Foghorn
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- This page is about the navigational aid called foghorn, for the Ray Bradbury science fiction short story, see The fog horn
The noise produced by a foghorn is very deep, due to the fact that deep sounds are audible to human ears at a greater distance than higher pitched ones. It is also very loud so ships a considerable distance away can heed its warning. Legend suggests that Robert Foulis heard his daughter playing piano in the distance on a foggy night, he noticed the low notes were more audible than the higher notes.
All foghorns use a vibrating column of air to create an audible tone, but the method of setting up this vibration differs. Many older foghorns, especially those on land, used diaphones to create the audible sound, producing a distinctive, deep and penetrating tone followed by an all-too-audible 'grunt', resulting in the famous two-tone sound that most people acquaint with foghorns and the sea. Other horns used vibrating plates, similar to a modern electric car horn, or air forced through holes in a revolving cylinder, much in the same manner as a siren (noisemaker).
When diaphone foghorns were still being constructed, the U.S.A. lighthouses usually had a two tone horn. Such horns usually sounded an E♭ quarter note followed by an A♭ half or whole note. English-built foghorns were usually single-tone horns, producing a long single note, usually an F or a G. Many Canadian foghorns are single-tone.
