Klaxon

A horn is a sound-making device that can be equipped to motor vehicles, buses, bicycles, trains, trams (a.k.a. streetcars in North America), and other types of vehicles. The sound made usually resembles a "honk". The vehicle operator uses the horn to warn others of the vehicle's approach or presence, or to call attention to some hazard. Motor vehicles, ships and trains are required by law in some countries to have horns. Bicycles like trams, trolley cars and streetcars in many areas are also legally required to have an audible warning device, but not universally, and not always a horn.

Bicycles often have a classic bulb horn, operated by squeezing a rubber bulb attached to a metal horn. Squeezing the bulb forces air through a steel reed located in the throat of the horn, making it vibrate, producing a single note. The flaring horn matches the acoustic impedance of the reed to the open air, radiating the sound waves efficiently, making the sound louder. Other types of horns used on bicycles include battery-operated horns (sometimes even car horns on 12-volt circuits are incorporated) 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. Car horns are usually electric, driven by a flat circular steel diaphragm that has an electromagnet acting on it and is attached to a contactor that repeatedly interrupts the current to that 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 of typical car horns are approximately 107–109 decibels, and they typically draw 5–6 amperes of current.

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