A dynamic speaker and a dynamic microphone are the same reversible device: coil, magnet, and diaphragm

A dynamic microphone and a loudspeaker share the same electromagnetic transducer: a coil of wire attached to a diaphragm sitting in a magnetic field. In a microphone, acoustic pressure moves the diaphragm and coil, generating a voltage; in a speaker, an electrical signal drives the coil, moving the diaphragm and generating acoustic pressure. Because the physics is identical and reversible, a speaker can serve as a large, idiosyncratic microphone and a dynamic mic can serve as a quiet speaker. The reversal works only for dynamic (coil-and-magnet) transducers: condenser and electret mics use a different principle and require battery or phantom power, so they must not be reversed. Related devices are variations on the same idea — headphones are tiny speakers, and a record-player cartridge is a microphone with a needle. Collins names this the Eighth Rule of Hacking, noting the caution that some things are reversible with interesting results while others are reversible only with irreversible ones: a dynamic mic driven backwards as a speaker is fragile and should not be pushed hard.

Examples

Plug headphones into a recorder’s or mixer’s microphone input and speak into them — you’ll record your voice. Place a large speaker in front of a kick drum as a subwoofer-style microphone (as Motown engineers did). Use a guitar pickup as an antenna to sound out a circuit board.

Assessment

A student plugs a dynamic microphone backwards into a speaker cabinet — predict the result. Then explain why you can use a headphone as a microphone but must NOT use a condenser microphone as a speaker, referring to the difference in transduction principle.