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Old 20th December 2004, 04:02 PM   #1
Few is offline Few  United States
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Default experiences with ESL directivity?

I'm interested in hearing about the successes or failures others have had with trying to make electrostatic loudspeakers (ESL's) with something approximating constant directivity (distribution of sound independent of frequency). Here "constant" directivity means the speaker has the same dipole radiation pattern from a few hundred Hz on up---the pattern doesn't grow narrow and pick up more lobes as the frequency increases.

For practical and theoretical reasons I'm inclined to avoid curved panels (sections of an upright cylinder). Since I'd like to make something a bit narrower than the 20"-wide ESL/dynamic woofer system I now use, I don't think approximating a cylinder with multiple flat panels would do the trick either. That leaves me feeling that a narrow, tall tweeter, flanked by wider midrange sections, etc. would be the way to go, but of course there are many ways that could be implemented.

The other alternative might be a cylindrical version of the Quad ESL 63's point source approximation. That would involve delay lines so the signal emanates first from a narrow, tall central section of the diaphragm and then emanates from regions progressively farther from that central section. I'd be inclined to try to do this actively but I'd have to come up with a low-cost amplifier/transformer scheme that I could afford to duplicate several times--once for each delayed section. I believe it was Bill Waslo who showed that cheap transformers can be used to drive small sections of ESL panels. I haven't yet tried to duplicate his results. I also have yet to complete my chip amp project so I haven't tried it with a capacitive load. I'd be interested in finding out if one of those would tolerate a very narrow section of a larger ESL panel. Even if the amp and transformer details could be worked out, there's still the pain of making sectioned ESL stators, but I guess I'm going to run into that no matter what I do.

The tweeter/midrange approach certainly seems cheaper and simpler. Anyway, I'd like to make new mistakes instead of repeating old ones so if anyone has experiences or insights pertinent to the problem of controlling ESL directivity, I'd love to hear about them. BTW, I'm more likely to do another hybrid system rather than trying to go ESL fullrange.

Thanks in advance.
Few
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Old 20th December 2004, 05:00 PM   #2
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The line-source analog of the Quad 63 arrangement was, IIRC, one of the tricks Acoustat used in the Spectra series. And narrowing down the radiating area with increasing frequency is an old trick; there was an old paper reprinted in the back of Wagner's book showing an implementation.
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Old 20th December 2004, 06:41 PM   #3
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Thanks for the reply. I'm aware that there are commercial incarnations of the narrow tweeter, wider midrange... approach (starting with the original Quads), but I haven't read much about diy versions recently. Most people seem to go the Sanders route and just allow the treble to become increasingly directional as the frequency increases. That's what my current ESL's do, and I'm growing tired of the effect. You haven't heard beamy treble until you've heard a 20" wide speaker putting out 15 KHz!

Interesting Acoustat comment. I'm not an Acoustat expert but what I've read about them suggests they used the "tweeter" approach rather than the full-range delay line approach for some of the speakers in the Spectra line. Perhaps I'm just misinterpreting the limited information I've seen.

So, is there a reason there seem to be fewer homebrew attempts to broaden the ESL's distribution of high frequencies? Is it just because everyone avoids the construction hassles, or is it because people have tried tweeters or delay lines but the results have been unsatisfying? ESL's seem particularly well suited to this sort of tweaking since there are no bulky magnets and pole pieces to contend with. It provides lots of design flexibility and less trouble with the cavity resonances found in most ribbon or quasi-ribbon tweeters.
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Old 20th December 2004, 06:53 PM   #4
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My info on Spectra is limited, too. I use some converted Acoustat 1+1 panels (8" wide), but they beam like crazy, too. Narrow sources need high excursion, and that's difficult for diy panels.

Reality: people who have big panels rarely have anyone else around to worry about who gets the sweet spot.
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Old 16th January 2005, 07:48 PM   #5
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I've been building ESL's for 20 years and found that making them as narrow as possible to be the best way to control beaming.
Ultimately, I found 4X36 inches to be the best of both worlds, 2 cells of this size stacked to make a 6 foot line source.
A stereo pair can play to 100 db at 18 feet, full range as a dipoles or mounted in a walll as an infinite baffle. I use 1/8 inch spacers.

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Old 16th January 2005, 08:16 PM   #6
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Thanks, Jack, for the reply. That's very interesting. Based on recent reading and thinking I was considering *decreasing* my currently used diaphragm-stator spacing of 1/16" when making my next pair of speakers, but your 1/8" spacing is clearly a vote for going the other direction. And you're running them full range? Hmmm...

Can I ask a few questions about your system at the risk of being nosey?

1) What are you using for a transformer? (Step-up ratio, brand...)?

2) What bias voltage do you typically use?

3) What kind of amplification do you drive them with and have you found 100 db to require lots of watts?

4) What is the resonance frequency of the panel? I'm assuming you don't have any spacers other than the 1/8" spacers around the periphery of each 4" x 36" panel, is that true?

Thanks again for a thought-provoking reply.

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Old 16th January 2005, 08:40 PM   #7
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I'd be happy to supply you with answers to your questions.

The transformer I use is made by Tranex, it has a step up ratio of 1/150.

My bias voltage is around 5 kv, I can control the output if I wish.

The driving amplifier is an old Hafler DH-500.

The fundamental panel resonance is around 40 hz.,no other spacing is used except the periphery.

I don't wish to upset the die hard esl builders here, but I do not use
mylar for the diaphragm, I would never be able to get that low a resonance frequency. I use "Saran Wrap". Yes, plain old Saran Wrap, it has just the right properties for my use. "No" other films will work quite like it.
I can provide additional details on construction, just ask.

Jack
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Old 16th January 2005, 08:52 PM   #8
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Jack,
Saran Wrap--more food for thought! (No food/food-wrap joke intended.) I hope there's no need to worry about upsetting the die-hard enthusiasts. If we were all interested in doing everything the standard way, one of the motivations for the DIY approach would disappear--as would much of the fun.

I assume you use Saran Wrap because it drops the resonance frequency compared to a stiffer Mylar diaphragm. Have you found any evidence of high-end roll-off because of the more massive diaphragm material? Any problems with the diaphragms losing tension over time?

If I'm remembering correctly the original Quads used something Saran-like for the bass panels, and then polyester film or something like it for the mids and highs

With such narrow panels do you run into problems compensating for front-to-back cancellation? If you do that electronically, at what frequency do you start the compensation?

Thanks for your willingness to entertain more questions.
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Old 17th January 2005, 01:29 AM   #9
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Few,

I would have to say that Saran Wrap's ability to provide a lower resonance has more to do with it's elasticity than mass.
Classic Saran Wrap is 0.5 mil in thickness, surely half that thickness would be nice but not imperative. Another reason for the lower resonance is the fact that I use 1/8 inch thick double sided foam tape for spacers. It has a little give to it and has dampening qualities.
Since the active area of my cells is just 4 inches wide, I needed a baffle to provide extra width for cancelation.
I wound up using a flat baffle about 22 inches wide and 80 inches long and cutting a slot to fit two cells in a vertical line. I added 4 right angle supports to hold things straight up. To this I added hinges on the outside edges of the main baffle and added foldable wings to double the baffle width.
I have always been the sort to use common every day materials to build things. Ask me what I use for a conductive coating on the diaphragm. No, it's not graphite, although it is easy to find in any hardware store.

Jack
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Old 17th January 2005, 01:32 AM   #10
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Saran does have a higher density than PET, so there may be something in that. It can be a devil to coat permanently without proper surface treatment- I'd be curious what you found that would stick.

I use Clysar, a shrink-wrap material. It's impractical for a commercial design (needs restretching too often) but it has that nice soft non-crinkly character that I like in Saran compounds. I do use a Saran-based conductive coating on it.
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