Most loud speakers consist of a circular permanent magnet surrounding a freely moving coil, which is attached to a cone shaped diaphragm. In the speaker below, the circular magnet is partially cut away so that our visitors can see how it operates.

Alternating current, generated by a microphone, a pickup head, amplifier, radio, or another source, flows through the coil of the speaker. You can observe the operation of a microphone at our Condenser Microphone Java Tutorial.

The current, alternating at the same frequency as the sound waves that generated it, induces an alternating magnetic field in the coil. As the polarity of the magnetic field of the coil alternates, it is alternatively attracted to and repelled by the permanent magnet. This causes the coil to vibrate. The vibrating coil causes the attached cone shaped diaphragm to vibrate and reproduce the sounds generated by the original source.