There are a couple of types of bass traps; resonating absorbers and porous absorbers. Both types are effective, but differ in their band action.
Porous absorbers control a broader band, absorbing sound all the way across the audible band, including low, mid, and high frequencies. Resonating absorbers control a narrow band action and absorb a narrow band of sound frequencies.
Frequencies tend to build up in the corners of a room. Since most studios do not have the space or the funds available to be able to treat full walls with thick, bass trapping foam, and since sound can be 6dB louder in a corner than in any other part of a room, it just makes sense to install bass traps in the corners.
There are two types of acoustic foam surfaces; plain faced or profiled foam. The advantage of plain faced foam is that there is more mass to it, therefore it is more likely to absorb the bass frequencies. However, since it is so smooth, there is a chance that the sound wave may ‘glance’ off of the smooth surface and be reflected back into the room.
The advantage of profiled foam is that it can catch or trap the sound waves striking it, thus reducing the reflections. Profiled foam will tighten up those high to mid frequencies, but if the room requires more bass trapping to be able to cope with the low frequencies, you will need to add more bass trap treatments A fairly inexpensive solution would be combination of both smooth and profiled Acoustic foam in the Auralex ATOM12CHA ATOM-12 Bass Trap Kit (retails at $499.00 for four corner treatments). This kit includes (4) – 12” x 12” 12” cornerfill cubes and (12) 12” x 12” x 17” – 2 foot long LENRD bass traps. Installation’s a snap and what you end up with, in each trihedral corner of your room, is a column; one smooth cube at the base and three profiled bass traps stacked on top of each other. Installed in this way, you’re REALLY going to have serious low frequency accuracy in your room.Mineral Fiber Insulation is a great way to tighten up a room’s sound. Aurelex Mineral Fiber sound insulation (retails at about $2.69 sq ft) is WAY more dense then “the pink stuff” and is specifically designed for acoustical usage. It yields much better bass trapping, overall absorption, thermal characteristics, moisture absorption and sound transmission loss (isolation) than standard fiberglass insulation. Mineral Fiber vs. Standard Fiberglass: better sound isolation, better acoustic absorption, lower moisture absorption, better flame spread and smoke density, 1200° burn point vs. 650°.
For a simple Do-it-Yourself project – build a simple wooden framework (2’ x 4’) and insert Aurelex Mineral Fiber sound insulation.
1. Build a 2 x 4 ft frame out of light weight lumber with corner gussets for strength. Keep the unit as light as practical; screw and glue the frame together and counter-sink with a pilot bit to keep the screws from splitting the wood.
2. Insert a sheet of 2’ x 4’ x 4” mineral fiber, and then upholster the frame work in a breathable fabric (burlap works well) and staple to secure.
3. Add hanger hardware, such as standard screw hooks or eye hooks. You can choose to hang this bass trap from the ceiling with chain or ‘hover’ it out from the wall using door stops and picture frame wire.
Another simple method of fabricating a bass trap is; build a 2’x 8′ wood frame, topped with a 1/8″ to 1/4″ sheet of plywood, backed with (2) 2″ thick mineral fiber panels. The 1/4″ plywood bass trap handles the deepest bass frequencies, while the 1/8″ plywood handles upper bass range.
For an informative book on the subject, check out: Acoustic Diffusers and Absorbers: Theory, Design and Application by Trevor Cox, Peter D’Antonio.
For anyone dealing with room acoustics or sound control, you may want to access to this unique and authoritative text describing how to effectively measure, model, design, optimize and apply diffusers and absorbers including the Helmholtz device.