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Appendix III:A-3. Effects of the Environment on Instrumentation
There are various environmental factors that can affect the performance of noise-measuring instruments and their readings, including:
• Temperature

Sound-measuring equipment should perform within design specifications over a temperature range of -20 °F to 140 °F (-29 °C to 60 °C). If the temperature at the measurement site is outside of this range, refer to the manufacturer's specifications to determine if the sound level meter or dosimeter is capable of functioning properly.

Sound-measuring instruments should not be stored in automobiles during hot or cold weather because this may cause warm-up drift, moisture condensation, and weakened batteries.

• Humidity

Most noise instruments will perform accurately as long as moisture does not condense or deposit on the microphone diaphragm. If excessive moisture or rain is a problem in an exposure situation, refer to the manufacturer's instructions or other noise professionals for technical support.

• Atmospheric Pressure

Atmospheric pressure affects the output of sound level calibrators. When checking an acoustical calibrator, always apply the corrections for atmospheric pressure that are specified in the manufacturer's instruction manual.

• In general, if the altitude of the measurement site is less than 10,000 feet above sea level, no pressure correction is needed. If the measurement site is at an altitude higher than 10,000 feet, or if the site is being maintained at a pressure greater that its surroundings (for example, in underwater tunnel construction), use the following equation to correct the instrument reading:
Air Pressure Correction Equation

 C = 10 log [( 460+t 528 ) 0.5 ( 30 B )]
C = correction, in decibels, to be added to or subtracted from the measured sound level
t = temperature in degrees Fahrenheit
B = barometric pressure in inches of mercury

NOTE:

For high altitude locations, C will be positive; in hyperbaric conditions (above atmospheric pressure), C will be negative.
• Wind or Dust

Wind or dust blowing across the microphone of the dosimeter or sound level meter produces turbulence, which may cause a positive error in the measurement. A wind screen should be used for all outdoor measurements and whenever there is significant air movement or dust inside a building (for example, when cooling fans are in use or wind is gusting through open windows).

• Magnetic Fields

Certain equipment and operations, such as heat sealers, induction furnaces, generators, transformers, electromagnets, arc welding, and radio transmitters generate electromagnetic fields that can induce current in the electronic circuitry of sound level meters and noise dosimeters and cause erratic readings.  If instruments must be used near such devices or operations, the extent of the field's interference should be determined by consulting the manufacturer's instructions.

• Sound

For sound level meters and noise dosimeters equipped with omnidirectional microphones, the effects of the microphone placement and orientation are negligible in an environment that is typically reverberant.

If the measurement site is nonreverberant and the noise source is highly directional, consult the manufacturer's literature to determine proper microphone placement and orientation.

For determining compliance with the impulse noise provision of 29 CFR 1910.95(b)(1) or 29 CFR 1926.52(e), use the unweighted peak mode setting of the sound level meter or equivalent impulse precision sound level meter.