Vehicle horn

A horn is a sound-making device installed on motor vehicles, trains, boats, and other types of vehicles. The sound it makes usually resembles a “honk” (older vehicles) or a “beep” (modern vehicles). The driver uses the horn to warn others of the vehicle's presence or approach, or to call others’ attention to a hazard. Motor vehicles, ships and trains are required by law in some countries to have horns. Public transit vehicles and even bicycles are also legally required to have an audible warning device in many areas.

Modern car horns are usually electric, driven by a flat circular steel diaphragm that has an electromagnet acting on it in one direction and a spring pulling in the opposite direction. The diaphragm is attached to contact points that repeatedly interrupt the current to that electromagnet causing the diaphragm to spring back the other way, which completes the circuit again. This arrangement opens and closes the circuit hundreds of times per second, which creates a loud noise like a buzzer or electric bell, which sound enters a horn to be amplified.

A traditional style automobile horn includes an expansion chamber cast into its body, once spiral shaped, to better match the acoustical impedance of the diaphragm with open air, and thus more effectively transfer the sound energy. Sound levels of typical car horns are approximately 107–109 decibels, and they typically draw 5–6 amperes of current.

Horns can be used singly, but are often arranged in pairs to produce an interval consisting of two notes, sounded together; although this doubles the sound volume, the use of two differing frequencies is more perceptible to the human ear than two horns of the same frequency, particularly in an environment with a high ambient noise level. Typical frequencies of a pair of horns of this design are 500 Hz and 405–420 Hz (approximately B₄ and G♯₄, minor third).

Most cars, motorcycles, and motor scooters have for some time used a cheaper and smaller alternative design, which, despite retaining the name "horn," abandons the actual horn ducting and instead relies on a larger flat diaphragm to reach the required sound level. Sound levels of such horns are approximately 109–112 decibels, and they typically draw 2.5–5 amperes of current. Again, these horns can be either single, or arranged in pairs; typical frequencies for a pair are 420–440 Hz and 340–370 Hz (approximately G♯₄–A₄ and F₄–F♯₄) for this design.

A horn grille is a part of some designs of car or other motor vehicle that has an electric horn, such as a motor scooter.

Larger, louder air horns, as found on trucks (lorries) and buses, are driven by air compressors, or supplied by reservoirs charged to operate their air brakes. The compressor forces air past a diaphragm in the horn's throat, causing it to vibrate. Usually two are used, with varied frequencies generally lower than those of automobile horns, in the 125–180 Hz (approximately C₃–G₃) range. Sound levels are approximately 117–118 decibels.

In order to be heard at the longer distances that trains require to warn of danger or stop, their locomotives have pneumatic train horns that are both louder and lower in frequency than motor vehicle horns. Operated by compressed air from the train's air brake system, their sound level is 146–175 dB. In the United States, train horns are required to have a minimum sound level of 96 dB and a maximum sound level of 110 dB at 100 ft (30 m) in front of the train.