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.