How does a microphone turn sound into a signal?

A recorded voice begins as moving air, but a recorder needs an electrical signal. In a dynamic microphone, a thin diaphragm moves back and forth as sound pressure changes. A light coil of wire is attached to it, so the coil follows that movement.

The coil moves in a magnetic field supplied by a magnet. Its movement through the field induces a voltage across the coil's ends. The voltage changes as the motion changes, carrying a changing electrical pattern related to the arriving sound. This conversion from one physical form to another is called transduction.

Direction matters. In the chosen wiring convention, the illustrated inward movement gives a relative signal of +1; reversed movement gives −1. These are illustrative signed values, not volts measured from a particular microphone. Swapping the output connections would reverse the labels without reversing the sound.

Movement matters too: a diaphragm merely held in a displaced position does not keep producing that same induced voltage. Sound makes it move repeatedly, and the resulting changing signal can be amplified, stored or sent onward. The electrical trace can preserve a sound pattern even though no pocket of the original air travels through the cable.