Electrostatic stator design

[martinv]

Hi People,

I'm designing some electrostatic loudspeakers at the moment. I have looked around for materials to use for the stators. The other day at the hardware store I found some sheet aluminium with pre-drilled holes. I was very excited. Looking through the sheet however the hole to metal ratio seemed quite low. I took a brochure and at home worked it out a little better.

The hole size is 1.5mm which is ok. The sheets are 0.7mm thick which is ok.
The hole to area ratio however is 0.23 which falls short of the 0.40 recommended (by Sanders) minimum.

Does anyone have experience using a lower ratio? What can I expect? Lower sensitivity or other things as well?

The material is sold as Kantoflex brand Bleche from the Bauhaus chain here in Germany.

Regards,

-Martin

[MarkMcK]

Martin,

I feel your pain. I too have searched high and low for the perfect plate material and not found anyone making exactly what I want.

You analysis is correct. As the diaphragm moves toward the plates, the air mass between them will compress. Fewer holes of equal size will produce more compression, more diaphragm loading and less output. With increased pressure and holes of the same size, at some point you may even get whistling. In addition, the increased diaphragm loading will alter its resonance characteristics and thereby change its frequency response in complex ways.

We also don't know if Sanders was absolutely right. What if 40% isn't the minimum? If you decide to try a test panel using this plate and you have access to some test equipment, those of us interested in ESLs would love to know the results. It would let us get an experimental check on one boundary of Sander's rules of thumb.

Also, remember the Acoustats, much more hole to fill than 40%. I still have a couple of old panels around and I should rip one apart and measure the exact ratio. I will post the numer to this thread soon.

Thanks for the question (and hopefully for some future experimenting),

Mark

[jam]

Martin,

Why don't you try wire stators?

Jam

[Johan_Ch]

I have been thinking about the same thing for a while, and I have a suggestion.

What about the metal kind of speaker fronts that you mostly see on PA systems? Would that work?

Would the paint/plastic that the panels är covered wit cause problems?

Comments?

/Johan Ch

[Lonestar]

It will be rather difficult to determine the "hole ratio" for the Acoustats. Acoustats used wire stators glued to plastic grids (similar or the same as those used in fluorescent light fixtures).

To properly assess the Acoustat's ratio, you would have to measure the total area that the wire presents in comparison to the diaphragm. The plastic support grid's contribution would be minimal in comparison to the wiring and would only contribrute where the panel is not coincidental with the wiring.

The suggestion in the post above to use wire for the stators rather than metal panels is probably the easiest to achieve for the DIYer. OTOH, I suspect that Acoustat's plastic egg crate for the stator support adds some resonance of its own.

Lonestar

(Acoustat owner for the last 20 years).

[mwmkravchenko]

Whay not try this.... Take an aluminum bug screen and stretch it tightly over a flourescent light grate. By judicious heating in an oven you can warm up this assembly and press the screen into the plastic grate on a nice flat surface thereby bonding the two together. If your not worried about solvents there are a couple like acetone that will soften plastic. You would need to fabricate a pan that would hold the solvent in which you would have your screen allready stretched out and press your plastic grid into the puddle a whola a bonded assembly. The aluminum screen is almost an ideal open ratio and it has been sucessfully applied before.

Mark

[Lonestar]

For additional electrostatic resources, check out


The Electrostatic Circuit

[martinv]

Originally posted by jam
Martin,

Why don't you try wire stators?

Jam

I may well try wire though I believe it takes a lot longer to build a wire based stator (plus they're usually larger and heavier and I was hoping for something thin and elegant). Just to start wth I'd like to put together some prototypes quickly. Of course if I can't find any material suitable wire will become the number one choice.

Originally posted by Johan_Ch
I have been thinking about the same thing for a while, and I have a suggestion.

What about the metal kind of speaker fronts that you mostly see on PA systems? Would that work?

Would the paint/plastic that the panels �r covered wit cause problems?

Comments?

/Johan Ch

I have wondered about this too. Lately I've been noticing all sorts of perforated matial. Office desk sets, partition material in offices etc. :-) I believe the challenge is to find something reasonably flat as the stator to diaphragm spacing needs to be on the order of 2-3mm (80-120 thou). A lot of the metal grid (PA type etc. as you mention) I've seen is rather coarse, being made from wavey wire welded together rather than a sheet with holes drilled in it (um, difficult to explain!). Some parts of the stator will therefore be much closer to the diaphragm than others. Result I guess is loss in sensitivity. I believe the thickness of the paint would not cause great problems, though will reduce maximum excursion.

Originally posted by mwmkravchenko
Whay not try this.... Take an aluminum bug screen and stretch it tightly over a flourescent light grate. By judicious heating in an oven you can warm up this assembly and press the screen into the plastic grate on a nice flat surface thereby bonding the two together. If your not worried about solvents there are a couple like acetone that will soften plastic. You would need to fabricate a pan that would hold the solvent in which you would have your screen allready stretched out and press your plastic grid into the puddle a whola a bonded assembly. The aluminum screen is almost an ideal open ratio and it has been sucessfully applied before.

Mark

Something like this may well be quite successful. The screen door material would certainly exceed the 40% open guide. The challenge will be building the supporting frame. I think next time I'm at the store I'll check out these flourescent light grates!

Incidentally, the perforated metal I found in the shop appeared quite 'dark' and difficult to see through. Later closeup investigation revealed the 'transparency ratio' (for want of a better term) to be 23%. This might be a useful reference point for others looking for perforated material. To be 40% transparent it would have to look quite transparent.

At the risk of rambling, I thought of another way of measuring the transparency ratio for awkward to measure perforations. Weigh the perforated sheet, then work out the weight of a sheet without holes (based on thickness, size, and density of material). Still the above method of looking for something fairly optically transparent is probably fine!

In closing, I'm possibly being too paranoid about achieving the 'right' ratio! I should probably just experiment!

Thanks for the answers! Happy DIYing!

[peranders]

Have you checked this link?
http://www.ele.tut.fi/~artoko/audio/...rs/hybrid.html

[catapult]

When in doubt, Google. I typed in "perforated metal" and got tons of hits. This one was the first on the list.

http://www.mcnichols.com/products/perforated/

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