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
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
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.
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.
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.
Reality: people who have big panels rarely have anyone else around to worry about who gets the sweet spot.
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.
Jack
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.
Jack
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.
Few
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.
Few
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
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
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.
Few
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.
Few
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
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
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.
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.
I'm the sort of person that loves to use common easy to find items.
I wanted a coating material that would of course stick to Saran, I also wanted it to be invisible, and I wanted something with high resistance that actually worked.
After trying a myriad of products I came across a technique that works reliably and is repeatable.
After stretching and mounting the Saran to the spacers, I ran a thin strip of copper foil around the perimeter. I did this in such a way as to have copper just about in the middle of the spacer. A few inches is saved at the end for termination to the bias supply.
Using a clean lint free cloth, I apply my coating on the copper foil all around the periphery working my way to the center. I do this sparingly but a little more is used around the copper.
I'm getting to it!!......The coating I use is the "Family Dollar" brand of
acrylic floor polish, it works beautifully!
According to my tests, it does decrease in resistance with higher humidity conditions, such as my basement. I haven't found this to have any detrimental effects on performance.
Of course there are lots of professional materials out there, but this is a DIY group and it's a good way to get great results for very little money.
Jack
I wanted a coating material that would of course stick to Saran, I also wanted it to be invisible, and I wanted something with high resistance that actually worked.
After trying a myriad of products I came across a technique that works reliably and is repeatable.
After stretching and mounting the Saran to the spacers, I ran a thin strip of copper foil around the perimeter. I did this in such a way as to have copper just about in the middle of the spacer. A few inches is saved at the end for termination to the bias supply.
Using a clean lint free cloth, I apply my coating on the copper foil all around the periphery working my way to the center. I do this sparingly but a little more is used around the copper.
I'm getting to it!!......The coating I use is the "Family Dollar" brand of
acrylic floor polish, it works beautifully!
According to my tests, it does decrease in resistance with higher humidity conditions, such as my basement. I haven't found this to have any detrimental effects on performance.
Of course there are lots of professional materials out there, but this is a DIY group and it's a good way to get great results for very little money.
Jack
Jack: I agree the lower resonance of Saran Wrap is probably more a result of its elasticity than its greater mass (as compared to the more typically used Mylar). I intended my mass-related question to be aimed at the high frequency end of the spectrum. I wondered whether you have heard or measured any evidence of a roll-off in the treble.
I haven't tried your coating method, but it certainly sounds like it's worth a shot. Have you tried it on any other diaphragm materials? Just wondering whether it would adhere well to most of them, or whether the Saran is just particularly well suited.
Your wings approach to reducing the cancellation effects seems like a good one. It's sure too bad physics forces you to give up the narrowness of the 4" panel. I'd be happy to regain some room in my house by having speakers that are narrower than the two feet wide ones I've been using for the last few years. Interesting ideas; thanks for sharing them.
Sy: What do you mean by a "Saran-based coating"?
Few
I haven't tried your coating method, but it certainly sounds like it's worth a shot. Have you tried it on any other diaphragm materials? Just wondering whether it would adhere well to most of them, or whether the Saran is just particularly well suited.
Your wings approach to reducing the cancellation effects seems like a good one. It's sure too bad physics forces you to give up the narrowness of the 4" panel. I'd be happy to regain some room in my house by having speakers that are narrower than the two feet wide ones I've been using for the last few years. Interesting ideas; thanks for sharing them.
Sy: What do you mean by a "Saran-based coating"?
Few
A conductive coating will typically consist of a dispersion of conductive particles (in this case, carbon) in a carrier resin (in this case, Saran) and a solvent (in this case, butyl carbitol acetate). The coating is painted on, then the solvent dried out to leave a cured coating comprising the carbon particles and the Saran.
Saran is a particularly interesting material because of its profound water resistance. The water vapor permeation rate is among the lowest of all polymers.
Jack's floor-polish idea is a good one for most diy, but you'll have to keep an eye out for long-term stability.
Saran is a particularly interesting material because of its profound water resistance. The water vapor permeation rate is among the lowest of all polymers.
Jack's floor-polish idea is a good one for most diy, but you'll have to keep an eye out for long-term stability.
The unfortunate part about the Family Dollar floor polish is the actual manufacturer is unknown. This stuff has worked for almost two years so far.
One other thing I wanted to mention about Saran is that it is possible to get it larger widths. I need to do some cleaning up on my bookmarks, but I did find a place that can provide widths up to 24 inches. I'll see if I can find a URL on that.
BTW, my setup uses a one ohm resistor in series with the input side of the transformer for EQ.
The0.5 mil thickness of the Saran can easily do 20k with no problem.
Jack
One other thing I wanted to mention about Saran is that it is possible to get it larger widths. I need to do some cleaning up on my bookmarks, but I did find a place that can provide widths up to 24 inches. I'll see if I can find a URL on that.
BTW, my setup uses a one ohm resistor in series with the input side of the transformer for EQ.
The0.5 mil thickness of the Saran can easily do 20k with no problem.
Jack
I’m not a regular on this forum, but I had few comments on this thread: Any speaker that uses either R-C or L-C low pass filters for the side panels (while running the center full full-range) will achieve broadening of the directivity by two simultaneous effects: 1.) the highs are restricted mostly to a narrower emitter, and 2.) the low-pass filters also delay whatever highs make it through to the side panels, having the effect of setting them slightly behind the center panel. These effects combine to improve “dispersion”. I believe it is correct to refer to either effect when using such filters, which is how I think that the Spectra worked. By the way, for comparison, the outer rings of the Quad 63 and recent successors are also somewhat low-pass filtered. The highs are attenuated a bit more with each successive ring as well as being delayed. The two effects go hand-in-hand quite smoothly.
There is a bigger problem than just narrow directivity with using big panels at high frequencies. There will be rising high frequency response as highs are concentrated into the narrow beam. Sanders saw this in his early Audio Amateur article and built an op amp filter to reduce the highs accordingly. I don’t know what InnerSound is doing today in this regard, but there must be a passive filter of some kind in the cross-over.
To me it always made sense to put multiple panels together into a partial cylinder (or sphere), a la Sound Labs or the old RTR tweeters. You can run them all full range and still achieve broader directivity and less of the high frequency rise as you approximate the surface of a cylinder. Of course, this means more panels and more capacitance to drive, but it also means potentially greater SPLs and smoother response.
Finally, heavy films like Saran Wrap will most definitely roll-off highs prematurely as the mass reactance of the diaphragm exceeds the radiation resistance of the mid-range, causing a 6 dB per octave roll-off. It’s just physics. This is why some people strive for such thin films. With ½ mil and ¼ mil membranes, one can typically achieve bandwidths out to around 20 KHz or 25 KHz at the -3dB point. Now if you have a large beamy panel also made with heavy Saran Wrap, then you might get an accidental cancellation of the two problems, but it won’t be exact. Heavy films have their place in ESL woofer panels, IMHO.
There is a bigger problem than just narrow directivity with using big panels at high frequencies. There will be rising high frequency response as highs are concentrated into the narrow beam. Sanders saw this in his early Audio Amateur article and built an op amp filter to reduce the highs accordingly. I don’t know what InnerSound is doing today in this regard, but there must be a passive filter of some kind in the cross-over.
To me it always made sense to put multiple panels together into a partial cylinder (or sphere), a la Sound Labs or the old RTR tweeters. You can run them all full range and still achieve broader directivity and less of the high frequency rise as you approximate the surface of a cylinder. Of course, this means more panels and more capacitance to drive, but it also means potentially greater SPLs and smoother response.
Finally, heavy films like Saran Wrap will most definitely roll-off highs prematurely as the mass reactance of the diaphragm exceeds the radiation resistance of the mid-range, causing a 6 dB per octave roll-off. It’s just physics. This is why some people strive for such thin films. With ½ mil and ¼ mil membranes, one can typically achieve bandwidths out to around 20 KHz or 25 KHz at the -3dB point. Now if you have a large beamy panel also made with heavy Saran Wrap, then you might get an accidental cancellation of the two problems, but it won’t be exact. Heavy films have their place in ESL woofer panels, IMHO.
I'm smiling to myself on that one Sy, my 60'sh ears can barely do 13k
these days.
I am somewhat confused by Brian's remark of categorizing Saran as a heavy film. Perhaps my decimal point doesn't show up clearly, Saran has a thickness of 1/2 mil. This is not a heavy film and can provide plenty of high end, even for younger ears.
IMHO 20k is plenty for anyone, using Saran is a great way to get quality DIY ESL sound.
There are other reasons for using EQ beyond the physics of beaming in a wide panel. ESL's are a very high impedance device, a matching transformer doesn't come close to even things out. Direct drive ala tubes makes a difference, but opens up a whole new can of worms.
Finally, Roger Sanders and I have talked about ESL beaming 20 years ago. We both agree that narrow directivity is better, there is less room interaction. I found 4 inches to be a nice compromise, but still use the flashlight method of setting up the panels for my sweet spot.
I heard from Roger yesterday, by the way, he seems no worse for wear since leaving Innersound. He's a man of integrity and I respect that.
Jack
these days.
I am somewhat confused by Brian's remark of categorizing Saran as a heavy film. Perhaps my decimal point doesn't show up clearly, Saran has a thickness of 1/2 mil. This is not a heavy film and can provide plenty of high end, even for younger ears.
IMHO 20k is plenty for anyone, using Saran is a great way to get quality DIY ESL sound.
There are other reasons for using EQ beyond the physics of beaming in a wide panel. ESL's are a very high impedance device, a matching transformer doesn't come close to even things out. Direct drive ala tubes makes a difference, but opens up a whole new can of worms.
Finally, Roger Sanders and I have talked about ESL beaming 20 years ago. We both agree that narrow directivity is better, there is less room interaction. I found 4 inches to be a nice compromise, but still use the flashlight method of setting up the panels for my sweet spot.
I heard from Roger yesterday, by the way, he seems no worse for wear since leaving Innersound. He's a man of integrity and I respect that.
Jack
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