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A foghorn is a device which sounds a prolonged deep tone during conditions of low visibility to alert navigators to various hazards. Because the sound of a foghorn is so distinctive, many people associate foghorns with the sea, since they are often heard on visits to coastal regions. You may also hear the term “foghorn” used to describe someone with a penetrating, booming voice.
Various devices have been used for centuries along coastlines and on board ships to sound warnings during conditions of low visibility. While lighthouses can make excellent navigational aids, they are limited by the visibility in their area, and during especially foggy or snowy conditions, mariners might not spot the light of a lighthouse. As a result, many nations started firing cannons or ringing bells to warn mariners of things like shoals and rocks. By the 1800s, an early version of the automated foghorn had been developed, and today foghorn technology is quite advanced.
Modern foghorns emit laser beams periodically to test for visibility. If the laser indicates that visibility is low, a computer activates the foghorn, while will continue to sound until tests indicate that conditions have cleared again. The notes of foghorns are extremely low because low sounds carry better, providing more of a warning. For large ships which are hard to maneuver, the more warning of navigational dangers, the better.
You often see foghorns mounted on buoys and bridges, and they are also mounted on ships to prevent collisions. Although most ships have advanced navigational software which alerts them if they approach areas which are dangerous to navigate, ships have no way of knowing about approaching boats without foghorns and other signals. Therefore, ships regularly test their foghorns to ensure that they are in good operating condition.
For small boats which lack fancy navigational systems, coastal foghorns can be lifesavers when conditions get unpleasant on the water. Foghorns are also very useful in areas with rapidly changing coastlines, because they act as a reminder to proceed with caution. Incidentally, many foghorns are two-toned because early foghorns used diaphones, specially designed pipes which would produce a long tone followed by a grunting noise as the air in the horn stopped resonating. Although diaphones are no longer used, people find them friendly and familiar, so the distinctive sound has been retained.