I'm drawing up plans for a pair of speakers in which each channel consists of a stretched wire ESL mated with a vertical array of six Parts Express RS225 8" woofers mounted in an open baffle. The lowest 1 - 2 octaves will be handled by an old NHT 1259 woofer in a sealed box (~20" cube).
I've decided to make the ESL element out of paper/phenolic boards (called Garolite XX in the McMaster-Carr catalog) and magnet wire. I can post a SketchUp rendering if it turns out it would be helpful, but my real question concerns the dimensions of the ESL panel. I'm going to replace my current perforated steel stator ESLs which are about 20" wide and a little over 6 feet (~2 meters) tall because
(1) they're too big
(2) they beam like crazy
(3) I need something to improve the midbass, and
(4) I'd like to try something different.
The woofer array plus ESL may end up as large as my current ESL panels so reason (1) may not hold any water; reason (2) leads me to segmented wire stators; reason (3) leads me to the dynamic woofer array; I don't think I need to justify reason (4) to this crowd.
I was going to build wire stators that are between 9 and 12 inches wide, about 5 feet tall, and with a 1.5 mm stator-diaphragm gap, but the Garolite doesn't come in anything longer than 4 feet. I can come up with some sort of joint to make 5 foot long Garolite sections, but I worry about the flatness of the result, and I would like to avoid going through the trouble. I could make two small ESL panels and stack them vertically, but the middle seam between two ESL panels is one of the "features" of my current ESLs that I'm trying to eliminate in the new design.
I've pretty much decided not to use MDF or plywood instead of Garolite even though it would be easier to make long pieces out of those materials. This decision is partly motivated by a small prototype I made out of Masonite (HDF) that has let lots of slack accumulate in the stretched wires. I think that it was humid when I put the prototype together and the dry winter air has let the Masonite shrink enough to make the wires slack. In addition, the Garolite is due to arrive in a couple of days so now I'm committed!
Do those of you with ESL panels shorter than 5 feet (~150 cm) run into any difficulty? I like the idea of a true line source so I'm concerned about using an ESL panel that's so much shorter than the distance from the panel to the listener. Also, I'd like to be able to hear full range sound while standing so the top of the panel needs to be over my head or they need to be tilted back a bit. I noted that Martin Logan's Summit model has an ESL panel that is about 44 inches (112 cm) tall and about 12 inches wide, tilted back so that standing listeners don't lose the treble.
So should I go with 48 inch tall 12 inch wide panels, which can be done easily with the Garolite I've purchased, or go through the trouble of making taller panels? Suggestions or reassurance are welcome.
Reassurance: Since the material is on its way and its max dimension is 4ft, then that's what you're dealt, height-wise. Could you go wider than 12 inches?
I'm assuming the 6 RS225s stand on top of each other next to the ESL?
I'd be happy to put this in SolidWorks to get you going, if you're not in a hurry. How about some more background on your current ESL?
I'd like not to go wider than 12 inches because the vertical array of six woofers will be mounted on a baffle that's about twelve inches wide as well (it might actually be closer to 14 inches...). If I put the ESL and woofer array next to each the pair starts to become pretty imposing looking. I'm hoping , though, that two separate ~14" baffles will feel less imposing than the single ~24" baffle I now have. The bottom line is that for space reasons I'd like not to go any wider.
Also, one of my goals for this project is to achieve a frequency response that doesn't vary much as a function of horizontal angle. The SPL will diminish off axis because of the dipolar radiation pattern, but I'd like all frequencies to diminish equally. (I'm trying not to use the word "dispersion" in this context because it already has another meaning when applied to waves.) I think this goal might be easier to achieve if the effective width of the ESL and the woofer array are similar so there isn't a big discontinuity at the crossover frequency.
Finally, I'm planning to try a fairly high crossover frequency, maybe something like 500 Hz, to minimize the amount of air the ESL has to move. The wavelength is only 27" at that frequency so I don't want to make the distance between the center of the woofer array and the center of the ESL panel too great. So that's the reasoning that took me to this point.
You also asked about my current system. Each of my current ESL speakers consists of two 20" wide by 3 ft tall perforated steel stators, stacked vertically. Including the frame that holds everything together the speakers are nearly six and a half feet tall. They're made fairly conventionally, with 3M double-stick foam spacers (1/16" thick), graphite coating on the mylar, and an op-amp based active crossover. I used epoxy paint normally used for kitchen appliances to coat the stators. The transformer and mylar were bought from Barry Waldron.
I love the sound but they beam more than I can live with happily, and I can't run the ESLs low enough in frequency to make a smooth transition over to the 12" diameter woofers I'm using for the bass. I end up with less than satisfying mid-bass. I also blew out an amplifier after years of use with these ESLs and I have a feeling the massively capacitive load may have hastened its demise. A smaller panel that's segmented should reduce the capacitance quite a bit, even if I go with a slightly smaller stator-diaphragm distance.
Thanks very much for the generous SolidWorks offer. I've been making heavy use of SketchUp for brainstorming and Vectorworks for dimensioned drawings so I think I have that part covered.
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