Next on the list of simple electronic devices I think are cool is the Electret Microphone (also called the Electret Condenser Microphone). This simple, inexpensive, tiny device can turn sound waves into electrical signals that your phone or walkie-talkie can detect. Let's see how it works!

Electret microphones typically look like this...
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Electrets and Magnets

Before understanding the microphone, we need to understand the electret. Electrets are basically the electric equivelant of a magnet.

The magnets on your fridge are approximated as magnetic dipoles. An ideal magnetic dipole is essentially two opposite magnetic monopoles infinitely close together (magnetic monopoles are the electric charge equivalent of magnetism, and they don't exist as far as anyone knows). A permanent magnet acts somewhat like this: It has a magnetic dipole moment caused by the dipole moments of electrons lining up in the same direction. Magnets of course have a North and a South side - opposite poles attract, like poles repel (in general).

A permanent magnet
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An electret works just like this, but with electric fields and electrostatics instead of magnetic fields and magnetic dipoles. Many molecules are polarized, that is, they carry a net electric dipole. This is just like a magnetic dipole, but instead it acts like it has negative charge on one end and positive charge on the other end.

A common polarized molecule is water.
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If you melt a polarized material and apply an electric field, the electric dipoles will all experience a torque towards the electric fields lines. This will make most of the polarized molecules line up their electric dipole moments. When the material cools back down, the dipoles can't move as easily, and you now have a solid chunk of material with a net electric dipole: An Electret.

Electrets produce external electric fields just like magnets produce external magnetic fields. If you had two perfect electrets, they would act a lot like magnets - repelling and attracting depending on their orientation.

Note that electrets are somewhat permanent and stable - any material will polarize in an electric field, but electrets keep that polarization.
But electrets are going to act a little differently than magnets in practice because electric charges exist, and magnetic monopoles don't.

Capacitors

Now to change gears into the world of capacitors. Don't worry, this will be relevant as you'll see later.

If you take two conductive metal plates and hook up a battery across them, an electric field will be produced between the plates. This will induce equal and opposite charges on the plates depending on the voltage of the battery. This is the simplest capacitor, and said capacitor can store energy inside itself using the electric field: If you charge up a capacitor, remove it from the circuit, and connect it to another circuit, it will act somewhat like a very low capacity battery.

Parallel-Plate Capacitor
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This is the basic principle behind the thousands of tiny capacitors that allow essentially every single electronic device to work.

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Every capacitor has a quality known as its capacitance, which is based on its dimensions and material. Capacitance can be though of as how "big" a capacitor is - in more technical terms, it is the amount of charge a capacitor can hold per unit voltage. This is given by the most basic capacitor equation:

C * V = Q [5]

...Where C is capacitance, V is voltage across the capacitor, and Q is the charge stored on the capacitor.

It's this equation that will show us ...

How the electret microphone works

At the heart of the electret microphone is a parallel-plate capacitor like the one described above. One of the plates is made of a conductor and is fixed to the bottom of the microphone housing. The second plate is a thin film and is attached to a thin film of insulating material. This insulator is a permanent electret - it has a stable electric polarization and acts as an approximate electric dipole.

This elecret-modified capacitor works to produce voltage fluctuations (a signal) from sound waves when you talk into the microphone. Let's look at a brief example.

At the start, the capacitor has a fixed capacitance. This capacitance depends on how close together the two parallel plates are. Since the top plate has an electret embedded in it, charges are in induced in the bottom plate due to the external electric field produced by the electret.

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Imagine a sound wave comes in and hit the capacitor. Sound waves are just waves of air pressure, so this wave can move objects in the air. Remember that the top plate of the capacitor is a thin-film: Thus when the sound wave hits the film, it deforms it, causing it move towards or away from the other capacitor plate.

But capacitance depends on how far the plates are apart! If the top plates then moves closer to or farther away from the fixed bottom plate, the capacitance is going to change. However, the total charge Q on the capacitor doesn't change. By the above equation Q = CV, if C changes, V must also change accordingly to ensure that Q doesn't change.

By including the electret in the top plate, you don't have to apply the voltage to the capacitor to measure input sound waves. The electret handles polarizing the capacitor for you - no external field is required.

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This means that the voltage on the capacitor varies. Measure the output wires and you have a fluctuating voltage - an electrical signal carrying the sound in the sound waves. In this way, sound can be converted into an electrical signal! You can even harvest usable energy with this if the sound is loud enough.

Of course, it's usually a bit more complicated than that, as most things are (there's typically a transistor and some resistors inside to amplify and isolate the signal). But that's the basic working of one of the most common types of microphones.

Last time you made a phone call, you could have been using an electret.

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I hope you learned something from this today! If you have any comments, questions, or corrections, please let me know. I find this device pretty cool, as it uses a strange electric magnet analogue that doesn't come up very much, so I hope you found the post interesting.

Thanks for reading!

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