How does an electromagnet work in a headphone?

A speaker makes sound by rapidly vibrating a paper cone or diaphragm. There is an electromagnet attached to the diaphragm. When current flows through the coil one way it becomes a magnet . Since there is a permanent magnet behind the coil, the coil is pulled toward the magnet. When the current is shut off, the diaphragm springs back to neutral. When current flows the other way, the coil pushes away from the magnet. This cycle is repeated thousands of times in a second and creates sound that corresponds to the electrical signal that it is sent.

A dynamic microphone uses the same arrangement of a diaphragm attached to a coil near a magnet. The process is just the opposite. Instead of using electricity to make sound, sound is used to make an electrical signal that can be recorded.

Incoming sound waves push the diaphragm in and out which drives the coil towards and away from a magnet. An electrical current is created as the coil moves through the magnet's field. This current is a signal that corresponds to the sound received by the microphone.

As you can see, an electromagnetic coil is central to the operation of a speaker and a microphone. The magic of the coil is that it can act as a variable magnet, changing both in intensity and polarity. It also changes motion to electricity and electricity into motion.