§325.57 Location and operation of sound level measurement systems; stationary test.

(a) The microphone of a sound level measurement system that conforms to the rules in §325.23 shall be located at a height of not less than 2 feet (.6 m) nor more than 6 feet (1.8 m) above the plane of the roadway surface and not less than 3¹/₂ feet (1.1 m) above the surface on which the microphone stands. The preferred microphone height on flat terrain is 4 feet (1.2 m).

(b) When the sound level measurement system is handheld or otherwise monitored by a person located near its microphone, the holder must orient himself relative to the highway in a manner consistent with the recommendation of the manufacturer of the sound level measurement system. In no case shall the holder or observer be closer than 2 feet (.6 m) from the system’s microphone, nor shall he locate himself between the microphone and the vehicle being measured.