[Code of Federal Regulations]
[Title 49, Volume 5]
[Revised as of October 1, 2007]
From the U.S. Government Printing Office via GPO Access
[CITE: 49CFR325.37]

[Page 12-13]
 
                        TITLE 49--TRANSPORTATION
 
                      DEPARTMENT OF TRANSPORTATION
 
PART 325_COMPLIANCE WITH INTERSTATE MOTOR CARRIER NOISE EMISSION 
STANDARDS--Table of Contents
 
      Subpart D_Measurement of Noise Emissions; Highway Operations
 
Sec.  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 Sec.  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\1/2\ 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)(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/

[[Page 13]]

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.

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