FMSCA Regulation 325.73
325.73 Microphone distance correction factors.1
Table 1, in 325.7 is a tabulation of the maximum allowable sound level readings taking into account both the distance correction factors contained in 325.73 and the ground surface correction factors contained in 325.75.
If the distance between the microphone location point and the microphone target point is other than 50 feet (15.2 m), the maximum observed sound level reading generated by the motor vehicle in accordance with 325.39 of this part or the numerical average of the recorded maximum observed sound level readings generated by the motor vehicle in accordance with 325.59 of this part shall be corrected as specified in the following table:
| If the distance between the microphone location point and the microphone target point is | The value dB(A) to be applied to the observed sound level reading is |
|---|---|
| 31 ft (9.5 m) or more but less than 35 ft (10.7 m) | 4 |
| 35 ft (10.7 m) or more but less than 39 ft (11.9 m) | 3 |
| 39 ft (11.9 m) or more but less than 43 ft (13.1 m) | 2 |
| 43 ft (13.1 m) or more but less than 48 ft (14.6 m) | 1 |
| 48 ft (14.6 m) or more but less than 58 ft (17.7 m) | 0 |
| 58 ft (17.7 m) or more but less than 70 ft (21.3 m) | +1 |
| 70 ft (21.3 m) or more but less than 83 ft (25.3 m) | +2 |
