FMSCA Regulation 325.9
325.9 Measurement tolerances.(a) Measurement tolerances will be allowed to take into account the effects of the following factors:
(a)(1) The consensus standard practice of reporting filed sound level measurements to the nearest whole decibel.
(a)(2) Variations resulting from commercial instrument tolerances.
(a)(3) Variations resulting from the topography of the noise measurement site.
(a)(4) Variations resulting from atmospheric conditions such as wind, ambient temperature, and atmospheric pressure.
(a)(5) Variations resulting from reflected sound from small objects allowed within the test site.
(a)(6) The interpretation of the effects of the above cited factors by enforcement personnel.
(b) Measurement tolerances shall not exceed 2 decibels for a given measurement.
