Originally Posted by wakibaki
Yes, a capacitor. The capacitance is dependent on the plate area. If you have charge distributed on both sides of the diaphragm instead of one, you effectively double the plate area. For any given voltage the charge will be doubled. This is why the foils in capacitors are rolled up, to take advantage of both sides of the plates.
I have read another time the way you put the diaphragm's movment into equation, and I still don't see how you can account for coating both sides of the diaphragm. The only change it would induce is a better symmetry but that is 5µm of diaphragm thickness regarding to the 500µm of DS spacing, so it is not really an issue.
The force one stator applies to the diaphragm is F_E1, given by : (F_E2 is the force applied by the other stator to the diaphragm)
where Σ is the surface of both, e is the distance from stator to diaphragm, U_0 is the bias voltage, u is the drive voltage and x is the distance from the middle between both stators to the position the diaphragme actually is (so it measures the diaphragm position from where it would be if there was no force at all)
Here you have a change in sign because you have two stators so there is a symmetry.
If you sum these two forces and say that the diaphragm is more or less a resilience (which has a certain constant named k), add a force induced by the air friction (which is, in the end, responsible for sound) also given by a constant h you end up with this equation, after simplifying a bit :
M is the mass of the diaphragm. That is the equation I was talking about a few posts earlier, you only have to consider voltages, the charges are hidden underneath
Maybe that explains a bit better why you can't think in terms of charges, it will lead you nowhere. If you read Roger sander's book, you will see he has the same opinion ; that is about double coating the diaphragm, he doesn't explain the equations I have just given, and they are not taken from his book.
(iirc the foil in a cpacitor isn't rolled to take advantage of both sides, only to manage to fit a lot of surface into a given volume but that is not really relevant here)
And of course, if you want to try to double coat your diaphragm, go ahead and let us know what you find out