Microphones that convert sound into an electrical signal by means of electromagnetism. When sound waves hit the microphone, the membrane moves to the rhythm of the sound waves, and the coil on its back moves along with it. The relative movement of the coil within its (stationary) magnetic gap induces a small signal voltage in this coil. There’s your microphone, a device that converts sound into an electrical signal. Moving coil microphones are often preferred for use on stage, because they are quite sturdy and do not require external power.

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