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A vehicle horn is a sound-making device used to warn others of the approach of the vehicle or of its presence. Automobiles, trucks, ships, and trains are all required by law to have horns. Bicycles are also legally required to have an audible warning device in many jurisdictions, but not universally nor always a horn.


Horn types


Classic bicycle horns usually consist of a single horn operating at a single resonance frequency, with a reed made of steel located in the throat of the horn, and supplied with air by a rubber squeeze bulb. Other variations include battery operated klaxon horns, and small air horns powered by a small can of compressed gas.


Oliver Lucas of Birmingham, England developed a standard electric car horn in 1910. Automobile horns are usually electric klaxons, driven by a flat circular steel diaphragm that has an electromagnet acting upon it and is attached to a contactor that repeatedly interrupts the current to the electromagnet. This arrangement works like a buzzer or electric bell and is commonly known as "sounding" or "honking" one's horn. There is usually a screw to adjust the distance/tension of the electrical contacts for best operation. A spiral exponential horn shape (sometimes called the "snail") is cast into the body of the horn, to better match the acoustical impedance of the diaphragm with open air, and thus more effectively transfer the sound energy. Sound levels are approximately 107-109 decibels, and current draw 5-6 amperes.

Horns can be used singly, but are often arranged in pairs to produce a chord consisting of two notes, sounded together; although this only increases the sound output by 3 decibels, the use of two differing frequencies with their beat frequencies and missing fundamental is more perceptible than the use of two horns of identical frequency, particularly in an environment with a high ambient noise level. Typical frequencies of a pair of horns of this design are 500 and 405–420 Hz (approximately B4 and G#4).

Some cars, and many motor scooters or motorcycles, now use 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 are approximately 109-112 decibels, and current draw 2.5-5 amperes. Again, these horns can be either single, or arranged in pairs; typical frequencies for a pair are 420-440 and 340–370 Hz (approximately G#4-A4 and F4-F#4) for this design.

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