Car Horns

A motor horn, like any other source of sound, must set up vibrations in the air; the variations in air pressure associated with these vibrations are perceived by the ear as sound. The pitch of the sound is determined by the frequency, i.e., the number of vibrations per second.

Depending on the acoustic pressure (measured in microbars, a unit of pressure corresponding to a water volume of 1/100 mm) and the frequency of the sound, the human ear perceives a certain loudness.

Loudness is measured in Decibels. For a given acoustic pressure, sounds differing in pitch are not perceived as having the same loudness. The range of audibility is bounded by the threshold of hearing at low values of the acoustic pressure, and by the threshold of pain at high values of this pressure.

At frequencies of 2000-5000 cycles/secs, audibility extends down to very low acoustic pressures, i.e., the human ear is particularly sensitive in this frequency range. It so happens that the acoustic pressure of the sound emitted by motor vehicles is relatively low in this same frequency range of 2000 - 5000 cycles/sec. It is therefore advantageous to ensure that the frequencies of the sounds made by a motor horn are within this range: the horn will thus better be able to make itself heard above the general noise of the traffic.

Most of the horns now used as warning devices on motor vehicles are either of the impact type or klaxons.

Both kinds have an electromagnet whose winding is energised when the driver presses the horn button. An iron plate, called the armature, is attracted by the magnet and, in doing so, opens a contact and thus cuts off the current to the electromagnet, which now releases the armature, allowing the latter to spring back to its initial position. The whole cycle of events now repeats itself. The oscillations of the armature are transmitted to a diaphragm which in turn sets up vibrations in the air. The number of vibrations per second is called the fundamental frequency of the horn.

In the impact-type horn the entire diaphragm assembly (comprising armature plate, oscillating beam and diaphragm) strikes the core of the electromagnet. Harmonic oscillations in a narrow frequency range are additionally produced. As a result of this, the sound spectrum of an impact-type horn mainly shows two narrow frequency ranges, so that it can penetrate effectively through noise.

In the case of the klaxon, with its rather softer note, the diaphragm assembly does not hit the core, but can oscillate freely. The sound spectrum of the klaxon has many overtones spread out over a wide frequency range; the pitch of the fundamental tone of the horn is determined mainly by the length of the horn.