English: Earphone unit, assembled (left) and disassembled (right). It consists of a permanent magnet (top center) with electromagnet windings around its poles (not visible, under the grease), with its poles near a steel diaphragm (bottom center). The magnetic field of the magnet attracts the diaphragm. When an audio signal current is applied to the electromagnet, it creates a varying magnetic field that alternately strengthens and weakens the magnet's pull on the diaphragm. So the diaphragm moves back and forth, pushing on the air, creating a sound wave.