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62 Appendix B
Glossary of Terms
053-644, Rev E Page 62 eg4 & eg3
PEL (Permissible Exposure Level)
The A-weighted sound level at which exposure for a Criterion Time, typically eight hours, accumulates
a 100% noise dose.
Projected Time
The variable amount of time used to make a projected dose calculation from an actual dose
measurement.
Random incidence (microphone)
This only applies to Class/Type 1 microphones. Measurements made in an area where sound waves
are coming from all directions, including reflections and diffractions. When the dosimeter is being used
as a sound level meter in an area where reflections and diffractions from nearby objects are present,
use the Random Incidence Corrector with the microphone, and angle the microphone at approximately
70 degrees. Most personal noise dosimetry applications do not require the random incidence
corrector, since they are not measured with a Class/Type 1 microphone.
Response Time
The response determines how quickly the unit responds to fluctuating noise. Fast has a time constant
of 125 milliseconds. Slow has a time constant of 1 second. Example: Typically, noise is not constant.
If you were to try to read the sound level without a response time, the readings would fluctuate so
much that determining the actual level would be extremely difficult. Using a response of slow or fast
simply smoothes the noise fluctuation and makes the sound level easier to work with. While the terms
slow and fast have very specific meanings (time constraints), they work very much as you would
expect. The fast response would result in a more fluctuating sound level reading than would the slow
response. The OSHA regulations require the slow response.
RMS (Root Mean Square)
The RMS voltage of a signal is computed by squaring the instantaneous voltage, integrating over the
desired time, and taking the square root. Simply put, the RMS values are the results from the
dosimeter with the response time and weighting settings taken into account.
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 3dB 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 82dB. A jet passes overhead taking 45
seconds with an average level of 96dB. 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 = 109.7dB, SEL for the jet = 112.5dB.