In a loudspeaker, a diaphragm is the thin, semi-rigid membrane attached to the central magnet. The magnet induces the diaphragm to vibrate, producing sound.

Using this in reverse converts air vibrations (sound) into electrical signals, as in several common designs of microphone.

Similarly, the eardrum uses this same principle, using a diaphragm to stimulate nerves to transmit a neural "image" of sound to the brain.