Posted by MAP on October 11, 2002 at 14:22:37:
In Reply to: Cardiode ESL Seaker .... posted by Terry Koehn on October 10, 2002 at 16:15:07:
Greetings TK,
I'm afraid a dual-diaphragm ESL such as you've been describing would be very impractical. Here's why:
1.) ESL diaphragms are poor attenuators (which is to say that they're good transmitters) of sound, due to their low mass per unit area.
2.) For this reason, any dual-diaphragm ESL would have to be separated by a small fraction of the highest-frequency wavelength of the speaker's bandwidth. One-eigth to one-quarter of an inch may be the largest practical separation.
3.) As I wrote in a previous post, acoustic stiffness goes as area^2/volume. In this case, the area would be huge, and the volume would be tiny. The net result is that you're fighting tremendous acoustical stiffness - which means that you need enormous force to overcome it (from Hooke's law in the compliance-controlled region.)
Conclusion:
For any dual-diaphragm ESL as you propose, where the front and rear diaphragms would create a differential pressure inside the enclosed volume, the enormous acoustical stiffness would render the speaker completly impractical, due to severely attenuated output with any realizable electric field, limited by (at least) the dielectric strength of air, which is roughly 1e6V/m.
Best,
MAP