§ 157.43. Location and operation of sound level measurement system.
(a) Microphone height. The microphone of a sound level measurement system that conforms to the provisions of § 157.31 (relating to types of measurement systems which may be used) shall be located at a height of not less than 2 feet (0.6 meters) nor more than 6 feet (1.8 meters) above the plane of the roadway surface and not less than 3 1/2 feet (1.1 meters) and not more than 4 1/2 feet (1.4 meters) above the surface on which the microphone stands. The preferred microphone height or flat terrain is four feet (1.2 meters).
(b) Monitor orientation. Requirements for monitor orientation shall be as follows:
(1) When the sound level measurement system is hand-held or is otherwise monitored by a person located near its microphone, the holder shall orient himself relative to the highway in a manner consistent with the recommendation of the manufacturer of the sound level measurement system.
(2) In no case shall the holder or observer be closer than 2 feet (0.6 meters) from the microphone of the system, nor shall he locate himself between the microphone and the vehicle being measured.
(c) Microphone orientation. The microphone of the sound level measurement system shall be oriented toward the traveled lane of the highway at the microphone target point at an angle that is consistent with the recommendation of the manufacturer of the system. If the manufacturer of the system does not recommend an angle of orientation for its microphone, the microphone shall be oriented toward the highway at an angle of not less than 70° and not more than perpendicular to the horizontal plane of the traveled lane of the highway at the microphone target point.
(d) Network and response mode. The sound level measurement system shall be set to the A-weighting network and ‘‘fast’’ meter response mode.
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