cross section of an electret microphone element.
pic.twitter.com/BoLbNqZlbr
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you can see the electret film coating on the backplate here. the metallized diaphragm is torn and distorted from the sectioning processpic.twitter.com/KCoxvnzW8C
how it works: the metallized diaphragm and backplate act as the two plates of a parallel plate capacitor. the electret material sits in between, along with a small air gap. the electret gives this capacitor a constant charge, Q...
...and since the capacitance changes when the diaphragm deflects due to sound coming in because C = e0*A/d, and because Q = CV, the voltage V = (Q/Ae0) * d. in other words, the voltage is directly proportional to the change in distance between the two plates.
the most fascinating part is the magical electret material. the name is a portmanteau of "electrostatic magnet." it's a material made of polar molecules (positive charge at one end, negative charge at the other) which are all aligned.
basically you melt the material while it's in a high voltage electric field, causing all the polar molecules to line up, then you let it cool and solidify. now one side is positively charged while the other side is negatively charged. 


Very cool! I had no idea that a JFET was included within he electret element.
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