(a) Highway operations.

When measurements are made in accordance with Part 452 of this Title upon a test site which is “hard”, a correction factor of two 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 sound level limits set forth in section 386(3)(a) of the New York State Vehicle and Traffic Law.

(b) Stationary test.

When measurements are made in accordance with Part 453 of this Title upon a test site which is “soft”, a correction factor of two 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 sound level limits set forth in section 386(3)(b) of the New York State Vehicle and Traffic Law.