bi-polar electrostatics?

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Joined 2002
A thought occured to me. What about bi-polar electrostatic speakers?

Would that work?
just put two esl's together so that is like so:

+ = + charged mesh
- = - charged mesh
I = insulator
F = film


This would then create two like-phased signals one from the back and one from the front and in between the two diaphragms would be a sealed space.

Has anyone thought about this and if it would work?
I myelf can't see any reason why it wouldn't and it would help the dipolar cancellation problem.
Joined 2001
Paid Member
I have some sketches of such a beast somewhere (i have 4 acoustat panels too :) - might just have to revist that idea). The volume of the inner chamber needs to be quite large so it is probably easier to think about it as two seperate ESL panels in a sealed or aperiodic box.

You would lose some of the boxless quality of an di-pole ESL, but you should get bass you don't usually associate with ESLs.

Disabled Account
Joined 2002
the dipolar cancellation problem occurs because the film moves back and forth
on one side it creates positive pressure and on the other it creates negative pressure. these two signals are out of phase obviously, and this cancels the other out to some degree
it is more appraent in lower frequncies which gves esl's the reputation of being very bass-less

but with bipolar speakers the signals are in phase thus increasing the volume which should help fix the loss of bass that dipolar speakers face.

and while on this topic, how about a sealed electrostatic speaker? rather than bipolar?
that woudl be very interesting too, should perhaps improve imaging?

I can see how adding the extra film might appear to allow more bass, but you may be adding another problem. The sealed enclosure between the two films will create a lower pressure when the films are away from each other and a higher pressure when they are close. This will definitely be worse than any dipole cancelation. The air no longer goes around the speaker to cancel, it just pushes back harder on the film itself.

This is exactly the reason that people don't seal their ESLs in a box. The electrostatic forces associated with an ESL are barely able to move the air as it is, they don't need a sealed box to fight against.

Roger Sanders' book claims that the cure is to use baffles around the speaker, which physically block the air as it tries to "run around" to the other side of the speaker. This will cause some aesthetic problems, but is much simpler in theory and construction. Also, it is easy to test existing speakers by making a cardboard frame around the speaker. Then you can tell if the addition is worth your time and effort and possibly sore eyes.
Perhaps you can make an unsealed box for the ESL to sit in. This will help eradicate the dipole cancelation, while not adding length or width, just depth to your speaker.

Good luck,
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Joined 2002
this is why i asked on here :) hehe could save me wasting my time :)

so these baffles are supposed to act as if the speaker were mounted in the wall, or in a large open baffle enclosure system to prevent the cancelling waves from actually doing so?

also has anyone ever found a DIY electrostatic amplifier schematic? i have looked everywhere and can't find anything
specialx said:
so these baffles are supposed to act as if the speaker were mounted in the wall, or in a large open baffle enclosure system to prevent the cancelling waves from actually doing so?

Precisely. Be sure to give the ESLs plenty of room away from the walls. And also angle them off of flat surfaces so that the sound bouncing off of the walls does not return to cancel the sound from the ESLs. I did a quick test with mine. I had someone hold a flat piece of wood a couple of feet behind the ESL parallel to the speaker. The difference was immense. I suggest you try this quick and simple test for yourself. I found that it sounded better if I positioned the speakers so that the sound bounced off of two walls and came around the speaker to my ears. If nothing else it is sure to increase the volume.

Are you looking for a direct drive amplifier or one that you plan on stepping up with a transformer? If you want to use a transformer then an amp that can deal with the low impedances and capacitive load is important. As for direct-drive, it is not for the faint of heart. I've gathered that they are difficult to design and find parts for, as the voltages are very high. Even those that build them sometimes find them to be unstable. But, I've never tried and you may have some luck with it. I suggest that you do some searches on this forum, as I know it has been discussed at length before.

Do you have a pair of ESLs, yet? Or are you looking for some feasability issues before beginning?

Good luck
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Joined 2002
i've already started purchasing and ordering parts to make the speakers. so far i ahve the perforated mesh, acrylic sheet for insulation and beautiful tasmanian oak dressed wood 50x50mm for the frame of the speakers. i plan to route a slot in the oak so the speaker components just fit in nice and snug.

i just got some 70% open area round hole mesh in staggered design 1mm thick, normal steel however, aluminium was $430 AUD for a 2meterx1meter sheet!!!

i am using acrylic for the insulation, each part is one piece, as i didnt want ugly joins in the corners. might be wasteful but looks a lot nicer. i got 1/4 inch thick as recommended by for full range speakers. the actual size i am aiming for is 250mm wide x 2000mm tall, minus the border around the speaker of 25mm leaves me with 0.39meter^2 surface area.
it might be less if i choose to implement a braced design, which i hope might change the resonancies of each section of panel

i also plan to stretch the mylar out in one go and make two panels at once, to hopefully avoid differences in film tightness(spell, grammar) in the two panels etc

i am investigating different coatings for the mylar film too, although i am told graphite is still the best.
i was looking into inks mainly,
has a silver based conductive ink which you can mix with a carbon based ink to get a specific resistance but as they replied to me with an email, the resistance would still be too low
so i think i will use ink, this will be my first ever DIY audio project of serious nature. i have made countless subwoofer boxes for friends cars and installed the systems complete, but nothing like this, so it will be a learning process :)

as for the amplifier i am looking at direct drive, and no i am not faint hearted lol. i want to try a direct drive because it will be unique, somethign different to learn about, plus the fewer components in the signal path seems very attractive for quality

in regards to the baffles, sound-lab have somethign like
looking at them they don't look very complex to make so i might try my hand as something similar.

thanks for all the help by the way :)

For a speaker only 250mm wide, I would not use a 1/4" thick insulator. At such a narrow width, you will never approach 1/2" excursion, so you will be giving up a LOT of sensitivity. You will also be quite disappointed with full range operation unless you plan to add some wide "wings" or other baffle to reduce the phase cancellation and bring up the low end.

If you plan to add a baffle, then you will end up with a speaker much wider than 250 mm. If you're going to have a wide speaker, why not just make the driver wider and get more volume out?

Before you make your full sized speakers, I recommend you make a few small test drivers- it is fast, easy, and cheap to do and will save you a lot of time, money, and headaches later on. Build a small driver with the diaphragm glued to 1/16" insulators and held together with clamps to see what sort of volume you can get out of it. Then, stack another set of insulators to bring the thickness up to 1/8", then add more insulators to bring the thickness up to 1/4". You will then have a good idea of how thick to make your speakers.

If you look at commercial full-range ESLs, you'll see they have one thing in common- they are all BIG. There is a good reason for that... they can't produce bass without being big. Actually, even big ESLs are likely to have anemic bass. That is why so many commercial ESLs use a conventional bass driver in a box to provide the bass output.

Disabled Account
Joined 2002
Firstly a correction for my last post, I meant that I will use graphite, not ink. Was in my head, just didn't come out that way.

Okay, upon later investigation I have decided on 1000mm H x 450mm W. This seems to be approx what a few other people with successful ESL's have chosen.

I will build my prototypes hopefully by the weekend, and I will get back with my results.

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Joined 2002
stretching rig will be the sticky tape, table, bike tube and pump method. it seems easy, and i have a few bike tubes laying about.
Unsure how well this will stretch it and how evenly.

Mylar source I am hoping to find tomorrow. I am also yet to find any sources local to me (Melbourne/Australia)

I am going to try that place, because where my dad works (and me too sometimes) they use DYmark line marking paints, so I can get the boss to order it in for me etc.

I sent an email off to them just before regarding the thickness of the film itself to make sure it isn't too thick, and the thickness I am looking up now on what is best, although I thikn thinner is better for higher frequency reproduction due to the lower mass.
Some mylar products here if you want to have a look aswell phishead8
At the bottom is says they are based in Lake Forest, California and a quick search says Pasanda is in CA so give it a go :)

Anyway, back to work for me.

The bike tube on a table method will stretch it VERY evenly and to VERY high tension. You will not be disappointed.

I came up with the table idea after trying out the metal bars with screws at the corners method with disastrous results. The problem with the bars is that tightening the screws at the corners puts huge tension on the film right at the corners, and decreasing tension as you move toward the center of the diaphragm. Flex in the bars only makes the situation worse.

I get a lot of email from people asking about the tensioning table and for some reason they don't seem to understand how fast and easy it is, or how well it works. I think the problem comes from the idea that the diaphragm is held in place with tape. It is easy to see that and say "how much tension can you get if tape can hold it?"

Here's a little experiment to try- tie a weight to the end of a shoe lace or piece of rope or string. Go out to a strairway or balcony railing. Hang the weight over the railing and pull on the string and see how much effort it takes to raise the weight. Now wrap the string around the railing once, and try raising the weight again. You'll see that the effort required is MUCH greater with the string wrapped around the railing. The friction between the railing and the rope is what makes the difference.

On the stretcher table, the film is bent over 3 corners of the edge of the table, greatly increasing the friction between the film and the table (the rubber tube adds to the friction, too). That bending is what allows relatively weak tape to hold the diaphragm under huge tension. Since the air in the tube is free to move about, it will tend to move to where the diaphragm has the least tension, and tighten it up. That means it will tend to even out the tension over the entire surface of the diaphragm, so even if you didn't tape the film down evenly, the air pressure in the tube will tend to even things out for you.

Disabled Account
Joined 2002
Hi ultranalog, I am the one who you recommended not to try the directly-coupled amp as I would probably end up putting myself 6 feet under. Thanks yet again however.

The ESL needs a high voltage DC bias supply. Generally speaking, the higher the bias voltage you use, the higher the sensitivity of your speaker. However, there are considerations beyond speaker sensitivity. If your speakers do not have plastic coated stators, then 1500-2000 Volts is about the highest voltage you will want to use, regardless of insulator frame thickness. Higher bias than that leads to corona discharge and its attendant whining sound. If your speakers use plastic coated stators, you can probably use higher voltages, but that will depend upon the insulator frame thickness also

Regarding that, what kind of properties should the stator coating posess? Insulative? Thick/Thin? Heat proof? I want to try and locate the most appropriate coating I can, as there are so many different paints and coatings these days, there is surely something suitable... and he also mentions plastic coating, I am not sure if he means plastic dipped, or some kind of platic paint coating or similar.
I was thinking powder coating, but am unsure if it is suitable?

Any ideas?

- Mike


Diaphgram is made of 6 micron Mylar and coated with graphite. Steel stator has polyethylene coating which has been tested with 15 kV. This allow rather high polarization voltages to be used. Current voltage is 6.8 kV but it is little too high at summer when humidity is high. Spacers have been made from optimally stiff foam material which has very low dielectricity factor
This guy uses a polyethylene coating. Anyone else comment on their choice of coating if any, and their voltages
For an insulating coating, try a high voltage spray on insulator. I found it at a local alternator repair shop. Just call around and ask them for an isulating varnish in a spray can. The stuff I had was red and it seemed to be the norm. I can't give you more info than that though, sorry. Ideally, after applying this stuff it will reduce the chance of the mylar arcing to the stator. I didn't get to try it out, as I bought the stuff right before a move and I haven't been able to start making ESLs again, yet. Of course, you can paint over the insulator to your taste.
Good luck
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