49 CFR 325.73: Microphone distance correction factors. 1

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:

Table 2—Distance Correction Factors

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 feet (9.5 m) or more but less than 35 feet (10.7 m) −4

35 feet (10.7 m) or more but less than 39 feet (11.9 m) −3

39 feet (11.9 m) or more but less than 43 feet (13.1 m) −2

43 feet (13.1 m) or more but less than 48 feet (14.6 m) −1

48 feet (14.6 m) or more but less than 58 feet (17.7 m) 0

58 feet (17.7 m) or more but less than 70 feet (21.3 m) + 1

70 feet (21.3 m) or more but less than 83 feet (25.3 m) + 2