Dan,

Two things I can tell you from first "ear" experience (with a Lawson L251...this mic lets you shut off the back diaphragm in a "cardioid only" position) is that the
mic gets quieter and louder with only the front diaphragm on. The proximity effect is a bit more than if both diaphragms are on, but still in the cardioid position (the
mic is infinitely variable from omni to bi directional). BTW, we are practically neighbors (studio in Bloomington). I'll have to stop by sometime (or vis versa).

Paul,

The back diaphram is still a functional part
of the accoustic circuit, even if it doesn't
have any polarizing voltage.

I'm more curious about the true single diaphram vs two capsule styles. For instance,
I have several Fairchild F-22's which are also the same mic as the Syncron AU-7A. Some of these are two diaphrams, and some are just
a single. The dual units are essentially two
halves back to back, the singles are one half
with a drilled plastic backing plate.

In casual listenning, they sound very similar.

In the dual units I can bias the back diaphrams and make multi-patterned mics just
as Gene does, or David does in the U95S that
I have.

But I can also block off some of the ports
on the single diaphram units and alter the
polar pattern that way.

Come on over anytime, I'm always here it seems.

If you guys want me to, I'll post something about this in my area... there are a lot of notable differences as well as very good reasons for the development of the twin membrane capsule... (Weber and Von Branmuhl patented the working model in '35 in Deutschland, and the patent was hurriedly filed here in '39 just before the war got really hot.

Didn't have time this morning, but if you post a request in my area, I'll try and put something up there...

The difference lies in low frequency directionality: at a given source distance, the dual membrane unit has a polar response that is less directional than it's single membrane counterpart.. Also, the dual membrane unit has reduced proximity effect and pop sensitivity. What does this mean to the mic user? The dual membrane unit is more suited for both the desired front incedence sound and unwanted rear sound are in the near field (vocals). This is well substantiated by traditional use. For far field use, the single membrane capsule has superior polar and on axis response. intermediate distances, either type may be used.
Those persons found using microphone in a manner different than this ruling shall be disciplined!