40 CFR 201.22: Measurement instrumentation.

(a) A sound level meter or alternate sound level measurement system that meets, as a minimum, all the requirements of American National Standard S1.4-1971 1 for a Type 1 (or S1A) instrument must be used with the “fast” or “slow” meter response chacteristic as specified in Subpart B. To insure Type 1 response, the manufacturer's instructions regarding mounting or orienting of the microphone, and positioning of the observer must be observed. In the event that a Type 1 (or S1A) instrument is not available for determining non-compliance with this regulation, the measurements may be made with a Type 2 (or S2A), but with the measured levels reduced by the following amount to account for possible measurement instrument errors pertaining to specific measurements and sources:

Table 1—Sound Level Corrections When Using a Type 2 (or S2A) Instrument

Measurement section Source Decibels 1

201.24 Locomotives 0

Rail cars 0

Locomotive load cell test stand 0

201.26 Retarder 4

Car coupling 2

201.27 Locomotive load cell test stand 0

Stationary locomotive 0

1 Amount of correction to be subtracted from measured level (dB).

(b) A microphone windscreen and an acoustic calibrator of the coupler type must be used as recommended by: (1) the manufacturer of the sound level meter or (2) the manufacturer of the microphone. The choice of both devices must be based on ensuring that Type 1 or 2 performance, as appropriate, is maintained for frequencies below 10,000 Hz.