Microphones and speakers: the same trick, both ways

A speaker and a dynamic microphone are built from the same three parts: a coil of wire, a permanent magnet and a diaphragm (a cone or thin sheet).

• Speaker: electric current in → coil and cone move → sound out.
• Microphone: sound in → diaphragm and coil move → electric current out.

Learn one direction properly and you get the other almost free.

"Describe how sound is produced by a speaker from a digital sound file", or the microphone version. Usually 3 to 5 marks.

The marks are for the chain: digital ↔ analogue conversion, amplification, coil and magnet, vibration, sound waves.

1. The digital sound data is sent to a DAC (digital-to-analogue converter), which turns it into a varying analogue electrical signal.
2. The signal is amplified.
3. The varying current flows through the coil, making it an electromagnet whose magnetic field keeps changing.
4. The coil's changing field interacts with the permanent magnet, so the coil is pushed and pulled back and forth.
5. The coil is fixed to the cone (diaphragm), so the cone vibrates too.
6. The cone pushes and pulls on the air, making sound waves that match the signal.

Step 4 is where marks vanish. "The current makes the cone move" skips the physics: it's the magnet pushing on the coil.

1. Sound waves (pressure changes in the air) make the diaphragm vibrate.
2. The diaphragm moves a coil back and forth inside the field of a permanent magnet.
3. Moving a coil in a magnetic field induces a small varying electric current: an analogue signal with the same pattern as the sound.
4. The signal is amplified, then an ADC (analogue-to-digital converter) samples it to produce digital data.