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  • § 325.1
    Scope of the rules in this part.
  • § 325.3
    Effective date.
  • § 325.5
    Definitions.
  • § 325.7
    Allowable noise levels.
  • § 325.9
    Measurement tolerances.
  • § 325.11
    Issuance, amendment, and revocation of the rules in this part.
  • § 325.13
    Inspection and examination of motor vehicles.
  • § 325.21
    Scope of the rules in this subpart.
  • § 325.23
    Type of measurement systems which may be used.
  • § 325.25
    Calibration of measurement systems.
  • § 325.27
    Use of a windscreen.
  • § 325.31
    Scope of the rules in this subpart.
  • § 325.33
    Site characteristics; highway operations.
  • § 325.35
    Ambient conditions; highway operations.
  • § 325.37
    Location and operation of sound level measurement system; highway operations.
  • § 325.39
    Measurement procedure; highway operations.
  • § 325.51
    Scope of the rules in this subpart.
  • § 325.53
    Site characteristics; stationary test.
  • § 325.55
    Ambient conditions; stationary test.
  • § 325.57
    Location and operation of sound level measurement systems; stationary test.
  • § 325.59
    Measurement procedure; stationary test.
  • § 325.71
    Scope of the rules in this subpart.
  • § 325.73
    Microphone distance correction factors.
  • § 325.75
    Ground surface correction factors.
  • § 325.77
    Computation of open site requirements—nonstandard sites.
  • § 325.79
    Application of correction factors.
  • § 325.91
    Exhaust systems.
  • § 325.93
    Tires.

Part 325
COMPLIANCE WITH INTERSTATE MOTOR CARRIER NOISE EMISSION STANDARDS

§ 325.37: Location and operation of sound level measurement system; highway operations.

(a) The microphone of a sound level measurement system that conforms to the rules in §325.23 of this part 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 1feet (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)(1) When the sound level measurement system is hand-held or is 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.
(2) 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 traveled lane of the highway at the microphone target point 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 toward the highway at an angle of not less than 70 degrees and not more than perpendicular to the horizontal plane of the traveled lane of the highway at the microphone target point.
(d) The sound level measurement system shall be set to the A-weighting network and “fast” meter response mode.

Citation: [40 FR 42437, Sept. 12, 1975, as amended at 41 FR 10227, Mar. 10, 1976]