Acoustic Insulation Q and A
Q: What insulation do you recommend for use making a bass trap?
A: In a broadband panel [appropriate for most applications in small rooms] we recommend that you use at least 4 thickness of semi-rigid fiberglass board such as Owens Corning 700 series semi-rigid insulation products [other companies such as Johns Manville and CertainTeed make similar products shop for best price in your local market]. Use a 3 lbs. density in all panels of 4 thickness or greater. In 2 panels we recommend a stiffer fiberglass board such as the 6 lbs. density Owens Cornering 705.
Fiberglass boards are the product of choice wherever else it may be available at a competitive price because they are lower in weight than most alternatives, have superior handling properties and are highly effective for acoustic purposes. But, mineral wool products such as Rockwool (Roxul), are also common and these materials function well acoustically and are sometimes available at very attractive prices. However, rockwool is not as easy to work with as fiberglass boards and you should use a higher density flavor [6 to 8 lbs. rockwool is a good choice] and thus using rockwool yields a heavier panel. There are also acoustic cotton and other insualtion boards which work well as acoustic absorbers but they generally are much more expensive and provide no acoustic benefit.
Q: What about Owens Corning 705 for thicker panels [4 or more]
A: You can use 6 lbs. OC705 in lieu of 3 lbs. OC703 in a thick panel, but it does not perform better than OC703. It does however cost more and weigh more than OC703. See the acoustical absorption coefficients for OC705 and OC703 data published by Owens Corning at the link below and that of many other manufacturers for their acoustic insulation products.
http://www.bobgolds.com/AbsorptionCoefficients.htmAs you may note there is no indication of superiority for the denser materials in a 4 panel.
Q: Yes, but I have read and been told by lots of folks on the internets that higher density acoustic insulation (like OC705) is better for low-end absorption.
A: The internet is a series of boobs... and that concept is simply not true when made as a general statement. Look at the data above, these are actual measurements taken in a laboratory and published by the manufacturer Owens Corning. Despite what you may have heard, the density of a porous material is NOT a defining element which determines its effectiveness as an acoustic absorber.
Porous absorbers work by capturing the vibrations of your rooms air volume in small cavities [interstices]. The defining characteristic is a property called Gas Flow Resistance [GFR]. A denser board of the same material will have fewer/smaller cavities and a higher GFR. A high GFR can be a good thing in a thin application, but over a certain panel thickness this is no advantage at all, in fact you are just wasting money on more material and fewer interstices. In fact at larger thicknesses a LOWER density material is appropriate. The intelligent design of a porous absorber requires matching a material of the proper GFR to the thickness of the device.
Porous devices thinner than 4" are not efficient absorbers of low frequencies regardless of the GFR of the material. You should use a panel at least 4 thick when you desire broadband performance [affective on lows, mids, and highs]. At 4 thickness OC703 has a GFR which makes for an efficient acoustic absorber paying roughly twice as much for OC705 to use in a 4 thick panels is not a rational design choice.
Q: How about 2 Absorbers?
A: OC703 will work, but OC705 is preferred as it is a stiffer board and easier to work with when thin. You can use an equivalent of OC705 as well: CertainTeed CB600, Johns Manville Spin-Glas 600 and other 6 lbs. pcf materials. You can also use Rockwool (mineral wool) with great results acoustically, but Rockwool is a floppier than fiberglass and this can be undesirable especially in a panel as thin as 2. If you use Rockwool in a thin panel, use a dense material, something at least 6 lbs. pcf or 100 kg/m3. This is the only application for which we recommend OC705 over OC703 and other 3 lbs. pcf fiberglass materials as the higher density material has a higher gas flow resistance suitable for a thin panel, also the denser material is more rigid and will hold its shape better. But.. if you are building full wood frames for your panels, then the additional stiffness of the 705 is of no event, in this case we suggest you stick with the less expensive 703.
Q: How thick should I make my broadband bass traps?
A: 4 total thickness (usually two 2 slabs, as most acoustic insulation materials are most readily available in 2 thickness) is an efficient choice. A 6 panel will work better at low frequencies, but if you have the sapce available to mount more panels it is more rational to use your insulation boards in a 4 format. Example: Using a stack of insulation boards a total of 48 thick to make panels, if we make Twelve 4 thick boards we will provide more overall absorption than if we were instead to make Eight 6 panels. But dont go less than 4 thick or you will reduce low frequency performance.
Q: Where can I get insulation
A: Shop at local commercial construction suppliers of insualation, drywall and/or steel framing such as Specialty Products & Insulation Co. (or similar companies as you may have in your area).
http://www.spi-co.com/If you can't find the material conveniently Ready Acoustics sells Owens Corning boards for you convenience at the web site... but driving a truck across town and picking the stuff up local will usually save money.
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