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Appendix B
55
Exchange Rate (ER
–
also known as the Doubling Rate)
Exchange rate refers to how the sound energy is averaged over time. (Also, referred to as Doubling Rate.) Using the decibel
scale, every time the sound energy doubles, the measured level increases by 3dB. This is the 3dB exchange rate that most
of the world uses. For every increase of 3dB in the time weighted average, the measured DOSE would double. Some
organizations such as OSHA in the U.S. have argued that the human ear self compensates for changing noise levels and they
felt that the 3dB exchange rate should be changed to more closely match the response of the human ear. OSHA currently
uses a 5dB exchange rate, which would mean that the reported DOSE would double with every 5dB increase in the time
weighted average. The exchange rate affects the integrated reading LAVG, DOSE, and TWA but does not affect the
instantaneous sound level.
Field Calibration (vs. Factory Calibration), pre-calibration, post-calibration
To be sure the dosimeter is functioning within specified tolerance limits, it should be returned to the factory annually for a
factory recalibration. To verify that the dosimeter is measuring properly during normal use, field calibrations should be
performed. Pre-calibration is performed prior to any data being recorded by the dosimeter. During a pre-calibration, the
dosimeter level may be adjusted to match the calibrator output level. Post-calibration is a simple verification that the
instrument has remained in calibration throughout the measurement period.
Free field setting (microphone)
Measurements made in an area without sound reflections, diffractions and absorptions caused by nearby objects (including
the operator). Sound waves are free to spread out continuously, like ripples on a pond, without reflection. When making free
field measurements with a dosimeter set up for use as a sound level meter, the microphone is typically pointed directly at the
sound source.
Hearing Conservation (HC)
The Occupational Safety and Health Administration (OSHA) and Mine Safety and Health Administration (MSHA) provide
regulatory standards for hearing conservation programs. In addition, the American Conference of Governmental Industrial
Hygienists (ACGIH) provides industrial standards for hearing conservation programs.
http://www.osha.gov/ ,
http://www.msha.gov/ , http://www.acgih.org/home.htm
Hertz (Hz)
Unit of vibration frequency in cycles per second.
Lavg (Average Level)
Lavg is the 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 cannot 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 any reading at all. 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).
Lep,d
Daily personal noise exposure level.
Leq (Equivalent Level)
The true equivalent sound level measured over the run time. The term LEQ is functionally the same as LAVG except that it is
only used when the Exchange Rate is set to 3 dB and the threshold is set to none.
Summary of Contents for Edge eg5
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Page 58: ...49 Appendix A Edge Block Diagram Analog Board ...
Page 59: ...50 Appendix A Edge Block Diagram continued ...
Page 68: ...59 Appendix C Appendix C Measurement equations APPENDIX C ...