May 2015 Page 42 of 51
When the working shift is less than 8 hours the TWA(8h) will be less than 90 dB. E.g. if the
working shift is only 5 hours, then the TWA(8h) will be 86.6 dB. In Figure 2 this can be seen
where the vertical green line (representing the working shift duration) crosses the blue line
(representing the TWA(8h) as the shift duration increases). This is shown by the dashed line.
Figure 2 shows the TWA(8h) value is less than 90 dB when working shift is 5 hours (TWA(8h) value is where blue line
crosses green line)
Likewise, if the working is shift is longer (e.g. 10 hours), then the resultant TWA(8h) would
be 91.6 dB. This is shown in Figure 3, where once again the dashed line shows the TWA(8h)
at the level where the vertical line of the working shift (green line) would cross the blue line
(if extended).
Figure 3 shows the TWA(8h) value is greater than 90 dB when working shift is 10 hours (TWA(8h) value is
where blue line crosses the vertical green line, assuming this green line is extended)
16.2 Projected Values
It is quite rare that a measurement would be carried out for exactly 8 hours. Also, it is
possible that a measurement is not carried out for the entire working shift (which equally may
not necessarily be 8 hours). It is therefore possible to use projected calculations. This
basically assumes that the worker is exposed to the same sound level for the remainder of a
particular time.
Projected values allow a shorter measurement to establish the noise level representative of
the workers full shift. The assumption is made that the level representing the energy
accumulated at a point in time (e.g. the Leq or Lavg value) will continue at the same level for
the remainder of the workers full shift.
The projected energy at the end of the workers shift is kept constant, but the time is changed
from the duration of the workers full shift to the standard 8 hours. Therefore, if the workers
shift is greater than 8 hours then the TWA(8h) will have a larger value in comparison to the
Lavg at the end of the full working shift. Likewise, if the full shift is less than 8 hours then the
TWA(8h) will be smaller than the Lavg at the end of the full shift.
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