MDF Frames for Acoustic Panels?

[Drums1225]

I'm about to take on the project of constructing (6-8) acoustic panels with OC 703. I've watched tons of videos on YouTube, read just about every thread I could, and after seeing countless variations, I've narrowed down the approach I plan to take. I plan on making frames and covering the wood completely with broadcloth (65/35 Polyester/Cotton) fabric I got on sale from Jo Ann Fabrics for around $2/yd, and leaving the backs open (covered with cheap fabric, or polyester landscape fabric).

I bought two 1" x 2" x 8' Pine boards from Home Depot, just to make a prototype, before I committed to buying and cutting all the lumber. Good thing, because I while I knew that 2" isn't really 2", when buying lumber, the OC 703 is ACTUALLY 2" thick, so I exchanged them for 1" x 3" x 8' (which presents another issue, but a surmountable one).

On New Year's Eve, I had some friends over, one of whom owns a business that designs and builds high end exhibits for museums in NYC. He also built a basement recording studio for his teenaged sons, so I knew he'd be interested in my project. I explained what I was doing and how I planned to go about it, he said "Just come to my shop, I have TONS of scrap MDF, and unlike your pine studs, you can cut them to exactly 2". You won't need to buy lumber or cut it. I'll cut all you need and then you can just bring it home and start assembling the frames. I'll even lend you my air stapler".

Almost TOO easy... I actually wanted to take on the project of cutting the wood, since I have bought a circular saw over a year ago, and I've never even plugged it in! However, one can't argue with the price of FREE!

Would anyone recommend against using MDF? I've read that there are some health concerns surrounding MDF; namely, that it's bonded with formaldehyde. The panels will be very light, so I can't imagine strength would be an issue.

I'd greatly appreciate any opinions/experience on using MDF to construct the frames for acoustic panels.

Thanks,
Chris

[robnold123]

3/4 inch MDF would be a wonderful material to build the frames with especially considering the cost. 1/2 inch MDF would work too if you don't stretch the fabric too tightly and bow in the sides. I usually cut my fiberglass or rockwool panels into 2 foot by 2 foot squares and put two of them in a frame with a piece of lumber across the middle to keep the sides from bowing in when you stretch the fabric covering. If you haven't already built bass traps, you might consider making these panels 4 inches thick as treating the lower frequencies too will generally give you the most acoustical benefit. Rock wool is about 1/4 the cost of fiberglass and is fairly similar acoustically so if you want to save money rock wool might be the way to go. I follow the Ethan Winer school of thought that denser is better for LF absorption--important. He has done some compelling tests to back this up. Be aware that rockwool labeled 8 pcf density is really 6 pcf. somewhere around 6 pcf. (actual density) would be good. Rock wool is more friable than fiberglass so if you go this route, do your cutting outdoors. Hope this helps I'd definitely jump all over the MDF and you might consider ripping them a little wider--maybe 1/2 wider in case you want to wrap the fiberglass or rock wool with polyester batting / or polyester craft felt to help keep the glass fibers out of the air. A little extra air space behind the panel never hurt anything either. I sure wish I had a friend with tons of MDF scraps laying around! Good luck and hope you enjoy building!

[Soundman2020]

that denser is better for LF absorption--important.

Actually, that isn't true, and you seem to be comparing apples with bananas.

The density of insulation, by itself, is meaningless as an indicator of acoustic absorption. What matters is the gas flow resistivity. That's what the sound waves "see" as the pass through it, not the density. Unfortunately, it is often hard to find the gas flow resistivity data for commonly available insulation products. Fortunately, as it turns out, for each class of insulation there is an approximate correlation between gas flow resistivity and density, but it is different for each type, and not entirely linear even within each type. For ordinary "pink fluffy" fiberglass insulation, the optimum gas flow resistivity occurs with density of around 30 kg/m3, but for mineral wool the optimum gas flow resistivity occurs with density of around 50 kg/m3.

And contrary to popular belief, higher density does not work better for lower frequencies: Optimum density works best. With density significantly higher or lower than the optimum, acoustic absorption falls off. And in fact, less dense absorption works better for low frequencies, to a certain extent, while more dense works better for highs, once again to a certain extent.

- Stuart -

[robnold123]

Thanks Stuart, I always enjoy reading your posts. Gas flow resistivity is still a little over my head at this point in time. Here is the link to density tests I was referring to; http://www.ethanwiner.com/density/density.html I'm aware that these tests are done with rigid fiberglass and not rockwool, however, to me this appears to be pretty compelling evidence that increasing density of an absorber could possibly have some advantages for bass trapping. Stuart I'd be curious to hear what you think about these tests/methods, etc. and also how density rockwool would fit into this picture. We probably ought to talk about this stuff on another thread though--I'm afraid I'm derailing this one. Sorry. To the original poster--I'd jump on that MDF though and build your absorbers thicker if you can. Stuart, if you've got some good links on gas flow resitivity, I'd love to see them. Thanks much, Rob

[Soundman2020]

Stuart, if you've got some good links on gas flow resitivity, I'd love to see them.

You could try playing around with Chris Whealy's porous absorber calculator: it gives you a better feeling for how these things work. You could also take a close look at the published data on 701, 703 and 705, and compare that.

Ethan did point out the limitations of his test rather rather clearly in that article, and to that I'd add that he only tested three different densities in a room with some very specific modal issues. So all that you can really surmise from that, is that 705 would be a good choice in that specific room for treating those specific low order modes. (Ethan also made that clear in his article). But to extrapolate that and say "higher density is always better for treating lows" is a rather large stretch!

- Stuart -