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Magic metamaterial diffusers -- almost!
Magic metamaterial diffusers -- almost!
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Old 18th April 2018, 09:25 PM   #1
bwaslo is offline bwaslo  United States
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Magic metamaterial diffusers -- almost!
Default Magic metamaterial diffusers -- almost!

(edit -- I originally said "wavelengths down to 1/46th.." Got wavelengths and frequency confused )

I ran into this paper - broadband diffusors that work up to wavelengths 46x the thickness of the diffusor. Looks like it would be a feasible 3D print (in a number or sections to get to the 52 inch width).

Click the image to open in full size.

See in particular, page 7 and table 4. (The deginitions of the parameters in table 4 are shown in Figure 1).

https://www.ncbi.nlm.nih.gov/pmc/art...ticle_5710.pdf

These could be made into wall hangings only a little over 1" thick and work down to 250Hz! Get some grille material the same color as the walls and should be an easy sell to significant others who would hate traditional diffusors. If I work up the STL files, I'll post them.
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Last edited by bwaslo; 19th April 2018 at 12:01 AM.
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Old 19th April 2018, 05:23 AM   #2
bwaslo is offline bwaslo  United States
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Magic metamaterial diffusers -- almost!
And here's the complication with that -- I made STL models for a 1ft high slice of this (in 11 prints of smaller parts to be combined) and the print time for that, with every speed up trick I could do, is still 28 hours. And that's with a 1mm nozzle and a 12"x12" bed. So, not a small project.

I may still do it, since my printer mostly sits there doing nothing most of the time. But to get to a 3ft x 4.5ft panel, its like four days of printing!
Magic metamaterial diffusers -- almost!-bed-placement-png
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Old 19th April 2018, 05:27 AM   #3
DPH is offline DPH  United States
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Print a mold and cast? Probably still too much work, if not more!
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Last edited by DPH; 19th April 2018 at 05:32 AM.
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Old 19th April 2018, 05:53 AM   #4
bwaslo is offline bwaslo  United States
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Magic metamaterial diffusers -- almost!
Yeah, and the structure doesn't look good for molding, or even extrusion for that matter. I'd be afraid of the overhanging wall things resonating. I put little bridge struts every 3 inches or so in the STL model to try to avoid that. Maybe some flat supports (or a screen?) could be attached to the top to keep it stable?
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Old 19th April 2018, 04:31 PM   #5
PRR is offline PRR  United States
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Quote:
Originally Posted by bwaslo View Post
...Looks like it would be a feasible 3D print....
Looks to me like 5/4" board, nice 1/16" veneer plywood, a fine table-saw, and some glue and jigs. Rip a whole lot of 8mmX13mm etc-etc slats, the 100mm wide exposed faces, and start gluing to a 5mm backer.

Inconsistency is *good*. A series of resonances may plot well but the ear is harder to fool. Bumping the saw fence 1mm every few cuts will spread the resonances around.

You are right, structurally it "flaps". The resonator slots need little fillers every few inches. A few columns inside the larger resonators to support the faces.
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Old 19th April 2018, 07:01 PM   #6
bwaslo is offline bwaslo  United States
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Magic metamaterial diffusers -- almost!
I'm thinking that maybe a sheet of 'hardware cloth' (square grid largely open screen') could be glued across it to tame it all down, without too much added labor effort.

You might be right about using board strips, additional length is nearly 'free' (labor wise) doing that. Note that in the model the thickness of the walls is under a mm! I did them all at 1.2mm (about the thinnest I could get with a 1mm 3D printer nozzle). But I suspect that could be cheated substantially.
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Old 21st April 2018, 05:10 PM   #7
DPH is offline DPH  United States
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P.S. Thanks for the link, Bill, I'm still trying to think of clever ways one could build this, but it's a tricky geometry. Or at least re-figure the dimensions to get something far more tenable to manufacturing
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Old 21st April 2018, 05:31 PM   #8
bwaslo is offline bwaslo  United States
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Magic metamaterial diffusers -- almost!
I think PRR has the right approach -- strips sliced on a table saw. Perhaps also make a bunch of little pedestals (back to 3d printing for those?) that could be placed where needed to help support the dangling sections and to break up resonances. I'd also 3D print small spacers to align all the parts for the right gap sizes.

It would be cool to use thin polycarbonate or lexan sheets for the stock material, but those can be tricky to cut (and tend to cut up your hands when handling the cut pieces!). It would be nice to have something available in ~4' length in one dimension. Open for ideas on cheap, easy to work with panels stock to do this with.

Sadly, in my living room, the place I'd most want to have one of these is behind our chairs, but there's a window there!
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Old 21st April 2018, 06:21 PM   #9
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I've used our 3d printer at work as much to make brackets and jigs as for actual parts, which would be a better use of the printer with the wood strips for everything else.
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Old 21st April 2018, 06:22 PM   #10
bwaslo is offline bwaslo  United States
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Magic metamaterial diffusers -- almost!
Agreed.

It also has the technical benefit of having SOMETHING for my 3D printer to do. Took months to get it working well, but now don't have a lot of plastic crap that I really need made!
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