[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: 49CFR325.25]

[Page 10-11]
 
                        TITLE 49--TRANSPORTATION
 
                      DEPARTMENT OF TRANSPORTATION
 
PART 325_COMPLIANCE WITH INTERSTATE MOTOR CARRIER NOISE EMISSION 
STANDARDS--Table of Contents
 
                        Subpart C_Instrumentation
 
Sec.  325.25  Calibration of measurement systems.

    (a)(1) The sound level measurement system must be calibrated and 
appropriately adjusted at one or more frequencies in the range from 250 
to 1,000 Hz at the beginning of each series of measurements and at 
intervals of 5-15 minutes thereafter, until it has been determined that 
the sound level measurement system has not significantly drifted from 
its calibrated level. Once

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this fact has been established, calibrations may be made at intervals 
once every hour. A significant drift shall be considered to have 
occurred if a 0.3 dB or more excursion is noted from the system's 
predetermined reference calibration level. In the case of systems using 
displays with whole decibel increments, the operator may visually judge 
when the 0.3 dB drift has been met or exceeded.
    (2) The sound level measurement system must be checked periodically 
by its manufacturer, a representative of its manufacturer, or a person 
of equivalent special competence to verify that its accuracy meets the 
manufacturer's design criteria.
    (b) An acoustical calibrator of the microphone coupler type designed 
for the sound level measurement system in use shall be used to calibrate 
the sound level measurement system in accordance with paragraph (a) of 
this section. The calibration must meet or exceed the accuracy 
requirements specified in section 5.4.1 of the American National 
Standard Institute Standard Methods for Measurements of Sound Pressure 
Levels (ANSI S1.13-1971) for field method measurements.

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