Considerations of Use for Sound Level Meters
Factors to consider with the use of a SLM include:
- When evaluating employee exposures, place the microphone in the hearing zone of the employee being monitored.
- Sound level readings in a non-reverberant environment should be taken in accordance with the manufacturer's instructions.
Special considerations for use and care may include:
- Always check the batteries prior to use. Be very careful with the microphone cable. Never kink, stretch, pinch, or otherwise damage the cable.
- Use the microphone windscreen to protect the microphone when the wearer is outdoors or in dusty or dirty areas. (The windscreen will not protect the microphone from rain or extreme humidity. Refer to the manufacturer's instructions when using equipment in extreme conditions.)
- Never use any type of covering over the microphone (e.g., plastic bag or plastic wrap) to protect it from moisture. Such materials will distort the noise pickup, and the readings will be invalid.
- Never try to clean a microphone, particularly with compressed air, since damage is likely to result. Although dirt and exposure to industrial environments will damage the microphones, regular use of an acoustical calibrator will detect such damage so that microphones can be replaced.
- Remove the batteries when the dosimeter will be stored for more than 5 days. Protect dosimeters from extreme heat and humidity.
- No field maintenance is required other than replacement of batteries.
Measuring Impulse/Impact Sounds
Some meters have a "peak" and "impulse" response for measuring transient sounds (sounds that decay or pass with time).
- The true peak value is the maximum value of the noise waveform. The impulse measurement is an integrated measurement. The true peak reading should only be used when determining compliance with OSHA's 140 dB peak sound pressure level [29 CFR 1910.95(b)(1) or 29 CFR 1926.52(e)].
- The user should not use "impulse" response when measuring true peak sound pressure levels.
Sound level meters used by OSHA meet the American National Standards Institute (ANSI) Standard S1.4-1971 (R1976) or S1.4-1983, "Specifications for Sound Level Meters." These ANSI standards set performance and accuracy tolerances according to three levels of precision: Types 0, 1, and 2. Type 0 is used in laboratories, Type 1 is used for precision measurements in the field, and Type 2 is used for general-purpose measurements.
- A Type 2 meter is the minimum requirement by OSHA for noise measurements, and is usually sufficient for general purpose noise surveys.
- The Type 1 meter is preferred for the design of cost-effective noise controls. For unusual measurement situations, refer to the manufacturer's instructions and appropriate ANSI standards for guidance in interpreting instrument accuracy.
- Use a dosimeter with a threshold of 80 dBA (A-weighted sound pressure level) and 90 dBA to measure noise exposures. Most modern dosimeters use simultaneous 80 and 90 dBA thresholds and may be used accordingly. Additional information (App III: A) on dosimeters is also available. A dosimeter with a threshold of 80 dBA is used to measure the noise dose of those employees identified during the walk-around survey as having noise exposures that are in compliance with Table G-16 of OSHA's noise standard 29 CFR 1910.95, but whose exposure may exceed the levels specified in Table G-16a [29 CFR 1910.95 Appendix A: Noise Exposure Computation]. In other words, the 80-dBA threshold is used to determine compliance with the 85 dBA time-weighted average (TWA) action level under OSHA's noise standard. The dosimeter with a threshold of 90 dBA is used to measure the noise dose of those employees identified during the walk-around survey as having potential noise exposures that exceed the sound levels in Table G-16 [29 CFR 1910.95] or Table D-2 [29 CFR 1926.52]. To put it simply, the 90 dBA threshold is used to determine compliance with the permissible exposure limit (PEL).
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