Originally Posted by lilmike
Keep in mind, 6" is one wavelength at 2.2 KHz.
Not exactly gonna trap any "bass" with something that is such a small fraction of a low frequency's wavelength.
Up against the boundary yes.
However, you place that same 6" of insulation ... well away from the boundary
and voilà, it becomes a tremendous effective absorber of the problematic modal range of typical residential rooms. A bass trap.
Some good advice given above, but lets examine the most important aspects of bass trapping with porous absorbers;
Ideally, place the porous material where it works the best .. at the 1/4 wave point off the boundary ... where the velocity is greatest.
If you chose to absorb, do so effectively. Poorly executed porous absorbtion of sidewalls filters the reflected energy in a manner whereby the resultant sound will be lifeless, muddy, bass heavy mess. Even sidewall panels need as much extension into the LF as possible. Oftentimes, prudent use of coverings can elicit a spectrally balanced effectiveness, however this is best confirmed with measurments.
Porous absorption can be effective quite deep, I've seen measured results down into the 30hz range. The thicker the better, if you go very thick, you use the less dense material. If you opt for less thickness, utilizing an air gap multiplies the effectiveness greatly.
As a rule of thumb, based on how much space off the surface you can use;
up to 4", use 3 lb fiberglass equivalent (4lb) mineral wool
4-6", use 4" 3 lb fiberglass or 4 lb mineral wool with a gap
6"-12", use 4" 3 lb fiberglass or 4 lb mineral wool with a gap off the surface, or fill the space with loose/fluffy
12" and more, fill the space with pink fluffy
One can either straddle a corner with a 4"-6" thick panel, or employ stacked fluffy, stacked rigid 703, superchunk style.
But be mindful that filling the corner is better than using a 4"-6" thick panel, however only somewhat better. Instead of filling the entire area, one may be better off utilizing the same material elsewhere, covering more corners.
Best case, fill all the corners as deep as you can spare. Remember, the thicker the porous absorber, the less dense (gas flow resistivity) the material is optimum. For example, if one could spare 18" across the entire back wall of a studio, pink fluffy will out perform 703 rigid fiberglass.
Acoustics is anything but intuitive. And from everything I've learned, the lowest range of frequency effectiveness extends much lower than typical 1/4 wave calculations would suggest. When including the effects of the porous material's speed of sound, the low end effectiveness becomes significantly better, and lower in frequency then previously understood. It's simple; the material is acoustically thicker than it's physical dimensions would suggest.
The 1/4 wave distance is shorter in pourous absorption, because the speed of sound in porous absorbent material is slower than it otherwise would be in air. Slowing the speed down, shortens the wavelength, thus shortening gap needed off the wall/boundary needed to affect any one frequency.
I've read individuals studying such things suggesting the speed/wavelength shortening can be as much a 1/3, correlating with a measured results showing that if the thickness of an effective porous absorber is at least 7% of the wavelength, then one could achive significant absorption approaching 100%.
I don't know for sure, but I seen measured results that show effectiveness much deeper than one would intuitively think would be possible ... using the typical 1/4 wave effectiveness mindset. So what I've since read about the speed of sound within the material, certainly is interesting and makes sense.
Largely, in the typical small rooms that comprise our HTs and listening rooms, one can hardly have too much bass trapping. However, enormous benefits are had with when one simply achieves an adequate level
of bass damping/trapping. And the cool thing is, the cheapest stuff (pink fluffy) is the most effective. Any large surfaces, ceiling included need treatment. One needn't be concerned about over doing it either, as the traps can be faced with diffusion, plastic, wood slats, anything that's invisible to the deep stuff. That way any amount of energy in the MF/HF can be retained, if one desires.
a nice example.