20.6 - Loudspeakers

How a loudspeaker works

A loudspeaker uses the motor effect to turn an electrical signal into sound. (Headphones work in exactly the same way.)

The structure and how it moves

1. A coil of wire is wrapped around the base of a cone and sits in the field of a permanent magnet.
2. An alternating current passes through the coil.
3. The current in the magnetic field experiences a force, pushing the coil one way.
4. When the current reverses, the direction of the force reverses, pulling the coil back the other way.
5. As the current keeps alternating, the coil and the cone attached to it move back and forth.

Producing the sound

• The vibrating cone makes the surrounding air vibrate, creating pressure variations that travel out as a sound wave.
• The frequency of the sound matches the frequency of the alternating current.
• A larger current creates a larger force, making the cone move further and producing a louder sound.