A horn is a sound generator controlled by a vehicle driver that can alert other drivers, often to dangerous situations and/or to convey one driver's disapproval of another driver's actions.
Disc Horn
One of the two common designs is the disc horn. When direct current voltage (DC Voltage) is supplied to the horn, the diaphram vibrates in and out to create a single tone. The tone's note and volume are affected by the amount of current passing through it. Since the resistance of the horn is effectively a fixed amount, the voltage applied will change the amperage, and therefore the note.
Trumpet Horn
A trumpet horn has more of a spiral appearance than the disc horn. The trumpet horn also contains a diaphram that vibrates, but the sound travels through the spiral and out of the end, resulting in both a more directed sound and a less harsh tone.
Horn Pairs
Some vehicles are equipped with a single horn, but others are equipped with two, which adds volume and a different note to make it more noticable. The second horn is typically tuned a minor third up or down, so that it is harmonic with the first horn. A pair of horns is usually placed next to each other, which reduces the amount of wiring needed and keeps one horn unit from being louder than the other based on where a listener is standing relative to the vehicle.
Klaxons
Many early car horns were actually klaxons, which gave them a unique sound compared to the uniform tones of modern car horns.