§ 157.32. Calibration of measurement systems.
(a) Calibration. Requirements for calibration shall be as follows:
(1) The sound level measurement system shall be calibrated and appropriately adjusted at one or more frequencies in the range from 250 to 1,000 hertz at the beginning of each series of measurements and at intervals of 5—15 minutes thereafter until it has been established that the sound level measurement system has not drifted from its calibrated level. Once this fact has been established, calibrations may be made at intervals of once every hour. A significant drift shall be considered to have occurred if a 0.3 decibel or more excursion is noted from the systems predetermined reference calibration level. In the case of systems using displays with whole decibel increments, the operator may usually judge when the 0.3 decibel drift has been met or exceeded.
(2) The sound level measurement system shall be checked periodically by its manufacturer, a representative of its manufacturer, or a person of equivalent special competence to verify that its accuracy meets the design criteria of the manufacturer.
(b) Acoustical calibrator. An acoustical calibrator of the microphone coupler type design for the sound level measurement system in use shall be used to calibrate the sound level measurement system in accordance with subsection (a) of this section. The calibrator shall meet or exceed the accuracy requirements specified in section 5.4.1 of the American National Standards Institute Standard Methods for Measurements of Sound Pressure Levels, ANSI § 1.13—1971, for calibrators for field type measurements.
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