(a) Microphone Orientation--The microphone shall be oriented in relation to the source of the sound in accordance with the instrument manufacturer's instructions. If the instruction manual does not include adequate information, a specific recommendation shall be obtained from the manufacturer.

(b) Meter Setting--The meter shall be set for the A-weighting network and fast response.

(c) Calibration Check--The sound level measuring system shall be calibrated with an external acoustic calibrator before and after each test sequence and at intervals of 5 to 15 min. at the beginning of each test sequence until it has been established that the system has not drifted more than 0.3 dB(A) from its previous calibration level. Subsequent calibration intervals shall not exceed 1 h for sound level measurements of vehicles operated on the highway.

(d) Meter Reading--The recorded reading shall be the highest sound level obtained as the vehicle passes by, disregarding unrelated peaks due to extraneous ambient noises.

(e) Ambient Sound--Measurements shall be made only when the A-weighted ambient sound level, including wind effects and all sources other than the vehicle being measured, is at least 10 dB(A) lower than the sound level of the vehicle.

(f) Wind--Sound level measurements shall be made only when the wind velocity is 19.3 km/h (12 mph) or lower with gusts not exceeding 33.2 km/h (20 mph).

(g) Precipitation--Except for snowmobiles in Section 1046, measurements shall not be made during precipitation, or when the road surface is wet or covered with snow or ice.