Appendix B
B: Glossary of Terms
Measurements must always be at least 5 dB above the Noise Floor to be valid. Therefore, the lowest
valid measurements of the dosimeter are approximately 40 dB on A-Weighting and 50 dB on
C-Weighting.
Peak Level
Peak is the highest instantaneous sound level that the microphone detects. Unlike the Max Level, the
peak is detected independently of the unit’s Response Time setting (Fast or Slow). Example: The
peak circuitry is very sensitive. Test this by simply blowing across the microphone. You will notice that
the peak reading may be 120 dB or greater. When taking a long-term noise sample (such as a typical
8-hour workday sample for OSHA compliance), the peak level is often very high. Because brushing
the microphone over a shirt collar or accidentally bumping it can cause such a high reading, the user
must be careful of placing too much emphasis on the reading.
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 smooths 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.
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