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between TWA and Dose is that TWA is expressed in decibels and Dose is expressed as a percent.
TWA is equal to a continuous level which if constant for eight hours would produce the same
noise dose as the dose accumulated over the run time. The average integrated sound level (Lavg) will be
equal to TWA at eight hours. If the time is less than eight hours, then TWA will be less than Lavg; TWA
will be greater than Lavg if time is greater than eight hours.
The M-27 automatically computes the TWA of a worker if the unit is worn by the worker for a full
workday. Even if the worker does not work exactly eight hours the TWA measurement will read the correct
TWA for that worker.
Lavg can be used with short term sampling to estimate the worker's TWA. If the noise exposure
during the sample period is representative of the noise throughout the workday and the workday is eight
hours, then the TWA for that worker is equal to the Lavg measured with the short term sample.
Estimating TWA from Lavg
A short term sample can be used to estimate worker exposure even if the workday is other than
eight hours by adding or subtracting a correction factor from the Lavg reading.
As an example, if a worker has a Lavg of 85.50 dB after a ten minute sample, and the sample is
representative of the noise throughout the workday, and the workday at that position is 7:15 then from
Table 4 under a 5 dB exchange rate find -.71 dB and subtract it from 85.50 to compute a TWA of 84.79 dB.
Accuracy of Readings
There is a tendency to overestimate the accuracy of digital readings. In the M-27, some values
such as Lavg and TWA are computed to 0.1 dB. The absolute accuracy of the reading is not 0.1 dB. (See
specifications for accuracy and tolerances.) However, the resolution of 0.1 dB is useful in determining the
sample time required for a short term sample. As an example, if Lavg reading is increasing .3 dB every
second then additional sample time may be required. If the Lavg reading remains stable within a few 0.1
dB's then the sample time may be long enough.
Hints and Good Practices
The M-27 is in the pause mode after the battery is replaced, when