Microphones

Quick Answer

A microphone converts sound pressure into a voltage. A dynamic microphone moves a coil through a magnetic field and generates its own signal from nothing. An electret holds a permanent charge on a capacitor plate and needs a supply for the amplifier built into it. A MEMS microphone is the same capacitor etched into silicon, amplifier included.

A loudspeaker run backwards is a microphone, and the dynamic type is exactly that: a diaphragm carrying a light coil that sits in a magnet's field, so pressure moves the coil, the coil cuts flux and a voltage appears across it. Nothing powers it — the sound does the work — which is why a dynamic microphone survives being unplugged, dropped and shouted into, and why its output is small enough to need a preamplifier before anything else can use it.

The other two are capacitors. An electret condenser microphone has one plate as a thin diaphragm and the other fixed a few micrometres away, with a permanently charged film holding the charge in place; sound moves the diaphragm, the spacing changes, the capacitance changes and the voltage across a fixed charge changes with it. That signal comes from a capacitance of a few picofarads, so it cannot drive a cable — every electret capsule therefore has a small FET inside it, and the two terminals you solder to are the FET's drain and source rather than the capsule's plates. This is why an electret needs a resistor to a supply on its output pin and a dynamic microphone does not, and why wiring one as though it were the other yields silence.

A MEMS microphone is that capacitor built by etching silicon, in a package a couple of millimetres across, with the amplifier on the same die and often a sigma-delta converter beside it — so the part that leaves the package is a digital stream rather than a signal you can spoil on the way. Two practical consequences follow. The port is a hole in the package that must line up with a hole in your enclosure, and its acoustic performance depends on that alignment as much as on the part. And because these arrive by reflow like any other chip, the heat profile is part of the specification, not an afterthought.