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High density fiberglass as bass absorber?
I've read for days concerning various DIY bass traps. Tubes, resonating panels, Helmholtz resonators, and all manner of rigid fiberglass designs with and without air gaps and with and without limp mass membranes. For the moment I am interested in quick and dirty bass absorption with no regard for aesthetics. Rolls of insulation stacked in the corner are said to be useful.
From what I've been able to gather, generally the effectiveness of fiberglass products increases with increasing density. Owens Corning Atticat fiberglass insulation comes packaged in plastic bags having dimensions of about 9"X 20"x 38" (about 4 cubic feet of volume). The weight is about 35lbs. Therefore, as packaged, it has a density of about 9 pcf. With OC 705 having a density of about 5 pcf, would we not expect Atticat to be a bit more effective at bass absorption than OC 705 per unit volume?
I don't find any mention of anyone trying to use this product as a bass absorber. I'm gonna try it, but before I do, perhaps someone out there can tell me that it doesn't work or why it won't work. Anyone?
From what I've been able to gather, generally the effectiveness of fiberglass products increases with increasing density. Owens Corning Atticat fiberglass insulation comes packaged in plastic bags having dimensions of about 9"X 20"x 38" (about 4 cubic feet of volume). The weight is about 35lbs. Therefore, as packaged, it has a density of about 9 pcf. With OC 705 having a density of about 5 pcf, would we not expect Atticat to be a bit more effective at bass absorption than OC 705 per unit volume?
I don't find any mention of anyone trying to use this product as a bass absorber. I'm gonna try it, but before I do, perhaps someone out there can tell me that it doesn't work or why it won't work. Anyone?
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See this Acoustic coefficents. Basically thickness or volume is FAR more important that pcf. I'd make sure you are above 1.5 pcf and not more than 6 pcf and get the maximum volume you can. Since you want broadband absorption it is the volume/thickness that is most important. |
That looks like a real effect. A bit unpredictable maybe, but real. Any idea how dense the insulation was as packed into the tubes? I will stack some bags iof this Atticat stuff into my window dormers, measure the low frequency levels in those dormers and at the listening position before and after, and we shall see what results. I'll also compare with a 20" ASC tube trap and a Realtraps Mondo Trap. Check back in a couple days. |
I couldn't find the rigid board so I bought the densest attic insulation I could find and stuffed it into Quik-Tubes (for concrete pillars) These are the results. -----no tubes------with one tube---with two tubes------------- 1K------------71db -------71db -------71-------------------- 200Hz--------73db -------74db -------75-------------------- 160Hz--------75db -------76db -------75-------------------- 125Hz--------76db -------76db -------69-------------------- 100Hz--------69db -------69db -------73-------------------- 80Hz---------69db -------73db -------73-------------------- 63Hz---------69db -------74db -------75-------------------- 50Hz---------69db -------76db -------68-------------------- 40Hz---------69db -------68db -------67-------------------- 31Hz---------73db -------67db -------70-------------------- 25Hz---------74db -------71db -------68-------------------- 20Hz---------73db -------68db -------65-------------------- |
The wrong "kind"? Knowledgeable people have recommended simple bags of rolled fiberglass insulation (thank you Ethan Winer). I find no reports on using this particular Atticat product or any other fiberglass of this density (if indeed there are any others). I suspect it has not been used due to the established practice of using rigid fiberglass OC 705/703 because these products work well and are easy to manipulate. And plastic bags bearing images of the Pink Panther are ugly. But I don't care about ugly, and a lack of reported success does not mean higher density fiberglass is known to be "the wrong kind". But it indeed might be the wrong kind. I am no acoustician. Hence the question. If someone has tried it, please let me know. If someone knows that fiberglass of this density will not work, please also let me know. I suppose at some density level compressed fiberglass will behave like a rock. It would be good to know if that begins to occur between 6 and 9 pcf. |
For present purposes I have no concern about reflections from the bags. FWIW, some very effective bass traps (e.g., realtraps) do not have any defined inner space, but they are said to work better with open space behind them. And they have a reflective material on the surface of rigid fiberglass, for that matter. But I want to keep it simple...just hoping to hear from someone who knows about the absorptive properties of fiberglass. |
I would be leery of anything in a bag, bags may be very reflective confusing soundstaging, perhaps affecting tonality. The reason I take the reflective foil off the semi rigid fiberglass. I too have read of people using the bags of insulation, hard to tell how effective or ineffective these schemes are without hearing them. If the insulation would hold together out of the bag it would be better. The guys I've seen using the roll insulation most often attach it to rather rigid screening so that it holds it's shape. You need a well defined inner space to be effective, that seems hard to attain with loose fiberglass. |
The product is a bag of fiberglass, pretty tightly compressed and intended to be taken out of the bag and spread in the attic. No foil or backing. Its not a sheet or a roll or anything like that. Its unique feature is the high density as packaged. If absorbance increases with density (I believe it does), I am hoping that stacking these bags in a bass-heavy corner will be effective. Shoot, I've heard of people using rolls of insulation still in the bags, and even trying to compress the rolls to increase density. In Atticat, we have pretty cheap and very dense fiberglass product on the Home Despot shelves. So I'm rooting for it. |
