How to Make a Microphone … From a Face Mask


Suppose this parallel board capacitor is connected to a 9 volt battery. A volt is a measure of the electric potential difference. In short, this is the potential electrical energy per charge: it is a measure of how much energy a charge would gain by moving through that potential. Therefore, this 9 volt battery will create a 9 volt potential change across the boards.

But what if I pushed one of the plates so that the distance between them decreases a little? Well, since the capacitor is still connected to the 9 volt battery, the potential should still be 9 volts. However, if the electric field stays the same, a shorter distance would mean less potential. The only way to compensate for the reduced space would be to increase the load on the plates. This extra charge would come from the battery and look like an electric current. On the other hand, if you move the plates away, then the charge will come out of 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 one of the plates into the condenser microphone to create a changing electric current. You can then record this stream and save it for later, and send it to an amplifier and speaker to produce louder sounds.

The best thing about a condenser microphone 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 may not be surprised that many high-end microphones are of this type. Of course, a small drawback is that these microphones need an applied voltage, that is, they need a power supply. This could be from a small microphone battery or, more likely, from the power supplied from the audio receiver / amplifier.

Now let’s look at a slightly different type of microphone: the electret microphone, which is sometimes called the electret condenser microphone. What the hell is an electret? The name should remind you of something familiar: a magnet. Although it is possible to create a magnetic field with an electric current (as with an electromagnet, as Wile E. Coyote demonstrates), most people probably think of something as a permanent bar magnet. These are made of 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 pole and a 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 like when a sock comes out of the dryer with a static electric charge and sticks to things. (Well, a sock doesn’t stay charged, but an electret does.) While a sock may have an excessively negative charge due to some extra electrons, or a positive charge due to missing electrons, an electret may be neutral. Even if an object has an equal number of positive and negative charges, it can still produce an electric field if there is a “charge separation.” Imagine a molecule with a slightly positive side and a negative side. It will remain neutral, but it will create an electric field.



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