How does a microphone turn sound into electricity?

A microphone is a transducer: a device that changes one kind of energy into another, here sound into an electrical signal. Sound is a pattern of changing air pressure, so most microphones start the same way, with a thin diaphragm that the sound pushes back and forth. What differs is how the movement becomes electricity.

In the dynamic microphone, a small coil of wire is attached to the diaphragm and sits in the field of a permanent magnet. When sound moves the diaphragm it moves the coil through the magnetic field, and that produces a varying voltage by electromagnetic induction. This is exactly how a loudspeaker works, only reversed. In a condenser microphone the diaphragm is one plate of a capacitor, and the vibrations change the capacitance, which is measured as the signal. A third kind, the contact microphone, uses a piezoelectric crystal.

The oldest kind was the carbon microphone, the first microphone that made proper voice telephony possible. Its diaphragm squeezes carbon granules, and the changing pressure changes their electrical resistance, so the current flowing through them rises and falls with the sound. It was developed independently in England by David Hughes and in the United States by Emile Berliner and Thomas Edison, and the patent was fought over in court. Most historians credit Hughes with the invention.

Because a speaker has a diaphragm, a coil and a magnet, it works as a microphone too. Studios even use this: a woofer placed in front of a bass drum makes a microphone for the kick drum.