May 2015 Page 48 of 51
Figure 14 shows the 10 hour measurement (blue line); but also the same noise level continued for the
rest of the 12 hour shift (green line)
Projecting the 10 hour measurement for the remainder of the 12 hour shift means that the
Lavg for the 12 hours equals the Lavg for the 10 hours = 88.0 dB.
In order to calculate the normalised TWA(8h), this time the measured energy represented by
the green line is kept constant. Once again the duration was changed (from 12 hours) to 8
hours.
Figure 15 shows the 10 hour measurement (blue line); but also the same noise level continued for the
rest of the 12 hour shift (green line). The TWA(8h) is also shown (red line) which has the same measured
energy as that represented by the green line
This has assumed that effectively the level was 88.0 dB for the entire 12 hours and this was
then normalised back to 8 hours (which ends up the same situation as scenario 3 above).
Additional Considerations
It is also possible to use other durations for normalising to (e.g. TWA(12h)). However, care
should be taken with this as this effectively changes the criterion duration from the standard
8 hours to 12 hours. In order to ensure that the amount of allowable noise exposure is not
increased then it is necessary to reduce the criterion level. This ensures that 100% noise
dose is the same amount of energy, regardless of the duration.
Using the standard 90 dB for 8 hours gives 100% dose; then changing the duration to 12
hours results in an allowable level of 97.1 dB (which keeps the 100% dose the same).
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