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, then the horn amplifies the sound making it 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.
An air horn consists of a flaring metal or plastic horn or trumpet (called the "bell") attached to a small air chamber containing a metal reed or diaphragm in the throat of the horn. Compressed air flows from an inlet line through a narrow opening past the reed or diaphragm, causing it to vibrate, which creates sound waves. The flaring horn serves as an acoustic "transformer" to improve the transfer of sound energy from the diaphragm to the open air, making the sound louder. In most horns it also determines the pitch of the sound. When vibrated by the diaphragm, the column of air in the horn vibrates in standing waves. The length of the horn determines the wavelength of the sound waves generated, and thus the fundamental frequency (pitch) of the note produced by the horn. The longer the horn, the lower the pitch.
Larger air horns used on ships and foghorns function similarly to a whistle; instead of a diaphragm the air escapes from a closed cylindrical resonator chamber through a precisely shaped slit directed against a knife edge (fipple). The air blowing past the knife edge oscillates, creating sound waves. The oscillations excite standing waves in the resonator chamber, so the length of the chamber determines the pitch of the note produced.
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.
Many fire trucks, ambulances, and other large emergency vehicles operate air horns as a means of warning vehicles to clear the right-of-way.
There are also electronic horns for emergency vehicles, which produce a similar easily recognizable sound. These are typically integrated into the same system as the vehicle's electronic siren, and sound through the same speakers. In the last several decades, electronic sound systems with more widely varying frequencies have been chosen as common supplemental warning systems.
Originally, diesel locomotives were equipped with truck horns. After an accident in which a driver mistook a train for a truck, the need for a unique-sounding train horn became clear. Consequently, North American trains now have at least two horns with different tones forming the airhorn, that sound simultaneously, creating a harmonic interval or chord. Each individual horn is called a "chime". Three and five-chime configurations are the most common, but two chime horns also exist.
Fifteen to twenty seconds before entering a level crossing, federal law requires locomotives to sound their horns in a standard warning sequence. This succession consists of two long, one short, and one long horn sounding repeated as necessary until the locomotive clears the crossing. Exceptions to federal law occur in locations with established quiet zone ordinances that prohibit sounding locomotive horns.
In recent years, it has become a fad for car and truck enthusiasts to install large air horns on their vehicles. Some jurisdictions do not allow an airhorn to be attached, whether or not it can be activated.
Portable air horns are also readily available packaged with a can of compressed air as the air source. These are often sounded by fans at sporting events such as American football, basketball, hockey and association football, and at other events such as graduations.
Small versions are sometimes used as bicycle horns, since it yields a louder warning sound than traditional bicycle bells or bulb reed horns.
Another use is as a non-lethal weapon for self-defense, mainly as an auditory distraction to get away from an attacker. For outdoor activities like hiking, hunting, cross-country skiing, canoeing, fishing, an air horn can be handy to frighten away unwanted or aggressive wildlife, signalling for help and to announce one's location.
At close range, they can also be used as freeze sprays (when the container is held upside-down).
Additionally, air horns (especially those that contain fluorocarbons) have the potential to be abused as a substitute for recreational drugs since many such refrigerants can be inhaled for a quick and dangerous intoxication.
The air horn is used for signalling in various sports:
NHL arenas normally employ two horns -- a high pitch horn (or a siren, as with the Boston Bruins and Montreal Canadiens' home rinks) that announces the end of the period (which is also used to indicate the start of instant replay reviews), and a much louder, lower pitch horn that is sounded when the home team scores and/or wins a game. Some MLB stadiums use horns for similar purposes, such as when a member of the home team hits a home run. In many places, air horns are used for signaling the end of the period or quarter on scoreboard systems. The entire idea of a goal horn in NHL ice hockey is said to have begun in 1974, when Bill Wirtz, then-owner of the Chicago Blackhawks, liked the sound of the horn on his yacht so much, that he had another example of it mounted inside of the "Madhouse on Madison" to be sounded whenever the Blackhawks scored a goal in their home rink.
Samples of various air horns have been used by NHL teams when the home team scores a goal, often accompanied by music.
The air horn is also a popular sample in reggae music. Jamaican dancehall music was the first musical genre to use the effect, and has been using the airhorn sample for over 26 years, in live shows as well as on mixtape recordings, and in Puerto Rican reggaeton, a reggae hybrid genre since the late 80s and 90s.  The sound effect has recently been used in hip hop music as well.
The "Air Horn Orchestra" gathered weekly every Wednesday from April 13, 2016 until the November 2, 2016 at the North Carolina Governor's Mansion to protest HB2 and Governor Pat McCrory by making a racket. The 30th and final performance is expected to set the Guinness World Record for the number of simultaneously sounding air horns, having had 342 participants. This was the last performance owing to the gubernatorial election 6 days later.
The Islanders and hockey fans everywhere can thank the Chicago Blackhawks for pioneering the now-ubiquitous trend during the 1973 Stanley Cup Final against Montreal. Hawks owner Bill Wirtz liked the sound of the horn on his yacht and had it installed at Chicago Stadium..."(He) decided it would be a great thing to have in his arena," Erick Kahlenberg, vice president of Kahlenberg Industries told the Daily News...Kahlenberg's uncle Karl sold Wirtz the yacht horn and now the company supplies more than a dozen NHL teams with their own goal horns, which are the same devices you find on boats, trains, trucks and in one case, an aircraft carrier. When the Blackhawks moved from Chicago Stadium to the United Center in 1994, they took the goal horn with them. By then, goal horns had become a staple of NHL games around the league.