FMSCA Regulation 325.75
(a) Highway operations. When measurements are made in accordance with the rules in Subpart D of this part upon a test site which is hard, a correction factor of 2 dB(A) shall be subtracted from the maximum observed sound level reading generated by the motor vehicle to determine whether the motor vehicle conforms to the Standards for Highway Operations, 40 CFR 202.20.
(b) Stationary Test. When measurements are made in accordance with the rules in Subpart E of this part upon a test site which is soft, a correction factor of 2 dB(A) shall be added to the numerical average of the recorded maximum observed sound level readings generated by the motor vehicle to determine whether the motor vehicle conforms to the Standard for Operation Under Stationary Test, 40 CFR 202.21.
