Vehicle horn

Horn of Rolls-Royce Phantom I Open Tourer Windovers (1926)

Modern electric horns mounted in engine compartment

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.

Diagram showing how a car horn works

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.

The Leslie S5T, a locomotive air horn with five 'chimes'

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.

To distinguish their sound from truck and bus air horns, train horns in the U.S. consist of groups of two to five horns (called "chimes") which have different notes, sounded together to form a chord.

In Japan, most modern trains like 209 series or E233 series from the first half of the 1990s onwards use electric horns as primary in passenger use. Although electric horns were used by Seibu 2000 series, air horns were primarily used until the 1990s. Modern Japanese trains may still be equipped with both air horns and electric horns.

Most modern streetcars, trams and trolley cars including low-floor vehicles around the world also employ horns or whistles as a secondary auditory warning signal in addition to the gong/bell which either use the sound of air horns or electric automobile car horns.

Steam whistle from a supertanker, in Merseyside Maritime Museum, UK

Ships signal to each other and to the shore with air horns, sometimes called whistles, that are driven with compressed air or from steam tapped from the power plant. Low frequencies are used, because they travel further than high frequencies; horns from ships have been heard as far as fifteen kilometres (ten miles).[1] Traditionally, the lower the frequency, the larger the ship. The RMS Queen Mary, an ocean liner launched in 1934, had three horns based on 55 Hz (corresponding to A1 ), a frequency chosen because it was low enough that the very loud sound of it would not be painful to the passengers.[2] Modern International Maritime Organization regulations specify that ships' horn frequencies be in the range 70–200 Hz (corresponding to C♯₂-G₃) for vessels that are over 200 m (660 ft) in length. For vessels between 200 and 75 m (660 and 250 ft) the range is 130–350 Hz and for vessels under 75 m (250 ft) it is 70–700 Hz.[3]

Smaller craft typically use electric diaphragm horns.

Portable air horns driven by canned compressed air are also used, as well as for officiating sports events and recreational activities.

Small battery-operated electric horns, as well as the traditional single-reed bulb horn, are typically used on bicycles.

Several languages have either borrowed or transcribed the name into their lexicons. In Japanese, the word "klaxon" (クラクション, kurakushon) refers to car horns in general. This is also true in languages such as French (klaxon), Italian (clacson), Greek (κλάξον), Dutch (claxon), Russian (клаксон), Polish (klakson), Spanish (claxon), Romanian (claxon), Czech (klakson), Turkish (klakson), Indonesian (klakson), and Korean (클랙슨).