33 C.F.R. § 86.05 Sound signal intensity and range of audibility.
Title 33 - Navigation and Navigable Waters
A whistle on a vessel shall provide, in the direction of the forward axis of the whistle and at a distance of 1 meter from it, a sound pressure level in at least one (a) 130–1200 Hz, for a vessel 75 meters or more in length; (b) 250–1600 Hz, for a vessel 20 meters but less than 75 meters in length; (c) 250–2100 Hz, for a vessel 12 meters but less than 20 meters in length.
Title 33: Navigation and Navigable Waters
PART 86—ANNEX III: TECHNICAL DETAILS OF SOUND SIGNAL APPLIANCES
Subpart A—Whistles
§ 86.05 Sound signal intensity and range of audibility.
Table 86.05---------------------------------------------------------------------------------------------------------------- \1/3\- octave For band level Audibility Fundamental measured at 1 meter range in Length of vessel in meters frequency frequencies in dB nautical range (Hz) (Hz) referred miles to 2x10 -5 N/m\2\---------------------------------------------------------------------------------------------------------------- ........... 130-180 145 ..........200 or more................................................... 70-200 180-250 143 2 ........... 250-1200 140 .......... ........... 130-180 140 ..........75 but less than 200.......................................... 130-350 180-250 138 1.5 ........... 250-1200 134 .......... ........... 250-450 130 ..........20 but less than 75........................................... 250-525 450-800 125 1.0 ........... 800-1600 121 .......... ........... 250-450 120 ..........12 but less than 20........................................... 250-525 450-800 115 0.5 ........... 800-2100 111 ..........---------------------------------------------------------------------------------------------------------------- Note. The range of audibility in the table above is for information and is approximately the range at which a whistle may usually be heard on its forward axis in conditions of still air on board a vessel having average background noise level at the listening posts (taken to be 68 dB in the octave band centered on 250 Hz and 63 dB in the octave band centered on 500 Hz).In practice the range at which a whistle may be heard is extremely variable and depends critically on weather conditions; the values given can be regarded as typical but under conditions of strong wind or high ambient noise level at the listening post the range may be much reduced.
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