Section 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 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 | Decibels1 |
| 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.