Air 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.

Thus, an outstretched hand reaching upward and pumping is a signal to the driver of an air horn equipped vehicle, requesting a toot.

[1] Consequently, North American trains now have at least two horns with different tones forming the airhorn, that sound simultaneously,[citation needed] creating a harmonic interval or chord.

[2] Fifteen to twenty seconds before entering a level crossing, federal law requires locomotives to sound their horns in a standard warning sequence.

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 bicycle, car, and truck enthusiasts to install large air horns on their vehicles.

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.

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.

[6] Since then, every NHL, AHL (with the exception of the Chicago Wolves, who instead use a siren and fireworks), ECHL, SHL, and CHL team has picked up a horn, including many more leagues.

In mixed martial arts, an air horn is commonly used to signal the end of a round as opposed to the bell used in boxing and professional wrestling.

[8][9][10][11] The 30th and final performance on November 2 was expected to set the Guinness World Record for the number of simultaneously sounding air horns, having had 342 participants.