The microphone of a sound level measurement system for all moving vehicle measurements for vehicles over 10,000 pounds GVWR and GCWR shall be located at a height of not less than 2 feet (.6m) or more than 6 feet (1.8m) above the plane of the roadway surface and not less than 3-1/2 feet (1.1m) above the surface on which the microphone stands. The preferred microphone height on flat terrain is 4 feet (1.2m). Stationary Vehicle Measurements for All Vehicles. 10,000 or less GVWR or GCWR. The same procedures as described in Regulation .05B of this chapter are used. Motorcycles. The same procedure as described in Regulation .05B of this chapter are used. Microphone Orientation. The microphone shall be pointed approximately straight upward so that the vehicle noise reaching the microphone from any point within the measurement site strikes the microphone at grazing incidence (parallel to the microphone diaphragm). Meter Operation. The sound level meter shall be operated according to the manufacturer's instructions and this section. The meter shall be set for A-weighting and fast response. The sound level meter shall be calibrated with the acoustic calibrator before the start of use and then at approximately 1 hour intervals when the instrument is in use. The operator may not stand at any point in the measurement site that lies on a line passing through the vehicle and the microphone. An operator, witness, or trainee may be positioned behind the microphone but no closer than 2 feet from the microphone. Background Sound Level. The sound level, including that from other vehicles, wind effects, and all other sources of noise except the vehicle being measured, shall be at least 10 dB lower than the maximum sound level of the vehicle at the time of the measurement. Meter Reading. The sound level reading of a moving vehicle or a stationary vehicle shall be at the highest reading observed for each measurement period.