Peter, just make sure you paint them with an "airless" spray gun. That way you won't clog up the pores of the foam (which would reduce its absorbtive qualities.)
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"...a random combination of columns..."
Neither the heights of columns nor their arrangement is random. They are determined by a precise mathematical formula in Number Theory.
"...stretch acoustically transparent fabric over the frame..."
My understanding is: That will reduce the effectiveness of the unit.
I did what you are describing using styrofoam. You can purchase styrofoam in 2"X2"X12" sections then cut to the desired length. The drawback with styrofoam is that you can not paint it. I also found a primative root mathematical formula that supposedly provides the optimum length and placement of the columns. I think it works quite well but with most things audiophile it is difficult to measure. If you are interested let me know and I can send you more information. Good luck.
Nsgarch - I'm not sure that the foam is meant to be absorptive due to open pores. RPG paints its diffusors, and I would imagine that this clogs up the pores. RPG does state that "the solid expanded polystyrene core of the skyline provides useful low frequency absorption", but also states that "the large prime number design offers the acoustic industry's largest reflection number density". Unless you are trying to make an "abfusor", I think you are after a reflective surface but with multiple angles of incidence and reflection, and not overly concerned with absorption.
John - you are right, the combination of columns is not random, and I was just being lazy in describing this. RPG uses "advanced primitive root number theory to design the most powerful two dimensional omnidimensional diffusing surface in the acoustical industry". Are their any skilled mathematicians out their (equally skilled in normal communication) who can explain what primitive root number theory is? I don't know how the link I provided arranges the various columns, but it is not purely random, as can be noted by the distribution of various column heights (at the bottom of the page). My hope is that it came directly off a Skyline, but I have no way of knowing!
Anyhow, I've referred to the discussion of diffusors in F. Alton Everest's "Master Handbook of Acoustics", specifically the discussion of "reflection phase-grating diffusors", or which the Skyline seems like a variant. At the time of writing the 4th edition (2001), the book seems to prefer quadratic residue diffusors to primitive root diffusors, and notes that diffusors built with separators between wells are most effective. The skyline doesn't use seperators, nor do the popular Auralex TFusors, but the RPG Omnifusor does. I'm not sure which of the 3 design is actually best. The RPG products are more evenly 2D than the TFusor, which seems to have less 2D effectiveness, but the TFusors are certainly most affordable. Back to the point: it would be great to know if the DIY design on the link is the same as the Skyline. It certainly isn't purely random, and recipes are available in Everest's book for both quadratic residue diffusors (which are symmetrical) and primitive root diffusors (which are not symmetrical, like the skyline and the DIY design). That just leaves me saying "hmmmmm".
Regarding covering a diffusor with fabric, yes I've heard that this compromises its effectiveness too (e.g. on the Audio Asylum acoustics forum). But the way it was stated seemed like here-say. Can anyone point to a basis for or against covering (seemingly ugly) diffusors with acoustically tranparent fabric?
The following "translation" of your primitive root link may be easier to follow: Let p be a prime number and let b be any number greater than 1 but less than p. Divide each of the following p powers of b by p: 1 (=b^0), b, b^2, b^3, ..., b^(p-1). If there are p-1 distinct non-zero remainders, then b is a primitive root of b. Otherwise, it is not a primitive root of p.
For p=7 and b=3, the 7 powers of 3 are 1, 3, 9, 27, 81, 243, 729 and the corresponding remainders are 1, 3, 2, 6, 4, 5 ,1. Since 6 (=7-1) of these remainders are distinct, then 3 is a primitive root of 7. However, if b=2, the 7 powers of 2 are 1, 2, 4, 8, 16, 32, 64 and the corresponding remainders are 1, 2, 4, 1, 2, 4, 1. Since there are only 3 distinct remainders, then 2 is not a primitive root of 7.
Why not just try using some sort of PVC tubing all linked together to creat a panel? The round surface should effectively diffuse, and if you keep them as full tubes, they can perhaps act as resonators too.
BTW, this is sort of a question rather than a statement of fact as I am curious about DIY acoustic projects-
I have a lot written up on diffusors, two main RPG kinds, one or two dimensional, both theory and practice, and I'm happy to send it out to people as MS Word attachments. This is before I've even read the original post in this thread, which I'll now do. In my approach, a table saw is called for, though someone at Pass Labs told me about using a hot wire to cut the 2" thick Styrofoam I use.
PS foam is available at Home Depot and Lowes.
EPS foam is expanded with steam and is not the same as EXTRUDED PS foam.
Owens Corning eps foam is pink, Dow is blue. Don't buy the white as it is just styrofoam.
A 4 ft by 8 ft sheet of EPS foam 2 inches thick is about $25 bucks. This is what you need to use. Lowes handles both at about the same price per sheet. Take your finger and push it into both products. The EPS foam is much more rigid and will do a better job.
If you cut it on a table saw you need to do it outside. The dust will be drawn to everything and it will be impossible to vacuum it out of your shop. If you have a band saw it will be much safer than a table saw. You can also make a jig using a VERY sharp knife to cut it; watch your fingers. If you have a good table saw and a 60 tooth carbide blade you can do the job best. You will need a friend to pull the material through the saw as you push it. You also need to know the importance of keeping the material tight to the fence.
Polystyrene, extruded or steam expanded, will melt and bond to a table saw blade if the material isn't pushed through fast enough. The result of too slow a feed will result in a VERY fast kick back of the material. Don't be afraid of the saw; take control and you'll have no problems. On the other hand, use a knife and a good cutting fixture and you'll be fine.
You can buy an adhesive from FOMO products that is available in a 12 oz can with a straw tube to adhere it all together with. Unfortunately it's over $200 bucks a case. Use Elmer's glue instead. Latex paint will bond and give you the esthetic look you need.
I plan on doing just this in a few months to finish off my music room.
Peter, IMHO, your thread can contribute sonic improvements to "Gonners" that may not be matched by any other DIY project on a cost to benefit ratio I can think of.
I'll look forward to hearing how your project works out.