This is where I pull the "Why ask me?" bit, but we're really getting into areas that Brent Casey or Stephen Paul should be answering this question, not me. I have a fairly good understanding of how large diaphragm mics work on a "more than a lot of people know" scale, but I've never designed an LD mic, and at my age, I wouldn't even try without someone like Stephen Paul looking over my shoulder (even with him laughing at my efforts).
Here's what I "think I know", and it may help (and it may also be wrong, too), but it's gonna be in really simple form:
A diaphragm connected to a solid backplate with a small spacer between them, would be a pure pressure device that reacts strictly to pressure changes, in other words, an omni. They usually add a very pressure relief hole as a vent. (I threw that in because someone will correct me if I don't mention it.)
Now, with a solid plate close to the diaphragm, the pressure is still pretty high, so we need to add a little more volume behind the diaphragm, so it can move a little more freely, but how to do it so you don't lose output is the question. If you put in a thicker spacer, the output will go down. Ahh, make the backplate thick and drill some holes PARTway thru the plate. That way the plate is still close and the holes increase the volume of space between the diaphragm and the trapped air behind it.
Ok, we want a cardioid pattern outta this whole mess, so how do we get it? WEll, we know (from reading the big thread), that a cardioid pattern is just an equal (50/50) mixture of omni and figure 8. A figure 8 pattern is also know as bi-directional or pure pressure gradiant, since it reacts to velocity, not pressure.
Well, we've got our omni done, so we're 1/2 way there.
We know a figure 8 is simply a diaphragm that is exposed to sound on both sides and responds to velocity (air MOTION, not air PRESSURE), but how do we get the sound right if it's coming from behind the mic? We need to get air MOTION from behind the mic to affect the diaphragm. So, how the hell do we make the other half of this mic a figure 8?
Hey, if we drill just the right number and size of holes all the way thru the backplate, we can get the mic to react like a pressure gradiant and an omni in a nice 50/50 mix, and we have....
....a cardioid pattern. Ta DAAA!!!
Now, since this is really an electrostatic device, we don't want to get a lot of dust coming in thru those holes we just made in the backplate, so maybe a cover over them which can move? Hmmm, a second diaphragm, but not connected to anything electrically.
Now this back diaphragm is a lot like any front diaphragm, it has some mass and it moves, so it resonates, and that can be used for damping some frequencies or boosting other frequencies, depending on how it's tensioned and fastened.
And that's what "I think I know" about that subject.
