The most common instruments used
for measuring noise are the sound level meter (SLM), the integrating
sound level meter (ISLM), and the noise dosimeter. It is important
that you understand the calibration, operation and reading of the
instrument you use. The user’s manual provided by the instrument
manufacturer provides most of this information.
A sound level meter (SLM)
consists of a microphone, electronic circuits and a readout display.
The microphone detects the small air pressure variations associated
with sound and changes them into electrical signals. These signals
are then processed by the electronic circuitry of the instrument.
The readout displays the sound level in decibels. The SLM takes the
sound pressure level at one instant in a particular location.
To take measurements, the SLM is
held at arm’s length at the ear height for those exposed to the
noise. With most SLMs it does not matter exactly how the microphone
is pointed at the noise source. The instrument’s instruction manual
explains how to hold the microphone. The SLM must be calibrated
before and after each use. The manual also gives the calibration
procedure.
With most SLMs, the readings can
be taken on either SLOW or FAST response. The response rate is the
time period over which the instrument averages the sound level
before displaying it on the readout.
Workplace noise level
measurements should be taken on SLOW response. A Type 2 SLM is
sufficiently accurate for industrial field evaluations. The more
accurate and much more expensive Type 1 SLMs are primarily used in
engineering, laboratory and research work. Any SLM that is less
accurate than a Type 2 should not be used for workplace noise
measurement.
An A-weighting filter is
generally built into all SLMs and can be switched ON or OFF. Some
Type 2 SLMs provide measurements only in dB(A), meaning that the
A-weighting filter is ON permanently.
A standard SLM takes only
instantaneous noise measurements. This is sufficient in workplaces
with continuous noise levels. But in workplaces with impulse,
intermittent or variable noise levels, the SLM makes it difficult to
determine a person’s average exposure to noise over a work shift.
One solution in such workplaces is a noise dosimeter.
The integrating sound level meter
(ISLM) is similar to the dosimeter. It determines equivalent sound
levels over a measurement period. The major difference is that an
ISLM does not provide personal exposures because it is hand-held
like the SLM, and not worn.
The ISLM determines equivalent
sound levels at a particular location. It yields a single reading of
a given noise, even if the actual sound level of the noise changes
continually. It uses a pre-programmed exchange rate, with a time
constant that is equivalent to the SLOW setting on the SLM.
A noise dosimeter is a small,
light device that clips to a person’s belt with a small microphone
that fastens to the person’s collar, close to an ear. The dosimeter
stores the noise level information and carries out an averaging
process. It is useful in industry where noise usually varies in
duration and intensity, and where the person changes locations.
A noise dosimeter requires the
following settings:
(a) Criterion Level: exposure
limit for 8 hours per day five days per week. Criterion level is 90
dB(A);
(b) Exchange rate: 3 dB or 5 dB
as specified in the noise regulation;
(c) Threshold: noise level limit
below which the dosimeter does not accumulate noise dose data.
Wearing the dosimeter over a
complete work shift gives the average noise exposure or noise dose
for that person. This is usually expressed as a percentage of the
maximum permitted exposure. If a person has received a noise dose of
100 percent over a work shift, this means that the average noise
exposure is at the maximum permitted.
For example, with a criterion
level of 90 dB(A) and an exchange rate of 3 dB(A), an eight-hour
exposure to 90 dB(A) gives a 100 percent dose. A four-hour exposure
to 93 dB(A) is also a 100 percent dose, whereas an eight-hour
exposure to 93 dB(A) is a noise dose of 200 percent.
Usually the manufacturer
electronically adjusts dosimeters to the criterion level and
exchange rate in use. You may have to adjust them to suit the
exposure guidelines/standards in force in your jurisdiction.
Dosimeters also give an
equivalent sound or noise level. This is the average exposure level
for noise over the time dosimeter was on. It has the same total
sound energy as the actual, variable sound levels to which a person
is exposed over the same time period.
Scientific evidence suggests that
hearing loss is affected by the total noise energy exposure.
If a person is exposed over an
eight hour work shift to varying noise levels, it is possible to
calculate an equivalent sound level which would equal the same total
sound energy exposure.
FSM
