40 C.F.R. § 201.21 Quantities measured.
Title 40 - Protection of Environment
The quantities to be measured under the test conditions described below, are the A-weighted sound levels for “fast” or “slow” meter response as defined in the American National Standard S1.4–1971.
Title 40: Protection of Environment
PART 201—NOISE EMISSION STANDARDS FOR TRANSPORTATION EQUIPMENT; INTERSTATE RAIL CARRIERS
Subpart C—Measurement Criteria
§ 201.21 Quantities measured.