TSI Quest EDGE EG-4P, Руководство пользователя

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Glossary of Terms B-5
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.
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 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 = 109.7 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.