problem at 100hz

I'm not even suggesting you do what I did. Just want you to understand that deciding on what is even effective involves a bit of calculations. It is why I suggested the room calculator as the start. I don't have your exact measurements, but I did input the approximate dimension into it. If I'm close, you've got a couple modes in the 110-120hz range.

Without spending a ton of time and effort, I'd work on the 4 corners. First figure out what you have available for materials. I use a porous absorber calculator in which you input the thickness of the absorber and its gas flow resistivity rating. The second is the number that is hard to come by. The fundamental idea is that as air (sound waves) travels through the material. Eventually at some point the material will become reflective rather than absorptive. That point is the depth limit where the absorber no longer is effective. These are two common rating for fluffy low density fiberglass. Rock Wool is more than double typically.

Step one, decide how much of the corners you are willing to give up. 400mm square? Step two, Go to your local home building center and figure out what is available. Step three, Compare using the GFR numbers using one of these calculators.

I've enclosed a picture that demonstrates what I am saying. Two similar absorber ratings, one 3600 and another 5000. Second with M2 Rockwool at the same depth.

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