An
air horn is a pneumatic device designed to create an extremely loud
noise for signalling purposes. It usually consists of a source which produces
compressed air, which passes into a
horn through a
reed or diaphragm. The stream of air causes the reed or diaphragm to vibrate, creating sound waves, and the horn amplifies the sound so it is louder. Air horns are widely employed as
vehicle horns, installed on large
semi-trailer trucks,
fire trucks,
trains, and some
ambulances as a warning device, and on ships as a signalling device.