If LEQ is greater than the criterion level, the 3 dB DOSE is greater than the
LTL DOSE. If LEQ is lower than the criterion level, the 3 dB DOSE is less than
the LTL DOSE.
The above may not be true if the noise is below the threshold level.
If the noise is continuous or impulsive, then L-AVG will be nearly equal to
LEQ.
If the noise is intermittent, then L-AVG will be several decibels less than
LEQ.
The slow time constant will cause the readings taken with other than a 3 dB
exchange rate to be greater than if there were no time constant. For 3 dB it
makes little difference.
If the unit has not been in the RUN mode, PEAK LEVEL and SLOW MAX
will be at their minimum values.
The PEAK LEVEL will be greater than SLOW MAX unless the peak
occurred while the instrument was in PAUSE and was changed to RUN a few
seconds later. Then SLOW MAX might be greater than PEAK LEVEL.
OL TIME does not count until the peak level has exceeded the overload
level for a total of one second.
In the preceding statements a reading which is "greater than" or "less than"
can be taken to mean the same as "almost equal to."
Use of L-AVG and TWA
Time Weighted Average (TWA) is measured in decibels and is equal to a
continuous level for 8 hours which would produce the same noise dose as the
dose accumulated over the run time. The average integrated sound level (L-
AVG) will be equal to TWA at 8 hours. If the time is less than 8 hours, then TWA
will be less than L-AVG. If time is greater than 8 hours, TWA will be greater than
L-AVG.
The MICRO-15 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 8
hours the TWA measurement will read the correct TWA for that worker.
L-AVG 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 8 hours, then the TWA for that
worker is equal to the L-AVG measured with the short term sample.
Estimating TWA from L-AVG
A short term sample can be used to estimate worker exposure even if the
workday is other than 8 hours by adding or subtracting a correction factor from
the L-AVG reading.
As an example, if a worker has a L-AVG of 85.58 dB after a 10 minute
sample, and the sample is representative of the noise throughout the workday,
and the workday at that position is 7 hrs., 15 min., then from Table 1 under a 5
dB exchange rate read across from a work day of 7 hrs., 15 min. to the 5 dB
column and note the correction factor of -.71. Subtracting .71 from 85.50 gives a
TWA of 84.79 dB.
Accuracy of Readings
There is a tendency to overestimate the accuracy of digital readings. In the
MICRO-15 some values such as L-AVG and TWA are computed to 0.01 dB. The
absolute accuracy of the reading is not 0.01 dB. (See specifications for accuracy
and tolerances). However, the resolution of 0.01 dB is useful in determining the
sample time required for a short term sample. As an example, if L-AVG reading
is increasing 0.03 dB every second then additional sample time may be required.
If the L-AVG reading remains stable within a few 0.01 dB's, then the sample time
may be long enough.
Hints and Good Practices
The MICRO-15 is in the pause mode after each of the following: the battery
is replaced; the unit is turned on; it is reset; or the PAUSE key is pressed. Wait
until the SOUND LEVEL reading has stabilized before entering the run mode.
Always remember to press RUN.
Press PAUSE before pressing PRINT. If in the run mode, the data printed at
the beginning of the printout will not agree with the data at the end.
The SOUND LEVEL on the printout includes the noise of the printer, and
should therefore be ignored.
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