May 2015 Page 46 of 51
Scenario 2: Criterion duration (Tc) = 8 hours; measurement duration (Tm) = 5 hours;
shift duration (Ts) = 5 hours
The noise which the worker is exposed to is measured for 5 hours, which is the entire shift
duration. This is the same situation as a worker who has a shift duration of 8 hours, but is
only exposed to noise during 5 hours of this shift.
Figure 8 shows the 5 hour measurement
Figure 9 shows the 5 hour measurement (blue line); there is no further noise exposure for the remainder
of the 8 hours (red line)
In order to calculate the TWA(8h), it is necessary to keep the measured energy (represented
by the blue line) constant, but change the duration from 5 hours to 8 hours. This will result in
the level decreasing. In this case a worker exposed to 88.0 dB for 5 hours is equivalent to
one exposed to 84.6 dB over 8 hours (therefore, normalised to 8 hours).
Figure 10 shows the 5 hour measurement (blue line); the TWA(8h) is shown by the red line. The
measured energy represented by the blue line is equal to the measured energy represented by the red
line.
Scenario3: Criterion duration (Tc) = 8 hours; measurement duration (Tm) = 12 hours;
shift duration (Ts) = 12 hours
The noise which the worker is exposed to is measured for 12 hours, which is the entire shift
duration.
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