May 2015 Page 41 of 51
Threshold Value is the threshold below which sound levels are excluded from
calculation. OSHA measurements use an 80 dB threshold and call for
a hearing conservation program to be implemented when the eight
hour TWA exceeds 85 dB (50% dose).
TWA is the Time Weighted Average. Used in OSHA measurements, the
TWA is the daily personal exposure level calculated from the LAvg
and measurement duration.
Z-Weighting is a linear or un-weighted measurement. In the dBadge2 system it is
used for the measurement of peak sound pressure level against the
OSHA standard.
16. Projected and Normalised Noise Measurements
Noise exposure limits are normally set nationally or more locally. These limits would define
that a worker/person is allowed to be exposed to a certain noise level (criterion level, Lc) for
a certain time (criterion duration, Tc).
A possible measurement for this is also to use percentage dose. 100% noise dose equates
to a noise level at Lc for a duration of Tc. E.g. if Lc = 90 dB and Tc = 8 hours; then 100%
dose equates to 90 dB for 8 hours.
16.1 Normalised Noise Measurements
Normalised noise values refer to measurements made over differing durations, but are then
normalised to a standard duration. The energy measured over a particular duration is equal
to the energy when the duration is changed to the normalised value. E.g. 90 dB measured
over 4 hours is equivalent to 85 dB over 8 hours (using a Q5 exchange rate). Therefore, 90
dB over 4 hours is 85 dB when normalised to 8 hours.
A type of measurement for noise exposure is the 8 hour time weighted average (TWA (8h));
or the European equivalent metric noise exposure level over 8 hours (Lex8h). The
measurements basically normalise the actual measurements to a standard 8 hour period.
This means that any measurements or calculations based on times other than 8 hours can
then be normalised to 8 hours, which allows for a direct comparison between different
measurement intervals.
Normalising the values to 8 hours, it can be seen that as the working shift duration increases
then the TWA(8h) also increases. Assuming exposure a constant noise of 90 dB, the
TWA(8h) increases up to 90 dB when the working shift reaches 8 hours and then increases
over this as the shift exceeds 8 hours.
Figure 1 shows how the TWA(8h) value changes as working shift changes
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