Hello,
I came across this thread several days ago. Reading the whole discussion took quite a while and was very informative. My compliments first to Wachara for his builds, his will to share so much knowledge and to tame and guide all the ideas in this thread. A big nod from my side!
I have been thinking about building my own ESL headphones since more than 20 years and never got to it -shame on me-. Many answers I had have already been answered here. What got me started was an article by Philippe Hiraga in L'Audiophile. Still being in school back than.
Now I own a Stax Lambda Signature and a SRM1/MK2 (thanks to Kevin G. here, he gave me the right advice how to repair it).
Reading the thread I gained a few ideas that I would like to share and discuss here, I also found some topics only described but not explained. Please correct me if I am wrong, I have just engineering knowledge, no building experience.
Please allow me a remark on the Resistance of the membrane:
The membrane gets moved by the voltage applied to the stator (Coulomb Law). The deflection of the membrane will be half-sine or parabolic like. If charges can move too easy on the membrane, they will be attracted by the stronger electric filed in the middle, where the distance is smaller. So the charge would shift to the center, reenforcing the pull in the center. This could lead to higher distortion and/or instability. Therefore you need to have a fairly high resistance.
For the stator I have the idea shown at
http://up.picr.de/24335672lm.jpg
based on the following: (sorry, I have no permission to post pictures yet)
I do not understand the center hole on the Stax 009. To me it seems too small to significantly reduce the forces acting on the center of the membrane. On the other hand applying more force to the edge of the membrane might help to move it more uniformly.
(I think that the center-hole is truly for acoustic reasons. As the membrane movement is biggest at the center, it makes sense to me, to open up the center)
The stator itself has an eigenfrequency within the audible range. Getting the stator more stable will always be good. So I have to make it stiffer, where most of the bending takes place and/or reduce the mass where most of the motion takes place.
I want to control/reduce the forces on the center of the membrane.
So I chose an oval stator (minimizing the dominance of eigenfrequencies of circular objects), started the copper about 3mm away from the spacer (to keep capacity low) and increased the hole size toward the center.
The active membrane would be 78x102mm, open area is 32%. But increasing toward the center, where the membrane moves most of the air, so it's hard to tell, how it would compare to an equal spaced stator.
I am curious to read what you think about this.
Many greetings
Georg