Aural sound signal

An aural sound signal is a sound made by a vessel’s whistle, bell, or gong to convey a maneuvering, warning, or restricted-visibility message under the COLREGs. Rule 32 defines the whistle and the prescribed blast lengths: a short blast is about one second, a prolonged blast four to six seconds. Rules 34 and 35 set the meanings, one short for an alteration to starboard, two short to port, five or more short and rapid for doubt or danger, and the fog signals at intervals not exceeding two minutes. Annex III fixes the whistle’s frequency band and audibility range by vessel length. The signals work by ear when sight is lost, which is why their timing is exact.