How does a car horn work?
While some modern car horns are actually specialized computer audio
systems, the old-fashioned electromagnetic car horns are still
common. An electromagnetic horn uses an electromagnet to attract a
steel diaphragm and turns that electromagnet on and off
rhythmically so that the diaphragm vibrates. In fact, it uses the
diaphragm's position to control the power to the electromagnet.
Whenever the diaphragm is in its resting position or even farther
from the electromagnet, a switch closes to deliver electric current
to the electromagnet. The electromagnet then attracts the
diaphragm's center. But when the diaphragm moves closer to the
electromagnet, as the result of this attraction, the switch opens
and current stops flowing to the electromagnet. Because of this
arrangement, the diaphragm moves in and out and turns the
electromagnet off and on as it does. The diaphragm's tone is
determined by the natural resonances of its surface.