FMSCA Regulation 325.79

§325.79 Application of correction factors.

(a) If two correction factors apply to a measurement they are applied cumulatively.

(b) The following examples illustrate the application of correction factors to sound level measurement readings:

(b)(1) Example 1 - Highway operations. Assume that a motor vehicle generates a maximum observed sound level reading of 86 dB(A) during a measurement in accordance with the rules in Subpart D of this part. Assume also that the distance between the microphone location point and the microphone target point is 60 feet (18.3 m) and that the measurement area of the test site is acoustically “hard.” The corrected sound level generated by the motor vehicle would be 85 dB(A), calculated as follows:

  • 86 dB(A) Uncorrected reading
    + 1 dB(A) Distance correction factor
    −2 dB(A) Ground surface correction factor
    ——
    85 dB(A) Corrected reading

(b)(2) Example 2 - Stationary test. Assume that a motor vehicle generates maximum sound level readings which average 88 dB(A) during a measurement in accordance with the rules in Subpart E of this part. Assume also that the distance between the microphone location point and the microphone target point is 35 feet (10.7 m), and that the measurement area of the test site is acoustically “soft.” The corrected sound level generated by the motor vehicle would be 87 dB(A), calculated as follows:

  • 88 dB(A) Uncorrected average of readings
    −3 dB(A) Distance correction factor
    + 2 dB(A) Ground surface correction factor
    ——
    87 dB(A) Corrected reading

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