Loudspeaker Design

A loudspeaker represents a way of converting electrical signals to sound signals. All speaker do this by having the electrical signal exert some sort of force on a "diaphram", a relatively large, more or less flat piece of material which is made to vibrate by the force applied to it. There are a number of ways of having the electrical signal exert forces on the diaphram, but by far the most common is the "dynamic" loudspeaker, in which an electrical current flowing through a coil of wire which is immersed within a magnetic field is used.

It was already discovered by Ampere about 200 years ago, that if you had a magnetic field and a wire carried a current through that field, then that wire would feel a force on it proportional to the current in the wire, and proportional to the strength of the magnetic field. That force was in a direction perpendicular to the wire, and also perpendicular to the direction of the magnetic field.

Thus, in a dynamic loudspeaker, a wire is wrapped around a cylinder many times (to increase the force since each piece of wire will feel the same force since each carries the same current.) That cylinder is then immersed in a strong magnetic field ( Good loudspeakers use some of the strongest permanant magnets available). The amplifier then, by raising and lowering the voltage, drives more or less current through the coils of the loudspeaker, producing more or less force on the diaphram.