An electric horn is a tapered sound guide designed to provide an acoustic impedance match between a sound source and free air. This has the effect of maximizing the efficiency with which sound waves from the particular source are transferred to the air. Conversely, an electric horn can be used at the receiving end to optimize the transfer of sound from the air to a receiver.
Classic horns usually consist of a single electric horn operating at a single resonance frequency, with a reed made of steel located in the throat of the electric horn, and supplied with air by a rubber squeeze bulb. Other variations of electric horn include battery operated klaxon electric horn, and small air horns powered by a small can of compressed gas.
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.
Truck horns may be electric horn klaxons of similar design, but often are purely acoustic, driven by air from an air compressor which diesel trucks have already on board to operate the air brakes. Such electric horn is often used as trim items, with chromed straight horns mounted on top of the cab. This design may also be installed on customized automobiles, using a small electrical compressor. Usually one electrical horn is used, sometimes more. The frequencies vary in order to produce a variety of different chords, but in general are lower than those of automobile horns; for instance 125 through 180 hz (approximately b2-f#3). Sound levels of an electric horn are approximately 117-118 decibels.