# Foam Diffuser



## aardvarcus (Oct 6, 2009)

I am curious, how much would you say a 1' by 1' skyline foam diffuser is worth? Most all pre-made diffusers seem fairly high priced, but after cutting one out of wood, I can see why. I was wondering what most people felt one square foot of skyline diffuser was worth.


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## eugovector (Sep 4, 2006)

I've wondered the same thing. Assuming the foam it closed cell, it should work. Can you link the foam you're thinking about using to make the diffuser?


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## bpape (Sep 14, 2006)

Functionally, not much realistically. It doesn't have enough depth to perform down even into the mids very well and in the range it is supposed to operate in, the foam will be somewhat absorptive (not a lot but some - which is a no-no with a diffuser).

Bryan


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## aardvarcus (Oct 6, 2009)

I am actually trying to make one out of expanding foam (AKA great stuff), by casting it into a wooden mold covered in release agent. Not nearly as easy as it sounds. I figure if i can make it work (pretty big if there), and I can make one square foot out of a can, I can essentially buy diffusers for the cost of great stuff per square foot. I have a test one curing right now. There is a huge difference in cure time between a 1" bead and a huge glob.

I know they won't go real low, but how low do you think it is before the diffusers turn into absorbers? I am using closed cell foam, so it is "hard" to the touch. I am currently using the regular great stuff in the red can, but if it doesn't work right, I am going to try to track down the stuff they call the "big gap filler" in the black can.


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## bpape (Sep 14, 2006)

They will do very little absorption as the depth doesn't allow it and the face seals to a hard surface. 

Try QRDude (great program by the way) and model what you're making and it will tell you the effective diffusion range.

Bryan


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## robbo266317 (Sep 22, 2008)

aardvarcus said:


> I have a test one curing right now. There is a huge difference in cure time between a 1" bead and a huge glob.


Most of these use moisture to cure. I saw a forum elsewhere that was casting large areas of foam and he would lay down one layer across the entire piece and then lightly mist with water before laying down the subsequent layers. hth


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## monomer (Dec 3, 2006)

That's urethane foam and water makes it less dense (more foaming or filling capacity)... its a condensation reaction and so some water is produced naturally during the cross-linking of the unsaturated bonds. Urethane foam can be open or skinning (closed cell), flexible or rigid... I would say you'll need some flexibility to remove the foam from the mold since I'm assuming it contains no draft angles and the draw (length to width ratio for the cells) is pretty deep. Flexibility to me (I'm guessing here) would denote the ability to absorb some vibrational energy... If you check into art supply vendors you'll find a huge choice of two-part urethane and polyether foam systems in cans that come in a variety of densities and flexiblity choices are available. However, I'm thinking building one in something like ultra-cal would probably have better reflection characteristics and be easier to de-mold... though it would weigh a good deal more than 2-1/2lbs density foam. Hey what about fiberglas in a simple vaccum chamber? or casting epoxy? Anyway, good luck on your project, post pictures, and give some updates, I'll be watching your progress to get some ideas.


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## aardvarcus (Oct 6, 2009)

I need to try the tip of water misting to help the curing. I am having several different problems right now, as I expected. I have angled the walls very slightly to help the release of the foam from the mold. I put petroleum jelly on as the release agent, however after the first foam diffuser, the second one started to stick to the mold. Also, some of the foam bubbles on the surface pop, and it makes the surface very bumpy. I will have to look up the epoxy, that could be promising.

Thanks for all the help. I will try to get some pictures up soon, though there's not much to see right now. If anyone has any other ideas about what or how to cast inexpensive diffusers, please share them.


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## bpape (Sep 14, 2006)

I suspect that no matter what you use as a release agent or a filler, you'll need to recoat the mold for each casting.

Bryan


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## aardvarcus (Oct 6, 2009)

I figured I would put up some photos of my first two attempts. Please, any suggestions/comments would be greatly welcomed. Hopefully I can churn out version 3 tonight.


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## aardvarcus (Oct 6, 2009)

Well, I have made a couple more using water to assist in the curing. This essentially solved all my previous problems, but introduced even more. Now, the foam over-expands the mold by quite a bit, but then shrinks up as it dries. It's still shrinking days later! The shrinking causes the side walls to cave in to convex shapes, and causes the taller stand alone pieces to curve towards the middle over time. So in essence, it looked a lot better at first, but over time it looks much worse than the original ones. Also, adding the water makes the foam even less dense, to the point where they are just barely "soft" to the touch. Not really soft, but not nearly as rigid as most rigid foam.


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