A Bit More Than A Microphone: The Electret Story

When designing a microphone assembly the other day, I reached for an electret condenser microphone capsule without thinking. To be strictly accurate I ordered a pack of them, these small cylindrical microphones are of extremely high quality for their relatively tiny price.

It was only upon submitting the order that I had a thought for the first time in my life: Just what IS an electret condenser microphone?

A condenser microphone is easy enough to explain. It’s a capacitor formed from a very thin conductive sheet that functions as the diaphragm, mounted in front of another conductor, usually a piece of mesh. Sound waves cause the diaphragm to vibrate, and these vibrations change the capacitance between diaphragm and mesh.

If that capacitance is incorporated into an RC circuit with a very high impedance and a high voltage is applied, a near constant charge is placed upon it. Since the charge stays constant, changing the capacitance causes a tiny voltage fluctuation that can be retrieved as the audio signal from the microphone. Condenser microphones built in this way can be extremely high quality, but come at the expense of needing a high voltage power supply to supply the charge and an amplifier to buffer and magnify the audio.

Electret microphone capsule schematic. wdwd [CC BY 3.0]
Electret microphone capsule schematic. wdwd [CC BY 3.0]
The electret condenser microphone that I bought for pennies is a capacitor microphone, so why is there no high voltage supply? The answer comes from the material that comprises its diaphragm, an electret is a material that carries a permanent charge without the requirement for a high voltage supply to maintain it. The electret microphone works in exactly the same way as any other capacitor microphone, but the permanent charge on its electret diaphragm is enough to produce a retrievable voltage across it. The capsule will contain an FET as a high-impedance buffer to provide a usable output with a DC supply from a single AA cell.

So having answered the question as to what an electret condenser microphone is, there remains a more fundamental question: just what is an electret? Surely it can’t be possible for a material to retain an electrical charge indefinitely, that’s magic, right? A web search finds some people who appear to think so, as it seems electrets have a crowd of “alternative thinkers” who are searching for means to harvest that static charge as a source of free energy. Clearly some meaningful information is required, and for that we head to the many chemistry and physics lecturers who have put their course materials online.

Dipoles, How Do They Work?

A dipole in a water molecule. Jü [CC0]
A dipole in a water molecule. Jü [CC0]
A dipole, in a chemical sense, is a molecule which possesses a difference in electrical charge between its different parts. At one end of it may be a functional group with a surfeit of electrons, while at the other may be one with an electron deficit. A commonly quoted example of a molecule with this property is water, it forms a V-shaped H-O-H molecule with the oxygen at its point having a greater negative charge and the hydrogens at its points having a greater positive one.

A solid made of molecules that contain dipoles does not have a static charge, because those dipoles are aligned in a random order such that no charge gradient occurs. But if all the molecules in a solid could be aligned such that their dipoles run in the same direction, then their negative and positively charged ends would be arranged back-to-back. Think of them as a bit like a big pile of miniature batteries in series, with the side of the material to which their negative poles point becoming rich in electrons and that to which their positive poles point becoming deficient in them.

It’s important to stress though that the battery analogy works only in so far as to help visualise how the dipoles align to create a unified charge gradient, and not to give an impression of behaving as if they were a real battery. For an electret material to possess a persistent charge it must also be an insulator, for if it were an electrical conductor the charge would simply conduct and neutralise itself. Thus no current can flow from an electret material, and the free energy dreams of our alternative thinkers on the web must inevitably come to nothing.

Electret DIY

Knowing now what an electret material is, the next question from any self-respecting Hackaday reader should be “How can I make one?“. To which the answer is surprisingly simple, and you will be pleased to hear one that you have a chance of doing yourself. A solid containing molecules with a dipole is melted and allowed do cool and solidify in the presence of a large electric field. This description of making an electret, written by our own [Steven Dufresne], should get you started in your well-equipped kitchen, assuming your kitchen is equipped with a 30 kV power supply.

A wide variety of materials can be used to produce electrets, including a host of commonly available polymers. There are also naturally occurring electrets of which quartz is an example, and natural substances from which they can be made such as carnauba wax, beeswax and rosin. This PDF has a list of some of the materials and in particular the polymers (page 5).

If you decide to make an electret material yourself, aside from replicating a few-cent microphone at great effort, what use could you make of it? A Google Patents search returns overwhelmingly acoustic applications, with a long list of microphones, transducers, and hearing aids. There are a few other used though, for example an air filter in which the charge held by the electret is enough to trap charged dust particles in the air flow. There are passing mentions online of their past use as focusing elements for CRT displays, and the Wikipedia electret page mentions their use in photocopiers, but neither of these applications have much other information. Perhaps it’s best to look upon home electret manufacture as one of those curiosities you might just try because you can.

If you make an electret successfully we’d implore you to write it up and submit it to our tips line.

Electret microphone capsules images in banner: Engineer’s Garage.

34 thoughts on “A Bit More Than A Microphone: The Electret Story

  1. Surely 1000s of us want to know about that picture. I assume it’s a electret material. Is it also food? The pieces that are lying down sure do look like a chocolate peanut butter cup. The piece viewed edgwise looks like peanut brittle.

    Mmmmm, chocolate and peanut butter microphone! Yum!

        1. The brown is due to the Carnauba wax, tastes like chicken because everything does. Interestingly, that batch included white rosin powder available in sports stores. The batch I did with D’Addario dark rosin instead didn’t have the inhomogeneity (see the link for the different batches tested).
          I knew those experiments would find a use someday!

    1. I was just thinking that, and I’m starving hungry as well. Could just go for a nice toffee bakewell or something like that. Even the wax is starting to look edible…

  2. About 1968 Popular Electronics had an article about them, with a sidebar about making your own. They suggested using the high voltage on a color tv set to charge it, though nowadays that’s not so common.

    Electrets are so cheap it’s well worth taking one part. When I did, I was surprised to find an FET in a small package, I was expecting a blob of epoxy over the die.

    I have seen projects for electrostatic speakers and headphones, but not electret, which would seem worthy of a try since it gets rid of the high voltage supply.

    “73”, the ham magazine, had an article about 1970 showing how to make an electrostatic speaker out of aluminum foil and newspaper. A gimmick, but you leave the newspaper around and suddenly sound comes from it.


    1. You beat me to it! I think the article was titled “The Not Altogether Forgotten Electret”, and IIRC the instructions they gave for making one called for latex paint as the material to be polarized. Strange to think that I was only 10 years old then, yet I remember it so well.

  3. Maybe it would be better to think of this like a magnet. Liquid iron will maintain its polarity when it solidifies under a magnetic field, This is doing the same thing except electrostatically. When you start talk about batteries it’s no wonder people start thinking about free energy.

    1. I believe William’s Grand Prix Engineering was doing something like this with magnetically loaded composites (MLC) in a flywheel configuration for regenerative breaking systems. I think they have since sold that branch of the business…

  4. The Amateur Scientist column in Scientific American magazine published some how-tos on making electrets back in the 60’s. See 1960 Nov, pg 202 and 1968 Jly, pg 122.

    IRIC, at least one of them embedded charged aluminum foil in parafin wax.

  5. “Back in the day” I consulted for a company making prosthetics for the deaf (I did the DSP parts). They wanted the very best small microphones – cost not really an object – for obvious reasons. One of the company principals made a test setup to measure mics – bought a few of every type of the desired form factor, and subjected them to an impulse response test. The impulse response was measured using a spark gap to make the pulse, in mid air far from reflective things, and then analyzed first on a PDP-8 (because we had one with good code for that) and later a PC after I wrote and debugged FFT code for the 286 (whoo-hoo, big-time stuff – the data aq for a PC was far from trivial…and even a modern sound-card has one heck of a crazy impulse response itself – then, you could hardly get 8 bits/11khz, not good enough).

    To avoid rambling too long on this, the hands-down winner was the 3-wire one from…Radio Shack (actually a Panasonic/Matsushita) that needed 3 wires as the FET was a source-follower design, with far less distortion and larger dynamic range than the two-wire devices above. And dirt-cheap even then, compared to all alternatives.
    Dunno if the 2 wire ones ever caught up…maybe…

    That was one of the things from Japan that really changed things in a lasting way. The other was cheap electrolytics – under a buck when American manuf’s were getting 6-10 times the price…
    (Oh, and cars, but this isn’t a car site, I hope)

    1. I didn’t know it at the time but the astounding 3D binaural sound I captured with a dummy head and two of these Rat Shack gems showed how right he found it. I made a recording of our local civic orchestra choir and bells and a battery of 105mm howitzers from a handful of meters away, blows the Telarc disc out of the 1812 overture war. On cassette!

      A lot of landline phone stuff from the 80/90’s has one of these things, two for a cordless phone with base intercom.

  6. What mic elements you getting, and where are you getting them from?

    I had been looking at making a DIY mic project off plans I’d seen based off the Panasonic WM-61A, but was then dismayed to find that they are not as available as they once were (at least at a good price).

    Anybody got some places for alternatives?

    1. That modification can likely be done with any electret microphone. I posted another message here with a youtube link that shows how to do it. I did it myself with a different electret and it worked fine.

    2. A garden variety capsule can be modified for split phase (balanced) operation, by cutting the ground trace to the FET source pin. Using a pair of equal value resistors wired in the classic way will complete the task. Schematically, the circuit resembles a split load aka concertina phase splitter, as used in push-pull circuits. This can feed a differential amplifier for further balanced feed.

      Years ago I made a pair of mics, that used a power box/buffer. Using a PNP transistor in the top of the circuit wired as an emitter follower, the base resistor feeds the element. When you unplug the mic, the circuit turns itself off. While the application data on these suggests a single 1.5V cell, it’s really surprising how much the sound opens up with a 9V battery. They hear *everything* !

  7. It seems to me, [permanent] kinetic energy, versus potential. But then, the high school physics teacher got me w a pulley-weights display, as I walked in… “kinetic” I think I announced, being the only junior in the class… He was a real card. He had a pulley jammed w a paperclip. LMAO! ;>)))))

    Great stuff guys! Keep it coming ! ! !

  8. The article statement: “..naturally occurring electrets of which quartz is an example.” Is incorrect. A quartz crystal has piezoelectric properties that produce a Voltage when a mechanical stress is applied to the crystal.

    BTW: I used a Calrad quartz earphone (I still have it more than 55 years later!) for my first crystal radio set, however it used a “real” diode (a Germanium 1N34), not a traditional “cat’s whisker” which relies on the Schottky created from a metal-metal oxide interface like an old copper penny. There was no amplifier, as this was a direct conversion AM radio, like all “real” crystal radios. That’s why it used 0.3V drop Germanium instead of a new (at the time) silicon diode for more efficiency. All piezo devices are bidirectional transducers — they create mechanical motion by applying an alternating Voltage, or as a mic by applying sound (pressure) to the crystal and using the resultant Voltage output.

      1. It’s likely to have been Rochelle salts crystal. Later piezoelectric materials used ceramic, these were widely used (and still are) in che…errr… inexpensive phonographs. The Chinese have made a gazillion Chuo Denshi CZ-800 cartridges for the headshells of Crosleys, Victrolas, and other retro players.

    1. Thanks for the great suggestion!

      Next time, please submit to the tips line. If your comment hadn’t gotten auto-moderated (for whatever reason), I wouldn’t have seen it, and we still wouldn’t have heard of him.

      But as I said, thanks.

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