[Code of Federal Regulations]
[Title 49, Volume 4]
[Revised as of October 1, 2004]
From the U.S. Government Printing Office via GPO Access
[CITE: 49CFR325.55]

[Page 798]
 
                        TITLE 49--TRANSPORTATION
 
                      DEPARTMENT OF TRANSPORTATION
 
PART 325_COMPLIANCE WITH INTERSTATE MOTOR CARRIER NOISE EMISSION STANDARDS
--Table of Contents
 
        Subpart E_Measurement of Noise Emissions; Stationary Test
 
Sec. 325.55  Ambient conditions; stationary test.

    (a)(1) Sound. The ambient A-weighted sound level at the microphone 
location point shall be measured, in the absence of motor vehicle noise 
emanating from within the clear zone, with fast meter response using a 
sound level measurement system that conforms to the rules of Sec. 
325.23.
    (2) The measured ambient level must be 10 dB(A) or more below that 
level specified in Sec. 325.7, Table 1, which corresponds to the 
maximum permissible sound level reading which is applicable at the test 
site at the time of testing.
    (b) Wind. The wind velocity at the test site shall be measured at 
the beginning of each series of noise measurements and at intervals of 
5-15 minutes thereafter until it has been established that the wind 
velocity is essentially constant. Once this fact has been established, 
wind velocity measurements may be made at intervals of once every hour. 
Noise measurements may only be made if the measured wind velocity is 12 
mph (19.3 kph) or less. Gust wind measurements of up to 20 mph (33.2 
kph) are allowed.
    (c) Precipitation. Measurements are prohibited under any conditions 
of precipitation, however, measurements may be made with snow on the 
ground. The ground within the measurement area must be free of standing 
water.

[40 FR 42437, Sept. 12, 1975, as amended at 41 FR 28267, July 9, 1976]