33 CFR § 62.47 - Sound signals.
(a) Often sound signals are located on or adjacent to aids to navigation. When visual signals are obscured, sound signals warn mariners of the proximity of danger.
(1) Sound signals are distinguished by their tone and phase characteristics.
(i) Tones are determined by the devices producing the sound (i.e., diaphones, diaphragm horns, reed horns, sirens, whistles, bells and gongs).
(ii) Phase characteristics are defined by the signal's sound pattern, i.e., the number of blasts and silent periods per minute and their durations. Sound signals emanating from fixed structures generally produce a specific number of blasts and silent periods each minute when operating. Buoy sound signals are generally actuated by the motion of the sea and therefore do not emit a regular signal characteristic.
(2) Where no live watch is maintained, sound signals are normally operated continuously. However, most are equipped with Mariner Radio Activated Sound Systems (MRASS) that are activated by the mariner by keying their VHF radio microphone five (5) times on the designated charted frequency. Channels 81a (157.075 MHz) and channel 83a (157.175 MHz) are the two most common frequencies, but others may be designated and charted. (Mariners may consult the appropriate U.S. Coast Guard Light List volume or local notice to mariners for specific activation frequencies and instructions.) Activated signals will normally operate for 45 minutes after the signal is triggered.
(b) Mariners should not rely solely on sound signals to determine their positions for the following reasons:
(1) Distance cannot be accurately determined by sound intensity.
(2) Occasionally sound signals may not be heard in areas close to their location.
(3) Signals may not sound in cases where fog exists close to, but not at, the location of the sound signal.
(4) As buoy signals are generally activated by sea motion, they may produce no signals when seas are calm.
(5) As previously noted, buoy positions are not always reliable. Therefore their sound signals cannot be assumed to be emanating from a fixed position.