Y
minutes of exposure to
sound level B to give
an overall level of
exposure.
It is important
to appreciate this,
especially when buying
equipment.
In the UK the
acceptable noise
exposure level for an
8 hour working day (L EP,d)
is 90 dB(A).
However, some
countries have an 85
dB(A) L EP,d
limit, so it is
important to be able
to set the dosemeter
according to the
criteria to be used if
it is to measure the
noise dose correctly. Similarly, it must include a facility to set the correct
doubling rate:
in the UK this
is 3 dB, ie., for
every 3 dB increase in
the sound level the
allowable exposure
period is halved.
Elsewhere rates
of 4 or 5 dB are used
(it is not universally
accepted that doubling
the sound energy
received necessarily
equates to doubling
the damage caused).
Most
dosemeters can be
adjusted to meet these
varying criteria,
either by the
manufacturers or in
some cases by the
user.
It is well
worth checking that
instruments are
correctly set to the
standards of the
country in which they
are sold in order to
avoid unexpected
results.
» Basic
Construction
» Principles
of Operation
» Dynamic
Range
» Calibration
» Use
of Dosemeter in the
Field
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