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Old 9th June 2008, 12:16 AM   #1
Few is offline Few  United States
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Default Starting a curved baffle dipole woofer array project

I'm (finally) getting my hands dirty with a new dipole woofer project that will ultimately mate with a pair of DIY electrostatic panels. I thought it might make more sense to post the woofer part here rather than in the ESL forum.

I wanted to try my hand at making curved baffles for several reasons:
1) It dramatically stiffens the baffle in one direction
2) It gives me a chance to try making kerfed boards for the first time
3) It gives me a chance to try working with fiberglass and epoxy
4) I'd like to break free from the tyranny of rectilinear speakers--in other words I thought the result might look cool.

I'm going to build two vertical arrays, each holding six Dayton Reference Series 8" woofers from Parts Express (RS225S-8).

Each array will end up between 5.5 and 6 feet tall (a bit under two meters tall, for those lucky enough to living metrically). I made the baffles oversized in both height and width so that I could have room to test techniques and adjust dimensions as I go along. So far I've only cut one woofer hole to see how my ideas will work in the real world.

The kerfed board made of a 7 foot long by 16" wide piece of 3/4" MDF:
Click the image to open in full size.

The kerfed board temporarily wrapped into its curved shape:
Click the image to open in full size.

The router support used to make flat cuts in a curved surface:
Click the image to open in full size.

The cutout designed to accept a woofer in its MDF mounting ring:
Click the image to open in full size.

An MDF woofer mounting ring placed into its hole in the baffle:
Click the image to open in full size.

A woofer mounted to its ring and placed in the hole. The woofers will be mounted from the rear (an unfortunate phrase) to hide the mounting holes and provide a cleaner look from the front.
Click the image to open in full size.

Ultimately, I plan to wrap the MDF baffle with fiberglass and epoxy to stiffen it further and smooth the transition from the mounting rings to the curved baffle. Here's a very rough test of what that might look like. That's my unprotected hand handling fiberglass. Nine years of college and still not too bright... Once the fiberglass is applied I'll cut through it to re-expose the woofer holes. I'm also trying to come up with a good way to fill the small gap between the fiberglass and the MDF at the sides of each woofer ring. Epoxy with talc as a filler is one option I'm toying with.
Click the image to open in full size.

That's as far as I've gotten so far. I'm hoping that posting this will force me to maintain construction momentum. I need to find easier ways to do some of the steps I've done so far if I'm going to mount twelve woofers. The Jasper router jig really works well for cutting circles, but you always need a stable center pivot hole about which the router can rotate. That's not always conveniently achieved. Here's a picture of the jig being used to cut the hole in the router support. I managed to make the hole the wrong size and then had to figure out a way to make it larger without having solid material to hold the Jasper jig's pivot pin. At least I made the hole too small instead of too large! The woofer mounting ring, also cut with the Jasper jig, is partially visible at the bottom of the picture.
Click the image to open in full size.

Few
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Old 9th June 2008, 03:20 PM   #2
Few is offline Few  United States
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One point of clarification: The 1/2" thick MDF woofer mounting rings will not provide a sufficiently stable platform for the woofers, and the baffle will only be 3/4" thick (plus fiberglass). To minimize the excitation of panel resonances and maximize woofer stability I'm planning to include a vertical support of some kind behind the curved baffle and mount the woofers to that support, probably by gripping their magnets. I'll then use a soft gasket to seal the woofers to the MDF mounting rings. One of Linkwitz's Orion modifications included a similar approach. I'd like the vertical support to be stiff along the woofer vibration axis but not so bulky that I lose the attractiveness of the thin front baffle. Has anyone seen an elegant solution to this sort of problem?

Few
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Old 9th June 2008, 09:15 PM   #3
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Default Re: Starting a curved baffle dipole woofer array project

Quote:
Originally posted by Few


Ultimately, I plan to wrap the MDF baffle with fiberglass and epoxy to stiffen it further and smooth the transition from the mounting rings to the curved baffle. Here's a very rough test of what that might look like. That's my unprotected hand handling fiberglass. Nine years of college and still not too bright... Once the fiberglass is applied I'll cut through it to re-expose the woofer holes. I'm also trying to come up with a good way to fill the small gap between the fiberglass and the MDF at the sides of each woofer ring. Epoxy with talc as a filler is one option I'm toying with.

Hi,
Looks like an interesting build. I've done a few curves myself.
You might want to reconsider the fiberglass wrap as it will be very time consuming to make smooth.

For what its worth, I would stack the rings to go completely through the opening, gluing them in. I'd also mount the woofers from the front, as this will be a lot easier and easier to brace the magnet.
Where the rings meet the baffle, you could use auto body filler (basically talc mixed with epoxy, except in a more convenient package) to fill and smooth the transition. Might be worth filling the kerfs with this too to increase the overall strength.

The front of this amp case is kerfed 5/16" HDF (masonite). sides and top are also 5/16" HDF.
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Old 11th June 2008, 03:44 AM   #4
Few is offline Few  United States
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Thanks for the response and the suggestions. You certainly might be right about the fiberglass concerns. On the other hand I'm not looking forward to cutting out twelve woofer rings out of MDF so if I had to do 24 or 36 rings in order to stack them, I think I'd really be discouraged.

As a result of the concerns you voiced I've started reconsidering an idea I had awhile ago. I could carefully sculpt the desired shape around the cutouts and MDF ring I've made already, perhaps using auto body filler to refine the shape. I could then use that shape as a plug, make a fiberglass mold from that plug, and then make twelve baffle fronts from the mold. I'd only have to carefully prepare the surface of the plug once, and if all goes well, I'd be able to make twelve copies whose surface finish wouldn't require lots of sanding before painting. I don't have any experience with this approach but it would be a MUCH simpler project than the one that inspired my thinking---a thread describing the diy fabrication of a carbon fiber motorcycle fuel tank, as described
here

I agree that filling the kerfs would be helpful. I've explored running some rubber sheet down each kerf in order to provide some damping, or I could just use filled epoxy in the way you described. I guess I'll have to experiment a bit more before finalizing a plan.

Thanks again for the thought provoking comments. By the way, I was inspired by your amp case photos in the high gloss thread. I hope to try your approach if I ever manage to get these things built.
Few
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Old 14th July 2008, 02:41 AM   #5
Few is offline Few  United States
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Well I guess slow progress is better than no progress. Work and life have been getting in the way, but I finally have the woofer rings and the holes to accept them cut into one curved baffle. Here are the various stages in the birth of a ring. I devised a jig that would hold the rings at various stages of their construction to simplify the use of my Jasper jig. Here's the jig and a few rings in various stages of their completion.
Click the image to open in full size.
Click the image to open in full size.

I ended up having to fill the kerfs in the curved MDF baffle before I could cut all the woofer holes because the baffle started to flex too much when more than one hole was cut into it. I filled the kerfs with West System epoxy thickened with talc. When I started to realize how much epoxy I was going to use up in this process ($$!), I quickly dug out my supply of popsicle sticks to help fill the kerfs. The thickness of the sticks turned out to be perfect, and I think I cut the volume of required epoxy down by more than 50%. Here's the concave side of the baffle.
Click the image to open in full size.

I refined the platform I made to support the Jasper jig so that I could cut flat holes into the convex side of the curved baffle. You can see the filled kerfs where the material has been removed to accommodate the woofer and woofer ring.
Click the image to open in full size.

With all six woofer rings resting (but not yet epoxied) in their holes, things start to take shape.
Click the image to open in full size.

Here's the side view of a ring sitting in its hole in the baffle. If I go ahead with the fiberglassing as planned (see earlier posts), I'll use the gap between the ring and the baffle to provide access to the space between the fiberglass and baffle. I'll need to fill that gap with sand or expanding foam, or...other suggestions? Now that I have a clearer idea how much epoxy costs and how much you can go through in short order, I know I'll not be filling that space with epoxy.
Click the image to open in full size.

I have to say that now I've put together a few photos it doesn't look like as much progress as it felt like when I turned off the light in my shop this afternoon. The good news is that things are moving much faster now that I've devised some jigs to crank things out reproducibly. I'm hoping the other channel will take a small fraction of the amount of time this one did. Before I deal with the second channel, though, I still have to delve into the world of fiberglass and epoxy. I've given a lot of thought to the suggestions that I just forget about the fiberglass, but at the moment I'm still leaning toward using it. It will stiffen the baffle, and part of the reason for taking on this project was to gain a little experience with some new (to me) materials and techniques.

Few
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Old 14th July 2008, 02:33 PM   #6
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Where will you cross the woofers to the panels?
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Old 14th July 2008, 02:37 PM   #7
Few is offline Few  United States
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I'm going to wait until I have some measurements in hand before making a firm decision, but expect it to be between 200 and 500 Hz. I was originally leaning more toward the high frequency end of that range, but I don't want the woofer-woofer or woofer line-to-ESL distance to cause major lobing issues so I may end up closer to 250 Hz.

Few
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Old 14th July 2008, 02:46 PM   #8
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I asked because I was trying to figure out if you could use one solid piece to mount the woofers on then mount the one piece smaller stiffer baffle to the curved baffle without mucking up the sonic benefit of the curved baffle.

I see you are using Russ's panels - NICE!
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Old 14th July 2008, 03:03 PM   #9
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Default Just a little note ...

Flex board (pre kerf'd) MDF is available.
Anderson international (aitwood.com) has it if you can't get it locally. Theyalso have a bunch of neat shapes and corner radii that hame life easier.
HTH
Jorge
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Old 14th July 2008, 07:10 PM   #10
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Thanks for the interest, Magnatar, but I'm afraid you lost me with the "Russ's panels" reference. Which panels do you mean? These are home-brew. I did look into buying some pre-kerfed panels but had trouble locating a local source; companies never got back to me after I emailed asking for information. Thanks for the aitwood.com link, Jorge. I did know about them but the larger sizes looked expensive to me when I was planning things out. Since I already had MDF hanging around I decided just to do it from scratch. Now that I know what fun it is to fill the kerfs with epoxy, and how expensive the epoxy is, a commercially available precurved panel of the right dimensions might be a good option.

Magnatar, when you mention the "smaller stiffer panel" are you referring to the panel grabbing the woofers by their magnets, or the electrostatic panel these will be mated with? When I first read your post I thought you were referring to the ESL panel so I wrote up the following response. As I reread things I realized I may have misunderstood. I've already got this typed up so I'll just leave it in here in case it's of interest to anyone.

Regarding attaching the ESL panel to the woofer panel:
I thought a bit about coupling the ESL panel directly to these curved baffles, but I'm worried that the front/back pressure difference generated by the woofers might force the ESL diaphragm into the stators. Leaving a gap between the ESL and the woofer array provides at least some chance to short circuit that pressure difference. Also, I want to minimize the chance that woofer panel vibrations will modulate the ESL's output. Of course the acoustic coupling risks causing the same problem, but others have gotten this configuration to work well so I'm going to give 'er a try. Here's one of the better known commercial examples:
ML Statement e2

If you were writing about a panel that the woofers' magnets would be mounted to and you have some suggestions about the geometry, can you tell me a bit more about what you have in mind?

Few
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