Room Treatment? How important is it to treat the wall behind/between the speakers?

@macg19  Thanks for taking the time/effort to take measurements with DIRAC. 

It appears that DIRAC is working as it's expected matching L and R speaker decibels levels as best it can based on whatever parameter limits you set.  But you're on step 4 when steps 1-3 aren't quite finished.  There is a sequence to treating a room and it generally goes like this: (1) begin with Early Reflections and L/R speaker's reflection symmetry within 0-10ms which usually involves 1st order reflections in most domestic sized rooms.  Energy Time Curve data is used for this step which REW provides but I don't know if DIRAC provides.  (2) Bass Decay time should be dealt with next to reduce masking of midrange details when bass decay times are excessive compared to those of mids/highs.  A T30 metric is found within REW and you can get pretty granular by using 1/3rd octave intervals to see where problems lie and what the best type of bass trap is to use (velocity or pressure). Again, not sure DIRAC measures this as it was designed more for frequency response and not the time domain (i.e. early reflections and decay times). (3) Mid/High decay times come next with the use of diffusion and reflection and appropriate T30 decay times suitable for your room size. (4) last comes frequency response and gentle EQ which is where DIRAC lives as I understand it. (I use Audiolense XO for FIR convolution filter creation myself.)  While Frequency Response is usually the default go-to chart to use, it is impacted by the sound bouncing off reflective surfaces between the speaker plane and your listening position.  Because the frequency response's informational content includes the effects of early reflections, it's important to get those locked down first otherwise you're chasing your tail.  Looking at your system picture I am not convinced that the wooden slats on the left sidewall and curtains on the right side are giving you symmetrical early reflected energy especially in the 1 and 2kHz octave regions where our hearing is most sensitive - the wooden slats would preserve the 1 / 2kHz octave energy while the drapes would attenuate it, and I hazard to guess that only through the use of DIRAC are the L/R frequency response curves very similar.  I'd encourage you to try the same type and thickness of absorption panel on each sidewall as a common denominator to different construction materials used for each sidewall which impacts tonal balance.  Play with absorption panel angles and distances from the wall behind it to achieve L/R Early Reflection symmetry before turning on DIRAC.

Reading your DIRAC graph:
> first off, the chart looks like it might be at 1/24th smoothing which shows all the nasty bits, so don't be surprised or discouraged. 
> the curves track each other nicely which is DIRAC's sweet spot but range from about -2 to +8dB for a +-5dB range which should ideally be at +-3dB or better.  Getting the sidewall's 1st order reflections symmetrical should help DIRAC out so that it doesn't boost/cut EQ as much.
> The large "U" shape in the 30 - 80Hz range is trouble as a kick drum lives in the 40-70Hz range as does bass guitar, and other instruments.  The R speaker at 45Hz is about 6dB louder than the L channel - not good. This could be caused by the R speaker being near an external wall with brick/concrete behind the drywall which reinforces these low frequency's energy whereas the L speaker is near an inside wall that is less stiff and flexes. Pressure traps can help here like GIK Scopus T40 / T70 traps as can multiple subwoofers.  Experiment further with speaker / chair positioning to see if that "U" hole can be remedied as it is beyond DIRAC's boosting limits.

@macg19   https://pmamagazine.org/early-reflections-101-the-first-10-milliseconds-that-make-or-break-stereo-imaging/

@kevinzoe - excellent article. Thanks for posting.

I have not used available Software to create the management of Sound within my room, it is carried out using different configurations for absorbent and reflective materials.

The placement of materials used is now concluded and having Triangle Shaped Absorbent Materials in all Room Corners is a must have in my room.

This set up has a Absorbent used behind each Speaker.

Rockwool with a Mass of 35Kg / m3 and used at 100mm (4") Depth is an affective  absorbent and a very cost effective method. It is good practice to create a small dimension cavity between inner face of Rockwool and the Rooms Wall. 

@macg19,

You can see updated pictures of my room in my profile now.

Long story on how to deal with the window... First thing I did was to use REW real time analyzer data/graphs (using the MiniDSP calibrated mic) to see where the resonant points of the room were when I played  low frequencies that I would hear the clear "booms" from the listening position. Just play the tones you want and walk through the room with the mic and find them while you are using the RTA or a SPL meter calibrated with C curve (A curve will not tell you true response for bass). Room bass nodes were partially located in the 4 room corners walls, from floor to ceiling, but they were much more concentrated inside the back sides of the closet - almost up to15db higher than in the listening position. So, I started to investigate what type of bass absorber was the best for the buck and ended using the Acoustic Fields designs. Worked like a charm... I made them as big as I could to fit the available space of the closet without tearing it apart. To make it happen, I had to repurpose the fractal diffuser that was inside the closet and moved it to the right-side wall and the small one from the right wall I originally had I moved to the window side. I did not want to remove the window, so what I did was to use a technique they use in mixing studios where they use portable treatment to control their recordings. I build a pair of sawhorses and created a support for my small fractal diffuser.

The bass traps significantly improved the entire bass response of the room because I could better integrate the subs with the main speakers using crossover & volume adjustments while maintaining a better response of the bass. With the 4 bass absorbers in place, I was able to lower the response of the room around ~35Hz by about 6db. Repurposing the two fractals, I also eliminated the problem of the window, since I eliminated the first reflection problems from both speakers adjusting the position of the middle of the diffuser height to the listening position. Soundstage got more precise and expanded to the sides of both speakers.

You can use a mirror to find first reflection points. Just ask someone to hold it in the wall and move it until you see the mid driver/tweeter – just pay attention to which speaker you are seeing... That is the area where you need to treat for first reflections. Same is true for ceiling and floor.

Hope this helps answers your question.