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Old 21st September 2002, 05:37 AM   #1
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Default bi-polar electrostatics?

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

+IFI-IFI+

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.
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Old 21st September 2002, 01:37 PM   #2
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I have no idea about ESLs but it's almost like an isobaric configuration isn't it...!
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Old 22nd September 2002, 04:52 AM   #3
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specialx,
Can you explain the dipolar cancellation problem?
I'm not exactly sure what you are trying to solve with this configuration.
Thanks
-Dan
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Old 22nd September 2002, 06:53 AM   #4
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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.

dave
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Old 22nd September 2002, 02:25 PM   #5
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phishead8:
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?
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Old 22nd September 2002, 08:39 PM   #6
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specialx,

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,
-Dan
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Old 23rd September 2002, 12:11 AM   #7
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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
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Old 23rd September 2002, 02:37 AM   #8
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Quote:
Originally posted by specialx
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
-Dan
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Old 23rd September 2002, 03:14 AM   #9
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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 http://www.amasci.com/esloud/eslhwto.html 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, www.conductivecompounds.com
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
http://www.soundlab-speakers.com/accessories.htm
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

-Mike
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Old 23rd September 2002, 02:47 PM   #10
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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.

MR
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