So I got big nulls in my room at 40hz and at about 80hz (mainly want to fix the 80hz). The image below is my listening room. Things to note is that at the far end of the room I cannot really place anything and the front wall is sloped (the green line indicates where the slope is) with a radiator on the right where I can't really put anything. Since I don't want my room to look like a studio I want to put bass traps in Ikea bookshelves etc. I also have no way to fix ceiling reflections.
In purple I've indicated possible places I can put stuff.
- On the bottom you see a bookcase I can put trapping material in (its about 30cm deep). I can also stick something to the side, because that's nicely out of sight.
- I can put a room divider such as Ikea Kallax behind my couch and fill it with material. This would be about 1.5m in height and can be about 30-40cm in depth.
- I can also put a bookcase next to the couch and fill that.
I plan to use cotton based insulation material. I can choose between 45kg/m3 and 25kg/m3 material. Thickness for the heavier type is up to 45mm and for the lighter material up to 200mm. But will all of this have any effect at all, because I cannot really put them in the actual corners...
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What did you do in the end?
When you consider the sound you will hear you have to divide up the frequencies into those above and those below the Transition (or Schroeder) frequency, usual around 250Hz but variable depending on the room size and shape. The biggest problems will undoubtedly be in the bass region, say 0-300Hz. This is the most difficult to solve but get this right and it will free your music to sound more as it should.
As you can't use lots of bass traps it may well not be worth using any and concentrate on using subs and DSP/EQ to try and solve the issues. On subs I don't know much but it seems a case of moving these around to get the best results (this applies to your main speakers and listening chair too if you can do this). With DSP/EQ you can adjust individual peaks and nulls (often very narrow nulls have little impact on the sound - it is the wider ones that cause problems). Machines like MiniDSP with Dirac are worth considering.
For the higher frequencies, say 300Hz to 20kHz, symmetry is one of the factors in a balanced sound and you haven't got it! How you listen to your music - on your own sitting anywhere on the sofa, or with someone else so that you are off centre - will have a bearing on what you hear. If you sit on your own then it would be wise to have the speakers at least as part of an isosceles triangle with you at the apex. It's true that your left speaker is closer to the side wall then your right one and this will effect the balance of the higher frequencies that you will hear at your chair. You can solve this by placing a suitable absorber on the left wall, and a moveable absorber on feet by the right one, to be stored elsewhere when not listening. You can also buy (or make) them in the form of an image so they look more attractive.
You mention you have a coffee table. You should check if there are reflections coming off that which will distort the sound. On ceiling reflections, you may not suffer from them. It will depend on your speakers dispersion characteristics and how far you are from them. If it is an issue, you could put some sort of absorbent material (that absorbs from say 300Hz to 20kHz) on top of the speaker so it sticks out a bit forward enough to stop a sound 'ray' going to the ceiling and bouncing back down to your ears.
Apart from the frequency response, there care two other aspects that are sometimes considered - phase, and sound decay times. Absorbers seem to help with both these whereas DSP/EQ has a more limited effect. Of these, decay times are a big influence on quality of sound.
Overall you shouldn't get carried away with trying to get the perfectly flat frequency response. If you clear up the worst damage to the lower frequencies and generally smooth out the higher ones that should give you a good sound. Above all of course it is what you hear that counts.