May 2015 Page 43 of 51
From the above, it can be demonstrated that if the working shift is exactly 8 hours and a
measurement is carried out for less than this time; the Lavg at the end of the measurement
will equal the TWA(8h).
The projected values are only appropriate when the measurement duration is less than the
full working shift duration. When the measurement duration occurs over the entire working
shift then the standard TWA(8h) (as calculated by the instrument) would be used.
As an example, assume that a working shift is 7 hours. However, the actual measurement
was only carried out over 5 hours. At the end of the 5 hour measurement the Lavg was 90.0
dB. It is assumed that this level remains constant for the remainder of the 7 hour working
shift. Therefore, the measurement is projected forward to the end of the working shift. This
level (90.0 dB for 7 hours) can now be used to calculate the TWA(8h), which results in 89.0
dB.
Figure 4 shows a measurement of 5 hours (green line); this is then projected forward to the end of the 7
hour working shift (red line); where the projected value crosses the original TWA(8h) (blue line) is the
actual TWA(8h), shown by the dashed line. (NOTE this is based on using levels of 90 dB and are only for
illustration. In practice the TWA(8h) would be calculated mathematically from the Lavg at the end of the
projected 7 hour working shift)
Another example is using a working shift of 10 hours. This time a measurement was carried
out for 6 hours. Therefore, the Lavg at the end of the 6 hours (90.0 dB in this example) is
projected forward to the end of the 10 hour working shift. This level (90.0 dB for 10 hours)
can now be used to calculate the TWA(8h), which results in 91.6 dB.
Figure 5 shows a measurement of 6 hours (green line); this is then projected forward to the end of the 10
hour working shift (red line); where the projected value crosses the original TWA(8h) (blue line) is the
actual TWA(8h), shown by the dashed line.
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