Below are the some notes on an ESL project I made back in the 60's. I'm currently concepting a new ES system. It's different! I'll keep you posted.
Back in the '60s I was quite involved in electrostatic speaker development - both for my employer (then Delco Radio, Div. GMC) and myself. Obviously, Delco was interested in the automotive application of the devices, but it was hard to displace the cheap dynamic speaker - then $1.75 a pop! But I quickly found out that the acoustical "source impedance" (a voltage source analogy, rather than the current source analogy of the heavier, moving coil devices) did great things for the audio quality for the quirky "room" impedance modes found in the interior of an automobile. Too bad they weren't cost-competitive......
My home electrostatics were the top end of a 2-way system. The ES units were used above 200 hz, and were operated in the then-popular constant charge mode (high resistivity diaphragm coating) advocated by Peter Walker and Arthur Janszen. The ES modules were approximately 2 " by 8". 24 units were used in a cylindrical array about 48" high. Their efficiency more than kept up with the pair of Bozak 12" woofers! Of course, the system was bi-amplified, with 100w amps being used for the woofers, and the same for the ES units.
The stators of the ES modules were acrylic castings, rendered selectively conductive with aluminum foil. The diaphragm material was DuPont 1/10-mil mylar (less than 3micron), rendered conductive with a commercial detergent coating. The system sound was outstanding, and capable of threshold-of-pain playback! The units also served as a studio monitor playback system for a few years when I operated a recording studio.
A word of caution, tho.... The acrylic material was a bad choice for the stators, as it seemed to be continually curing out - shrinking, if you will! Every few years called for re-diaphragming the modules. And forget about detergent coatings, as they will evaporate over a period of a year, or so. One could "refresh" the conductivity externally, but it was still a hassle!! I'm currently concepting a new ES system.
The modules are 2.25" x 7.75". I opened one up to show it's guts, including the (transparent) diaphragm. The stators are acrylic castings, rendered selectively conductive with an aluminum foil overlay. #2-56 hardware completes the conductive path to the outside world.
As I mentioned above, the modules were acrylic castings. A master part was made on a mill - from Lucite, I believe, altough one could make the master from anything, including wood. The airgap was integral to the casting/master, with no separate spacer being needed. Don't forget to spray a bit of mold release on the master!
The mold material was Dow Corning silicone rubber - Sylgard, I believe. I used this variant because of its wearability - as you'll be making well over 100 castings! (It's been a while, so forgive me if I forget some of the details.....) You'll get 25-50 castings from a mold before it goes to hell. About every 5-10 castings, "rejuvenate" the mold by brushing on a strong solution of diswashing detergent (Palmolive, Joy, etc...). The detergent tends to replenish components taken out by repeated exposure to the acrylic resin and/or catalyst. The casting resin was Castolite, available in any craft/hobby shop. The most stable material, however, is glass-filled epoxy resin. But I gave up on this stuff because of the drilling & tapping operations that come later. You'll go broke buying drills & taps as the epoxy/glass wears them out after a few frames! I believe I kept 4 molds going at once, to speed up the project, I also cured the acrylic more quickly by the application of heat: I used a frame with 4 each 25 watt bulbs underneath a marble slab (coffee table top!) to achieve a cure temp of about 125degF. Start 'em in the morning, and pop 'em out at nite! Make sure you do this in a well-ventilated area, as the curing acrylic fumes are hazardous!
To render the frames selectively conductive, spray the "working side" of the casting with a good quality spray glue, having gotten all your drilling & tapping done first. Then quickly burnish in place the aluminum broiler foil. Cut away the waste foil, and do the necessary electrical separation using an X-Acto knife. Mount the connection "studs" (#2-56 hardware), then spray a coat of hi-voltage dope over the stator conductor section, using a cardboard mask to keep the dope off the diaphram clamping periphery. If you're into the "constant Q" mode of operation, coat the mylar diaphragm with a solution of industrial detergent and isopropyl alcohol which will yield a resistivity of about 100k - 1meg per square. Let dry.
Flip a frame on its back, and stretch the diaphragm over it, using masking tape to aid in the stretching process). Position the mating frame over the lower one, and screw/clamp/ whatever, it in place. Run a bead of Pliobond around the outer periphery of the (upper) frame and let dry for about 15 min. Cut the diaphragm loose, leaving a 1/4" margin. Flip up the margin all around, letting the Pliobond do its thing. This operation will keep the thin diaphragm from slipping & losing tension in the future.
A word on rejuvenating the diaphragm coating from the outside of the speaker. Just make up a solution of Palmolive detergent & isopropyl alcohol. Since the modules are open to the atmosphere on both sides, just dip a soft brush into the solution and slop it all over the diaphragm area. Speed up the drying with a hair dryer. Any resistance paths to areas it shouldn't be will be blown away the first time you turn the system back on! No damage ever seemed to result from this technique.... I'll see what I can do about a system picture or two..... Keep in touch.
You can contact Moe Whittemore via The Audio Circuit.