Appendix B
B: Glossary of Terms
SEL/Lxe (Sound Exposure Level)
The sound exposure level averages the sampled sound over a one second period. Assuming the
sampled run time to be greater than one second, SEL is the equivalent one-second noise that would
be equal in energy to the noise that was sampled. SEL is typically measured using a 3 dB exchange
rate without a threshold. SEL is not used by OSHA. Example: Suppose you wanted to measure in a
location next to railroad tracks, which also happened to be in the takeoff path of an airport. A train
passes by taking 10 minutes with an average sound level of 82 dB. A jet passes overhead taking
45 seconds with an average level of 96 dB. Which of these events results in more sound energy?
You can answer the question by comparing their SEL readings, which compress each event into an
equivalent one-second occurrence. SEL for the train = 109.7 dB, SEL for the jet = 112.5 dB.
Threshold
The threshold affects the Lavg, TWA, and Dose measurements. All sound below the threshold is
considered nonexistent noise for the averaging and integrating functions. The threshold does not
affect measurements in the sound level mode. OSHA uses two different thresholds. The original
Occupational Noise Exposure Standard (1971) used a 90 dB threshold and called for engineering
controls to reduce the noise levels if the eight-hour TWA was greater than 90 dB. The Hearing
Conservation Amendment (1983) uses an 80 dB threshold and calls for a hearing conservation
program to be put in place if the eight-hour TWA exceeds 85 dB (50% dose). The Hearing
Conservation Amendment is the more stringent of the two rulings and is what most US industrial
users are concerned with. Example: With an 80 dB threshold, suppose you placed a 79 dB calibrator
on the unit for a period of time. Because all of the noise is below the threshold, there would be no
average (you can think of it as an average of 0 dB). If the calibrator were 80 dB instead, then the
average would be 80 dB. On histogram printouts, typically 1 minute (or other specified increment)
averages are printed. Because real noise fluctuates, it is quite possible to have an average level
below the threshold. This also applies for the overall Lavg.
TWA (Time Weighted Average)
The time weighted average always averages the sampled sound over an 8-hour period. TWA starts at
zero and grows. The TWA is less than the Lavg for a duration of less than eight hours, exactly equal
to the Lavg at eight hours, and grows higher than Lavg after eight hours. TWA represents a constant
sound level lasting eight hours that would result in the equivalent sound energy as the noise that was
sampled. Example: Think of TWA as having a large 8-hour container that stores sound energy. If you
run a dosimeter for 2 hours, your Lavg is the average level for those 2 hours - consider this a smaller
2-hour container filled with sound energy. For TWA, take the smaller 2-hour container and pour that
energy into the larger 8 hour TWA container. The TWA level will be lower. Again, TWA is ALWAYS
based on the 8-hour container. When measuring using OSHA’s guidelines, TWA is the proper number
to report provided that the full work shift was measured. Example: If the work shift is 6.5 hours long,
then measure for the entire 6.5 hours. TWA is the correct level to report to OSHA. It does not have to
be modified.
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