3M DL, SE user manual Level LFT, Lavg = ER log20RTIME 2LS ⁄ ER dt log2 Rtime dB

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Appendix B

Exchange Rate of 3 dB. The display will switch to ExpHrs if RTIME is greater than 3600 seconds.

EXP Sec = [ 2(Leq – 94) ⁄ 3.01 ] R TIME Pa2S

Level (LFT)

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 LAVG 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 LAVG resulting in a level perhaps around 40 dB (notably lower than the actual levels in the environment).

Lavg = ER log2∫0RTIME 2LS ⁄ ER dt – log2 (RTIME) dB

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.

Models SE and DL