[Scott Dorsey]

Tobiah wrote:
I googled a bit, but didn't hit much. I was thinking
that a diaphragm loose in space, would be a figure-8;
sound coming from the edge of the diaphragm would apply
pressure equally to both sides, whereas from the front
or the back would make the diaphragm move.0

Yes. It is sensitive to air velocity.

So what about cardioid and omni? What physical characteristics
of the mic cause the pickup pattern to change?

An omni would be a diaphragm in a sealed box, so it is sensitive to
air pressure rather than velocity.

All other patterns are combinations of those two.

This is discussed briefly in the FAQ, I think, but a better description
of the mechanism is in the Audio Cyclopedia.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."