(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 3 feet 1⁄2 (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 hand-held or otherwise monitored by a person located near its microphone, the holder must orient himself/herself 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/she locate himself/herself between the microphone and the vehicle being measured.
(c) The microphone of the sound level measurement system shall be oriented toward the vehicle at an angle that is consistent with the recommendation of the system's manufacturer. If the manufacturer of the system does not recommend an angle of orientation for its microphone, the microphone shall be oriented at an angle of not less than 70 degrees and not more than perpendicular to the horizontal plane of the test site at the microphone target point.
(d) The sound level measurement system shall be set to the A-weighting network and “fast” meter response mode.
Although we make every effort to assure that the information we provide is complete and accurate, it is not intended to take the place of published agency regulations. Regulations issued by the U.S. Department of Transportation and its Operating Administrations are published in the Federal Register and compiled in the U.S. Code of Federal Regulations (CFR). Copies of appropriate volumes of the CFR in book format may be purchased from the Superintendent of Documents, U.S. Government Printing Office, or examined at many libraries.
The CFR may also be viewed online at http://ECFR.gpoaccess.gov.