An air horn is a pneumatic device designed to create an extremely loud noise for signaling 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 signaling device.
In trucks, the air horn is powered with compressed air from the vehicle’s air brake system. A cord mounted on the ceiling of the operator’s cab is pulled to open the valve, supplying varying amounts of air to the horn. Thus, an outstretched hand reaching upward and pumping is a signal to the driver of an air horn equipped vehicle, requesting a toot.
In modern trucks the horn is actuated by a button on the steering wheel (just like a normal car horn). Some trucks have both electric and air horn, selectable by a switch on the dashboard. This is to prevent the use of the powerful air horn in populated areas.