Dynamic microphone

Sound is air being squeezed and stretched. The squeeze pushes a thin diaphragm. A coil glued to it slides through a magnet, and moving wire in a magnetic field makes a voltage.

• Sound crosses the 0.5 m to the mic in about 1.5 ms. We slow time down 500× so you can watch.
• The air barely moves. Each bit of air shuffles less than a micrometre back and forth.
• The diaphragm moves only tens of nanometres.
• Voltage = B × l × v: field 1.1 T × 6 m of wire × coil speed. A loud note gives about a millivolt.
• The coil and diaphragm act like a mass on a spring. They respond best near 900 Hz, so the recording sounds slightly different from the room.