§ 157.42. Ambient conditions.
(a) Sound. Requirements concerning ambient sound conditions shall be as follows:
(1) 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 noise zone, with fast meter response using a sound level measurement system that conforms to the provisions of § 157.31 (relating to types of measurement systems which may be used).
(2) The measured ambient level shall be ten decibels (A) or more below that level specified in 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 shall only be made if the measured wind velocity is 12 miles per hour (19.3 kilometers per hour) or less. Gust wind measurements of up to 20 miles per hour (33.2 kilometers per hour) shall be allowed.
(c) Precipitation. Measurements shall be prohibited under any condition of precipitation, however, measurements may be made with snow on the ground. The ground surface within the measurement area shall be free of standing water.
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