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Tuesday, April 16, 2024

How to Make a Microphone … From a Face Mask

I know everyone is sick of this pandemic, but I'm going to suggest that you keep your face mask. I mean, there's the whole thing about stopping your mouth-drops from getting into other people’s bodies, and also stopping their mouth-drops from getting into you. This is a nice feature in normal times, but when these drops can carry the Covid-19 virus, you probably want that mask. Plus, masks can even look cool. But there's something else you can do with one: You can use it to build a microphone.

How Does a Microphone Work?

There are different kinds of microphones, but they all do about the same thing, which is to turn acoustic sounds into electronic signals that can be amplified, modified, or recorded.

When you speak into a mic, the vocal cords in your throat oscillate back and forth. This pushes the air and compresses it. That compressed part of the air then pushes other parts of the air, so that you get an area of higher pressure traveling outward from your mouth. Boom, you just made a sound.

The primary goal of the microphone is to detect this changing pressure wave in the air and convert that to a changing voltage. Once you have a changing voltage, you can use this to make an electric current and send it through some wires. After that, you could either amplify this electric signal, record the signal, or do some analysis, like make a cool auto-tuned sound.

But exactly how do you transform an oscillation in the air to an electric voltage? There is actually more than one way to do this, but I want to go over two similar types of microphones: the condenser mic and the electret mic.

In physics, we don't really use the term "condenser," and instead we would call something like that a “capacitor.” The absolute simplest capacitor that you can imagine is just two parallel metal plates separated by some small distance. (Let's call this distance s.)

If you connect one of the plates to the positive terminal of a battery and the other plate to the negative terminal, then you get a charged capacitor. This means that one side has some positive charge (+Q) and the other side will have an equal and opposite negative charge (-Q). These two charged plates then create a fairly constant electric field (E) in the gap between them.

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Suppose this parallel plate capacitor is connected to a 9-volt battery. A volt is a measure of the electric potential difference. In short, this is the electric potential energy per charge—it’s a measure of how much energy a charge would gain by moving across that potential. So, this 9-volt battery will create a 9-volt change in potential across the plates.

But what would happen if you pushed one of the plates so that the distance between them decreases just a little bit? Well, since the capacitor is still connected to the 9-volt battery, then the potential would still need to be 9 volts. However, if the electric field stays the same, a shorter distance would mean a lower potential. The only way to compensate for the decreased spacing would be to increase the charge on the plates. This extra charge would come from the battery and it would look like an electrical current. On the other hand, if you move the plates farther apart, then the charge would come off the capacitor and also produce an electric current.

In other words, moving the plates back and forth creates a changing electric current. This is the basis of how a condenser microphone works. When you have a sound, it produces oscillations in the air. These oscillations then push on one of the plates of the condenser microphone to create a changing electric current. You can then record this current and save it for later, and you can send it to an amplifier and speaker to produce louder sounds.

The nice thing about a condenser mic is that one of the capacitor plates can be very thin and flexible. This means that it can move quite quickly in response to higher-frequency sounds, so you might not be surprised that many high-end microphones are of this type. Of course, one small downside is that these microphones need an applied voltage, meaning they need a power source. This could be from a small battery in the microphone or, more likely, power supplied from the audio receiver/amplifier.

Now let's look at a slightly different kind of microphone: the electret mic, which is sometimes called an electret condenser microphone. What the heck is an electret? The name should remind you of something familiar: a magnet. Although it's possible to create a magnetic field with an electric current (like with an electromagnet, as demonstrated here by Wile E. Coyote), most people probably think about something like a permanent bar magnet. These are made from materials that have tiny regions that also create magnetic fields called magnetic domains. When these magnetic domains are aligned in the same direction, you get a magnet with a north and south pole.

Instead of having permanent north and south poles to create a magnetic field, an electret makes an electric field using positive and negative electric charges. It's sort of like when a sock comes out of the dryer with a static electric charge and sticks to stuff. (Well, a sock doesn't stay charged, but an electret does.) While a sock might just have an excess negative charge due to some extra electrons—or a positive charge due to missing electrons—an electret can actually be neutral. Even if an object has an equal number of positive and negative charges, it can still make an electric field if there is a “charge separation.” Imagine a molecule with one side that’s slightly positive and the other side that’s negative. It will still be neutral, but it will create an electric field.

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One method for making an electret is to take some electrically insulating material, like plastic, and heat it up in the presence of an electric field. When it warms up, the plastic material allows molecules to move around more than they would in a room temperature solid. This allows the positive charges to move in the direction of the electric field and the negative charges to move in the opposite direction to create a charge separation. After that, when the material cools back down, these charges will essentially be "locked" in place. Now you have an electret.

Let me make a very rough sketch of an electret microphone so you can see how it works:

Note: This isn't exactly how these mics are set up, but it will give you an idea of how they work. Here we have two metal plates with an electret in the center. When a sound wave comes in, say from the left in the diagram above, it will push on the movable plate. This can change the distance from the electret to the metal plate and cause a change in the electric field. This changing electric field will cause charges to either flow away from or into the plate, producing an electric current.

This is indeed very similar to the plain condenser mic. The one big difference is that the electret mic doesn't need an applied voltage. It's like a capacitor in that it has two plates with charges, but with the electret the charge is always there. It doesn't need a battery to get it charged up. This means that you can make these mics really small, tiny enough to put in a smartphone or Bluetooth earphones, which are both common uses.

The Face Mask Mic

There's something else that uses an electret that we see quite a bit. N95 face masks have electret fibers in the mask. When tiny liquid drops come near these fibers, an attractive force causes the drops to get trapped among them. This protects the wearer from inhaling bad stuff, like the Covid-19 virus or other germs.

Maybe you can see where this is going: If you can make a microphone using electret materials, and there are electret fibers in an N95 mask, you can use a mask to make a microphone. Here’s what I did:

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I started with a face mask (the blue paper kind) and two old cans with different sizes. The small can acts as my stationary metal, and the large can has a cover of aluminum foil to act as my movable plate. The face mask is in between them. I stuffed some foam in between the two cans so that they would be separated and then connected my output wires to the two cans. That's it.

Instead of connecting the microphone to an audio recorder, I connected it to an oscilloscope. Don't worry: These oscilloscopes look complicated, but they really just measure voltages. The screen on the oscilloscope will display the voltage as a function of time to make a nice plot. This voltage will then be proportional to an actual audio signal that you could record—but it's nice to be able to see the output instead of only hearing it.

Then, to make some noise I used a recorder—you know, those flute-like things you used in your elementary school music class. Playing a note, I get the following output:

Notice the "squiggling" lines on the screen? Those represent the changing voltages due to the foil moving from the sound from the recorder. It works!

OK, I'll admit—it's not a very good microphone. But it is indeed an actual microphone. If you added an audio amplifier, I suspect you could even use it for recording your online meeting or something.

Does this mean you could make a microphone with pretty much anything? Yes, that's basically true. As long as you have something that moves due to sound to produce a changing voltage or current, you have a microphone. In fact, you can even make a microphone out of a cordless drill. The possibilities are endless—sort of like this pandemic.

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