Help treating my listening room

[Matthew J Poes]

That's good news! Otherwise it meant I wasn't understanding

So, towards the 35Hz peak and long decay time: any of the above models strikes you as clearly better?

Regarding the other peak, around 55-65Hz:
- 2nd length mode is at 70Hz
- 2nd width mode (panel door to window) is at 68Hz
- if we assume the panel door does very little, the extension of the room is the same width, so 68Hz would be f3 from brick to glass width.
- SBIR: I'm looking at the subwoofers given their xo at 80Hz. Both subwoofers cones are 68cm from the front wall so not aligning well with this peak as far as I can see. But their cones are 12" and centers are 23cm from the floor, so could be 5/4 of the wavelengths within 60 to 70Hz.
Seems 3 factors are piling up here: 1) f1 length axis, 2) f1 width axis, 3) 5/4 WL from SWs center to floor. Addressing this looks like my next target

The modes at those higher bass frequencies will be far more audible than the 35hz peak.

As for how to address the axial mode, the problem is that its so powerful (as this mode always is). A couple of corner bass traps or limp mass bass traps does very little. What I've found is that EQ is actually the easiest solution. That is my main recommendation. When that is not possible, then my next suggestion is to treat 100% of the front and rear wall with bass traps taht are heavily biased toward the frequencies below 80hz. One option is to use Helmholtz, panel, or membrane traps in the corners and covering the entire rear wall. Tuning to 35hz is tricky.

Take a look at this commercial offering:
http://www.gikacoustics.com/product/gik-acoustics-scopus-tuned-bass-trap-t40/
it's 4 square feet and 10" deep. Anything you build will likely end up with a finished depth very similar to this, to drop it further to 35hz, it would likely be even deeper. My own experience with these products is that trying to address this problem is not solved by 1 or 2 of these products. 4-8 square feet won't cover it. It ends up needing to be something like 20-30 square feet of coverage, basically most of the wall. I would be prepared to give up 10-12" of room depth and 16-20 square feet of wall space to make this work.

 

[Horacio Lewinski]

The modes at those higher bass frequencies will be far more audible than the 35hz peak.

As for how to address the axial mode, the problem is that its so powerful (as this mode always is). A couple of corner bass traps or limp mass bass traps does very little. What I've found is that EQ is actually the easiest solution. That is my main recommendation. When that is not possible, then my next suggestion is to treat 100% of the front and rear wall with bass traps taht are heavily biased toward the frequencies below 80hz. One option is to use Helmholtz, panel, or membrane traps in the corners and covering the entire rear wall. Tuning to 35hz is tricky.

Take a look at this commercial offering:
http://www.gikacoustics.com/product/gik-acoustics-scopus-tuned-bass-trap-t40/
it's 4 square feet and 10" deep. Anything you build will likely end up with a finished depth very similar to this, to drop it further to 35hz, it would likely be even deeper. My own experience with these products is that trying to address this problem is not solved by 1 or 2 of these products. 4-8 square feet won't cover it. It ends up needing to be something like 20-30 square feet of coverage, basically most of the wall. I would be prepared to give up 10-12" of room depth and 16-20 square feet of wall space to make this work.

I see what you mean.

Equalization is an inherent part of my system as DSP and room correction is the backbone of the active system. So I'm good with EQ!

My front wall is 122 square feet if covered completely. I'm assuming the 35Hz resonance will require more energy absorption than the 65Hz, and hence more surface. For the 65Hz I'm envisioning a perforated panel on the back wall, about 70 square feet.

If I take the first modeled panel from post #96: 10mm thick panel with 2.5mm perforations spaced 85mm, plus 90mm air gap, plus 50mm fiberglass= 150mm deep, or 6" deep. Am I missing something and should consider the absorber to be significantly deeper then? In your experience, a DIY gypsum or MDF wall perforated as above works down to 35Hz? Maybe the model is not accurate down at those frequencies?

 

[Matthew J Poes]

I see what you mean.

Equalization is an inherent part of my system as DSP and room correction is the backbone of the active system. So I'm good with EQ!

My front wall is 122 square feet if covered completely. I'm assuming the 35Hz resonance will require more energy absorption than the 65Hz, and hence more surface. For the 65Hz I'm envisioning a perforated panel on the back wall, about 70 square feet.

If I take the first modeled panel from post #96: 10mm thick panel with 2.5mm perforations spaced 85mm, plus 90mm air gap, plus 50mm fiberglass= 150mm deep, or 6" deep. Am I missing something and should consider the absorber to be significantly deeper then? In your experience, a DIY gypsum or MDF wall perforated as above works down to 35Hz? Maybe the model is not accurate down at those frequencies?

The models aren’t perfectly accurate. In my experience they get both the tuning frequency and damping wrong. They aren’t that bad however.

How deep partly depends on the mass of the diaphragm. I prefer a lighter diaphragm and deeper enclosure myself only because the heavier one is harder to move and as a result can sometimes act as more of a barrier. For example I wouldn’t personally go past a 1lb per square foot barrier.

My wall approach doesn’t involve perforating the wall. It uses the wall as a panel trap. A giant one. If you think of the wall and treatment panels as a compounded series of resonances that collectively give you a “flat” absorption curve, then the wall itself should be tuned to the lowest mode frequency for that axis. So your wall would ideally be tuned to 35hz. Ironically that is the tuning frequency you get with a 4” wall, 4lb density insulation, and two layers of 5/8” drywall with viscoelastic damping between, on each side. It’s close to what you get for a foundation wall with drywall on one side (because the concrete is a pretty solid barrier).

Now remember earlier I said tune to the lowest axial mode for that axis? I am not sure if the higher bass modes are length modes. I think you mentioned they might be width modes. What I would do is calculate the room modes and identify their source. Length, width, height, oblique, etc. tune for that. So if it’s the width mode, those 65hz traps should be on the side walls. Corners work too since they are related (corners “see” all modes).