Inside a stereo speaker, you will typically find a single permanent magnet attached to the frame of the speaker, and a voice coil attached to the cone. The voice coil is made of wire and is suspended in the gap between the magnet and the pole piece. When an electrical signal is passed through the voice coil, it creates a magnetic field that interacts with the permanent magnet, causing the voice coil and the cone attached to it to move back and forth, producing sound waves.