In brief, here's an excerpt from "Understanding and Using Directional
Microphones", an article published in the September 2000 issue of
"Sound on Sound" magazine
"...All microphones work by sensing the pressure difference on either
side of a thin sheet known as a diaphragm. Ultimately, there are
really only two fundamental microphone principles pressure-operated
(omnidirectional) and pressure-gradient (directional). In a
pressure-operated mic, one side of the diaphragm is open to the
atmosphere and is able to respond to the microscopic changes in
pressure representing sound...
...In a pressure-gradient mic, the diaphragm is still sensitive to the
difference in pressure on either side, but this time both sides are
exposed to the atmosphere, and therefore to the changing pressure
caused by passing sound waves. A sound arriving in the plane of the
diaphragm will present identical pressures on both sides and,
consequently, there will be no net movement. There is no pressure
gradient across the diaphragm and so the microphone is deaf to sounds
on this axis. In contrast, sounds arriving perpendicular to the
diaphragm will create a large pressure difference between front and
rear, and it will be moved a maximum amount as a result..."
Due to copyright, I can't reproduce the full article here, which goes
into much greater depth on directional microphones, but you can read
it in its entirety at this link...
You may also be interested in the use of directional microphones in
hearing aids. Among other sites, The Oticon site provides a good
overview and discussion on directionality. See...
Search strategy at http://www.kartoo.com: "directional microphones"