Vehicle horn

A horn is a sound-making device that can be equipped to motor vehicles, buses, bicycles, trains, trams (otherwise known as streetcars in North America), and other types of vehicles. The sound made usually resembles a 'honk' (older vehicles) or a 'beep' (modern vehicles). 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. Like trams, trolley cars and streetcars, bicycles are also legally required to have an audible warning device in many areas, but not universally, and not always a horn. A horn is a sound-making device that can be equipped to motor vehicles, buses, bicycles, trains, trams (otherwise known as streetcars in North America), and other types of vehicles. The sound made usually resembles a 'honk' (older vehicles) or a 'beep' (modern vehicles). 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. Like trams, trolley cars and streetcars, bicycles are also legally required to have an audible warning device in many areas, but not universally, and not always a horn. Bicycles sometimes 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 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. 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. 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 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 Hz and 405–420 Hz (approximately B4 and G♯4, minor third). 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 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. The radiators of modern cars no longer determine the shape of the grilles, which have become more abstract, the radiator being of different proportions from the grille and over 15 centimetres behind it. Now grilles are usually designed so the sound of a horn can readily come out through them. Those designs that echo the shape of the grille no longer have front fenders with rather large crevices that accommodate trumpet-shaped horns. Thus some cars, often British ones, have a pair of round horn grilles on either side of the radiator grille, with a horn behind each. A luxury car's horn grilles are usually chrome-plated. Cars with rear engines, such as the Volkswagen Beetle and the early Porsches, necessarily have no radiator grilles in front, and so have horn grilles placed below their headlights. Some motor scooters have this feature as well, placed below the handlebars. Their horn grilles may be made of cheap plastic. These vehicles and the cheaper cars have only one horn. Truck (lorry) horns may be electrically operated and similar to car horns, but are often air horns driven by air from an air compressor, which many trucks have in order to operate the air brakes. The compressor forces air past a diaphragm in the horn's throat, causing it to vibrate. Such air horns are often used as trim items, with chromed straight horns mounted on top of the cab. This design may also be installed on customised automobiles, using a small electrical compressor. Usually two or more are used, some drivers go so far as to install train horns. The frequencies vary to produce a variety of different chords, but in general are lower than those of automobile horns—125–180 Hz (approximately C₃–G₃). Sound levels are approximately 117–118 decibels.