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Appendix B
140
Models SE and DL
Exchange Rate of 3 dB. The display will switch to ExpHrs if RTIME is greater than 3600
seconds.
Level (L
FT
)
Notation used to represent sound pressure level measurements in IEC/ISO notation, where
the measurements are made with particular frequency (F) and time (T) response parameters.
For an actual measurement, the F and T parameters are replaced by standardized notation
for those parameters.
L
_AV
Average sound level measured over the run time. This becomes a bit confusing when
thresholds are used. Any sound below the threshold is not included in this average.
Remember that sound is measured in the logarithmic scale of decibels therefore the average
can not be computed by simply adding the levels and dividing by the number of samples.
When averaging decibels, short durations of high levels can significantly contribute to the
average level.
Example: Assume the threshold is set to 80 dB and the Exchange Rate is 5 dB (the settings
of OSHA’s Hearing Conservation Amendment). Consider taking a one hour noise
measurement in an office where the A- weighted sound level was typically between 50 dB
and 70 dB. If the sound level never exceeded the 80 dB threshold during the one hour period,
then the L
AVG
would not indicate a reading of zero. If 80 dB was exceeded for only a few
seconds due to a telephone ringing near the instrument, then only those seconds will
contribute to the L
AVG
resulting in a level perhaps around 40 dB (notably lower than the actual
levels in the environment).
L
_EQ
The true equivalent sound level measured over the run time. The term L_
EQ
is functionally the
same as L_
AV
except that it is only used when the Exchange Rate is set to 3 dB and the
threshold is set to none.
EXP
Sec
2
L
eq
94–( )
3.01⁄
[ ] R
TIME Pa
2
S
=
L
avg
ER log
2
2
LS ER⁄
td log
2
RTIME( )–
0
RTIME
∫
dB=