Pennsylvania Code
Title 67 - TRANSPORTATION
Part I - Department of Transportation
Subpart A - Vehicle Code Provisions
Article VII - Vehicle Characteristics
Chapter 157 - ESTABLISHED SOUND LEVELS
Subchapter D - INSTRUMENTATION
Section 157.32 - Calibration of measurement systems

(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.