[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: 49CFR325.35]

[Page 12]
 
                        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.  325.35  Ambient conditions; highway operations.

    (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 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 condition 
of precipitation, however, measurements may be made with snow on the 
ground. The ground surface within the measurement area must be free of 
standing water.

[40 FR 42437, Sept. 12, 1975, as amended at 41 FR 10227, Mar. 10, 1976; 
41 FR 28267, July 9, 1976]