An air horn is a horn which uses compressed air to create a very loud noise. Big trucks and emergency services vehicles use air horns to alert other drivers to their presence, as do trains and ships. Air horns are used at sports events since their volume level is high enough to be heard over loud audiences of fans. Some people also use personal air horns at sports games, or on their own vehicles, although the use of air horns on cars may not be legal in all regions.
A basic air horn includes a chamber of compressed air which is attached to a valve which allows air to pass through it into a resonant horn. You could think of an air horn as a souped up brass instrument, since these horns work on the same principle. Just like a trombone or a trumpet, an air horn can be calibrated to generate specific notes or tones. In some parts of the world, specific notes are reserved for particular classes of vehicle, and trains actually produce chords, not single notes.
The distinction between different air horn sounds allows people to know not only that something big is approaching, but what that something is. If you hear a chord, for example, you know that a train is on the way, while a stuttering tone is used on emergency services vehicles like fire trucks. On ships, air horns can be used as signals to communicate with other ships in the dark or in poor visibility conditions, when ships might not be able to see each other until it is too late to avert an accident.
The air horns used on trains and ships are quite large, and capable of generating a formidable sound. Personal air horns like those carried to sports games are much smaller; they typically consist of a canister of compressed air and a modest horn which can make a loud but not deafening toot. A personal air horn can also be used as a safety device; joggers, for example, could carry air horns to scare off would be attackers.
The use of air horns on personal motor vehicles like cars and trucks is often restricted or outlawed. This is because the use of air horns on non-commercial vehicles could be confusing or distracting for other drivers. This could potentially pose a danger. For drivers who cannot resist the temptation to install an air horn, moderation is advised; horns should really only be used in emergency situations. You may also want to consider avoiding the use of an air horn in residential neighborhoods, as this could trigger a noise complain to law enforcement.