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Return to Room Acoustics Forum by Rives Audio
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In Reply to: RE: DIY acoustic panels questions posted by David Aiken on February 03, 2008 at 19:19:10
on a post i made a long time ago, you said this:
"In any event I prefer to use polyester rather than Jon and Ethan's recommended fibreglass because of my background in health and safety. Fibreglass can present a health risk and I'd rather not use it myself in a DIY project."
can you tell me more about how these panels would be made?
-chris
I used polyester batts which were 430 mm wide by 1130 mm long (we're metric here in Australia so simply substitute whatever the measurements for your batts are.
I had a friend build me frames with an opening 1130 mm high and 860 mm wide so the frame would hold a double thickness of 2 batts side by side. Frame thickness was 100 mm, 4", which was about the thickness of each batt uncompressed. The frame sides extended about 15" below the bottom of the opening with cross pieces on the bottom (think inverted capital "T") for feet so they could free stand.
Across the open back of the frame I stretched and fastened a thin sheet of plastic. I sprayed spray adhesive onto the plastic and lay the first 2 batts side by side on top of the plastic, then sprayed them lightly with spray adhesive and lay the second pair of batts on top of them. I then covered the whole frame area with burlap. I lay the frame with batts in it on top of the burlap and tacked the burlap to the bottom of the lower crossbar of the frame. I then stretched the burlap tight up the back of the frame, over the top and back down the front pulling it as tight as I could, lay some board with a bit of weight on it on top to provide some compression of the batts, pulled the burlap tight and then tacked it to the bottom of the bottom crossbar on top of the starting row of tacks. Then I tacked the burlap to each side of the frame and removed the weighting. Panel finished.
The burlap provides the only compression and even though it is pulled tight and weighted during the covering process, it still bulges a bit once the tacking is finished and the weight removed. The end result is a panel that is 100 mm thick around the edges, the thickness of the wood, and thicker in the centre so looking at it from the side or from the top it will bulge in the middle slightly.
The plastic membrane seems to improve low to mid frequency absorption over simply compressing a double thickness of batts with wire mesh. You can improve absorption in the upper bass to mid area a bit more by replacing the first layer of batts with denser batts. We can now obtain quite dense (32 kg/cu metre = 2lb/ cu ft density) 50 mm = 2" thick polyester batts for soundproofing and home theatre use and they work quite well, either on their own with a think layer of normal polyester batting on top to provide some high to mid frequency absorption otherwise they're likely to be reflective at those frequencies, or as the first layer of batts with a second layer of more normal density batts on top. The dense batts are considerably more expensive here than normal polyester batts sold for thermal insulation, and they're still a bit more expensive than the normal batts when the price for an equal mass of each is compared. Despite that, if I were making more panels at this stage, I'd be using the denser batts for at least 1 layer and quite possibly for the second layer.
I have no idea how effective these are in comparison to any other design because I have only built them this way and I don't know anyone locally who has built any of the other designs for comparison. I also don't have the equipment to do measurements. They are quite effective when placed at first reflection points and they improve bass tightness in my room over and above the improvements I get with my DIY bass traps so they obviously seem to work down to a reasonable frequency, probably in the upper bass area, giving reasonably broad band coverage.
David Aiken
ok, yeah, polyester would cost way too much. so anyway, going back to your idea of a plastic sheet...assuming all this was in a wooden frame, would it be better to have (from back to front)
a) plastic/poly/fiberglass paper/fiberglass/poly/burlap
b) plastic/fiberglass paper/fiberglass/poly/burlap
c) plastic/poly/unfaced fiberglass/poly/burlap
d) plastic/unfaced fiberglass/poly/burlapthanks for all your help,
-chris
If I was going to use faced fibreglass and the facing was strong enough to handle the compression applied by a covering stretched tight, I would try the faced fiberglass with the paper tacked to the back of the frame and no plastic at all. A covering of polyester over the paper would be a reasonable safety precaution in case the paper splits at some stage and exposes fibreglass. I do think you need polyester over the front of the fibreglass.
I would only use polyester with unfaced fibreglass, and I would make sure that no fibres could escape between the plastic backing and the frame by both gluing and stapling the plastic down. You could then omit the polyester layer between the burlap and the plastic at the back of the panel but I would probably prefer to have some polyester there for HF to mid frequency absorption on that side also, otherwise the back of the panel will be reflective from mid frequencies on up.
David Aiken
in your second example, why does it matter if the back of the panel is reflective?
-chris
You get more absorption with the panel spaced off the wall and one of the reasons for that is that you expose a second surface for absorption. If you make it reflective, you lose some absorption efficiency at those frequencies where the rear surface is reflective.
Of course, if you only want the amount of absorption you get from the front of the panel it won't matter as far as absorption goes but the reflectivity at high frequencies coupled with the fact that it will still absorb at lower frequencies may create a problem with the tonal balance of the reflected sound field. Note that I said "may create", not will create, since that HF reflectivity has to be considered in the context of the overall reflectivity/absorption spectrum of your room.
David Aiken
assuming i went with the unfaced fiberglass, would you still recommend using spray adhesive (sticking fiberglass to plastic) in order to keep the fiberglass from sagging over time?
-chris
Yes, but not only to keep it from sagging. When I made my first pair of panels using polyester batts, I didn't glue them to the plastic. When I made the second pair, I did. My feeling was that I got better performance at low frequencies from the second pair. That's a judgement based on what I heard since I have no way of doing measurements.
David Aiken
thank you. your directions are quite clear. and as long as polyester isn't too expensive here in the states (which i have yet to look into) i think i'll be using your method.
-chris
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