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DIY planar magnetic + open baffle woofer array
DIY planar magnetic + open baffle woofer array
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Old 22nd December 2016, 03:33 PM   #1
Few is offline Few  United States
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Default DIY planar magnetic + open baffle woofer array

Way back in 2010 I started a project that has finally made its way into my living room. It started as a vertical array of open baffle woofers to go with a new wire-stator electrostatic loudspeaker (ESL). The early stages of the woofer baffle construction were posted here. Alas, the photo links are now broken. In the end I switched to making my own planar magnetic drivers instead of finishing the ESLs.

In the middle of it all my wife and I had a new house designed and built. Somehow the speakers have taken longer to build than our house.

And of course they're still not done. I'm using a miniDSP OpenDRC-DA8 for the crossovers and equalization. I'm new to that game so I think I'll try to get my act together a bit more before posting details on that part of the project. I have made some progress getting a reasonably flat on-axis response but haven't checked on polar responses, tweaked crossovers, or tried FIR filters to fix the phase. Lots left to be done.

The woofers are Dayton 8" reference series from parts express. They're gripped by their magnets rather than by attaching their fronts to the baffle (see the second photo below). The baffle was made with kerfed MDF that want bent into a curve. MDF disks were glued to the front, and then fiberglass and epoxy formed the curved surfaces. The rear spine to which the woofers are mounted was made with laminated fir, half of a PVC pipe to form a round counter, and again fiberglass epoxied around the whole mess to smooth the curves and hold the PVC in place.

The planar magnetic drivers consist of two 10 micron thick aluminum traces, one that covers most of the diaphragm, and a narrow central one that I'm hoping to use as a sort of tweeter. I say sort of because to start I'll run everything from about 150 Hz to 20 kHz through the tweeter, and run 150 Hz - 2 kHz (or something like that---still to be determined) through the wider pattern that surrounds the "tweeter." The aluminum is on 13 micron thick kapton tape and the edges of the kapton have thin stretch wrap material to serve as a sort of surround. The stretch wrap is mounted to the frames that are made of garolite from McMaster-Carr (it's a phenolic material).

The planar magnetic drivers are single-sided, with 42-grade NdFeB magnets glued with cyanoacrylate (Crazy glue) on to perforated steel sheets. Each magnet is 1" long, 0.25" wide, and 0.125" thick. The poles are the 1" x 0.25" faces.

I cut the aluminum traces using a Silhouette Portrait computer controlled cutter that I learned about on diyaudio. A thousand thanks to Bandsei for helping me get started on that and coming up with useful 10 micron aluminum-paper laminate.

The somewhat crazy dimensions of the planar magnetic towers evolved from 1) wanting tall line arrays because my living room, dining room, and kitchen are in a very long open space and I wanted decent sound throughout (without blasting someone sitting close to the speakers) and 2) mission creep set in as I assembled the drivers and then built the frame that houses them. I actually intended to end up with something at least a foot shorter but that's what happens when you design as you build, I guess.

I'll post some measurements once I make a bit more progress.

Few

P.S. Moderators: I actually intended to post this in the "planar and exotic" forum because of the diy planar magnetic component. The OB woofer project was first posted here but feel free to move this post if it would make more sense elsewhere.
Attached Images
File Type: jpg PC210113 speaker portrait in sun.jpg (132.4 KB, 1766 views)
File Type: jpg OB rear view.jpg (129.2 KB, 1693 views)
File Type: jpg PC210110 grill off planar.jpg (135.8 KB, 1653 views)
File Type: jpg planar trace with Silhouette.jpg (86.7 KB, 1637 views)
File Type: jpg close-up of planar traces.jpg (139.8 KB, 1625 views)
File Type: jpg Kapton on traces.jpg (107.3 KB, 817 views)

Last edited by Few; 22nd December 2016 at 03:38 PM.
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Old 22nd December 2016, 03:44 PM   #2
nc535 is offline nc535
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Impressive project! Something that few could do.
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Old 22nd December 2016, 04:20 PM   #3
jimbones is offline jimbones  United States
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whoa thats a serious diy project. very nice work indeed.
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Old 22nd December 2016, 04:42 PM   #4
Gadgety is offline Gadgety
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Wow, really nice both from an aesthetic and tech perspective.
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Old 22nd December 2016, 04:55 PM   #5
eriksquires is offline eriksquires  United States
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Beautiful indeed! I have a physics question. Usually with a di-pole you must keep the baffle width wide enough to prevent front to back cancellation. This doesn't seem to be a concern for you? At 150 Hz the wavelength is around 7', so I would expect the PM drivers to be radiating omnidirectionally, and therefore cancel considerably more than with a wide baffle.

Best,

Erik
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Old 22nd December 2016, 06:18 PM   #6
Few is offline Few  United States
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Thanks for the kind words, all. I think I spent as much time painting and sanding as I did making the planar drivers. I've decided I do NOT have a future career as a painter.

Erik: Yes there definitely is front-to-back cancellation. I'm using the miniDSP to equalize the response and compensate for that effect, while also providing the crossovers.

Few
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Old 22nd December 2016, 06:24 PM   #7
eriksquires is offline eriksquires  United States
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Few, Hmmmm, that I really want to see when done, especially the polar response.
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Old 22nd December 2016, 08:42 PM   #8
Few is offline Few  United States
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Erik,

If you have not yet seen Linkwitz's website pages on dipoles and equalizing their response you can take a look here. Half way down the page the equalization discussion begins.

Also, John Kreskovsky's website has some helpful technical background.

Few
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Old 22nd December 2016, 08:50 PM   #9
mwmkravchenko is offline mwmkravchenko  Canada
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DIY planar magnetic + open baffle woofer array
Nice work Few. Any measurements?
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Old 23rd December 2016, 12:25 AM   #10
Few is offline Few  United States
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Thanks Mark. I made some measurements in my shop as I was testing ideas but there wasn't enough room to allow much reflection-free time for the measurements. I've got more room now that the speakers are in their final-ish locations (they'll be pulled away from the rear wall for more serious listening). I've done some rough measurements just so I could get the miniDSP up and running and yielding a reasonably flat response at the main listening seat but they're not worth posting yet. I do plan to do some more systematic measurements and I'll share them as I make them. I need to learn the art of measuring tall dipoles in a highly reflective environment. I'm open to suggestions!

Part of my intent when designing the speakers was to reduce floor and ceiling reflections by making the speakers tall. I'm also hoping I can aim the dipole null at the wall of windows that's just to the left of the speakers and reduce the impact of that first side reflection. I guess we'll see how that all works out.

Thanks for your interest.

Few
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