How speakers turn electricity into sound
When things shake or vibrate, they make sounds that we can hear in the world around us. Most of the time the sound is invisible, but sometimes you can really see it! If you hit a kettle drum with a stick, you'll see the taut head jerk up and down a short while later -- blasting sound waves into the air. Speakers work in a similar fashion.
On the front of the speaker, there is a fabric, plastic, paper, or lightweight metal cone (sometimes called a diaphragm), similar to a drumhead (gray in the photo below). The outside of the cone is fixed to the outside of the round metal rim of the speaker. The interior is held in place by an iron coil (sometimes called a voice coil, shown in orange) in a hollow ring in front of a permanent magnet (sometimes called a field magnet, and yellow). When you connect speakers to your stereo system, an electrical signal travels through the speaker cable (red) and into the coil. This turns the coil into a temporary magnet or electromagnet. As current flows back and forth in the cable, the electromagnet attracts or repels the permanent magnet. This moves the coil back and forth, pulling and pushing the speaker cone. Like a drumhead vibrating back and forth, the moving cone pumps the sound into the air. This is why speakers are technically called drivers: they "drive" (move) air to produce sound.
