3M DL, SE - page 153

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 (LFT)
Notation used to represent sound pressure level m easurements in IEC/ISO notation, where
the measurements are made with particular frequenc y (F) and time (T) response parameters.
For an actual measurement, the F and T parameter s are replaced by standardized notation
for those parameters.
L_AV
Average sound level measured over the run tim e. 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 logarithm ic scale of decibels therefore the average
can not be computed by simply adding the levels an d dividing by the number of samples.
When averaging decibels, short durations of high lev els can significantly contribute to the
average level.
Example: Assume the threshold is set to 80 dB and th e 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 so und level was typically between 50 dB
and 70 dB. If the sound level never exceeded the 80 d B threshold during the one hour period,
then the LAVG would not indicate a reading of zero. If 80 dB was exceeded for onl y a few
seconds due to a telephone ringing near the instrum ent, then only those seconds will
contribute to the LAVG resulting in a level perhaps ar ound 40 dB (notably lower than the actual
levels in the environment).
L_EQ
The true equivalent sound level measured over the ru n time. The term L_EQ is functionally the
same as L_AV except that it is only used when the Exc hange Rate is set to 3 dB and the
threshold is set to none.
EXP
Sec
2
L
eq
94–( ) 3.01⁄
[ ] RTIM E P a
2
S
=
L
avg
ER log
2
2
LS ER⁄
td log
2
RT IM E( )–
0
RTI ME
∫ dB=