FMSCA Regulation 325.35
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 325.23.
(a)(2) The measured ambient level must be 10 dB(A) or more below that level specified in 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.
