(a) Microphone Orientation. The microphone shall be oriented in relation to the source of the sound as specified by the microphone manufacturer. Where the instruction manual does not include adequate information, specific orientation recommendations shall be obtained from the manufacturer.

(b) Meter Setting. The sound level meter shall be set for the A-weighted network and slow response.

(c) Calibration Check. An external calibration check shall be made before and after each period of use and at intervals not exceeding 2 h during continued use.

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

(e) Temperature and Humidity. Measurements shall not be made when the temperature is lower than 0° F (-18° C) or higher than 122° F (50° C) or the relative humidity is lower than 10% or higher than 95%. Calibration checks shall be made frequently enough to stabilize the readings under the particular weather conditions.