Yes, I again agree with Hexspa. Since you are handy with using REW, your can track your progress. Since LF tend to be focused into the corners, that's a good place to add more treatment. To get the most effective corners bass traps, make sure you use FRK or paper/plastic facing the room. As I mentioned in an earlier post, spacing the wall panels off the wall may be the easiest since the panels are already built. If you measure with REW after you space them, you'll see how much (or little) you have gained.
I will try first to increase the gap between my broadband absorbers and wall. They do have a little air gap, but its only just a tad over 1inch. I'm going to leave a 4inch gap. Probably will do at this weekend, still have to think how I'm gonna get them mounted with 4inch gap
One idea is to use a couple 2x4's mounted vertically (They are only 3 1/2 inches so that may be enough or you could attach a 1x2 furring strip to the front of it ?)
Anyway, if you go with the 2x4 idea, you'll want to bore a holes a little larger than the screwhead edgewise into the 3 1/2 about 2 3/4 deep so you don't need so long of a screw to mount the 2x4. With the 2x4's vertically attached the wall, you attach your panel to that.
Something else that just popped into my head is build trusses out of 1x2's instead of 2x4's; More labor but cheaper materials and more acoustically transparent.
If it were me I would first try this out without installing the panels i.e. I would first try placing the extra wall panels vertically on chairs spaced 4" out from the walls on both sides of the room. Once I got the effect I was looking for I would then consider installing them on the walls. I say this from experience of trying to rid small to medium sized rooms of lower mid nulls and peaks - it's very very difficult.
Thats what i did today with panels! now theres about 4inch gap between every panel. I had some old ugly panels i had built time ago, still had that good fabric to cover them and so i could put them on wall as well. I really didn't see that much difference in measurements, i think 110hz got bigger null now? But definetly when i listened quick listen i really thought i could hear much much more bass in listening position. (that will be clear when I'm there listening with fresh ears and can listen with time and mix something)
I suggested spacing your panels off the wall because it seemed it may be the easiest, not because it was the most effective. Traps in the corners along the side walls and the ceiling are generally more effective. For those use FRK facing the room.
Nigel and Hexspa are correct that it's not easy but without sufficient bass trapping it's even more difficult!
A demonstration you can do is to manually sweep a sinewave and you'll notice the level change. Set it at a given freq. say, 100Hz then walk all around your room and you'll notice it's different from spot to spot. The idea is that since the peaks and nulls are caused by room modes, eliminating the room modes would eliminate the peaks and nulls.
OK, so you're not going to eliminate the modes, so you do the best you can to minimize them. Corners are places where 2 or 3 boundaries come together so you get more "bang for your buck" when you treat there.
Another thing you may want to do is to verify the freq response of your speaker(s) at the typical distance of 1 meter. Test them one at a time (not both together... you probably only need to do one but since you'll be all set up, you might as well test both...just to see how well matched they are). The best place to do that is actually outside if it's quiet. Even better if you're on a slope so the ground falls away and will not interfere (much) with the measurement. As you should probably know/assume/guess, outdoors is the approximation of an anechoic environment...or vice versa
Well how i can manage to eliminate those modes? those 70hz and 110hz are axial length modes. Would be nice to get them little bit even, but how I'm gonna get them more even? Just more bass trapping sidewalls?
Yes, but in the corners first then more on the ceiling, sidewalls and backwall.
The following has been repeated here many times:
A rectangular room has 12 corners not only 4. To reduce room mode peaks, nulls and modal ringing, treat as many of the 12 corners as good as you can. Wideband absorbers on the flat walls and ceiling help too, but since corner traps are built with FRK, paper or plastic facing the room, they are more effective bass traps while at the same time, reflecting some mids and highs (which is not what you want for RFZ absorbers).
Should i make bass trap panels on these spots? I don't really want any mid/high absorption anymore. I currently have only 0.1s rt60 time... So should i put that paper over glasswool before fabric? But aint that fabric itself also reduce high frequencies?
It would make L / R symmetry also better. These are just above first reflection points, but i shouldn't worry cause my speakers aint that high ?
Heres an update! I've been making new panels, new speaker stands and other stuff whole summer. Here's how graphs from today looks! I just put my focal's vertical position and did some placement experiment and thats the best compromise i came up.