With proper performing treatments, the results will be noticeable right away, so no need to treat every sq inch if that’s not the route you want to take. With some bass trapping & treating reflections on 4 walls, you can have a very good sounding room (& even 2 side walls or side walls & behind the listener can have big improvements). This is all I’m doing currently, nothing on the floor (not even a rug) or ceiling.
Couple pointers, if looking at GIK do not look at their 1", 2", 3.6" panels, these IMO are a waste of time (& how you end up with a dead sounding room). As a min I’d do their 4" sidewall & 5.25" on front/back walls but would prefer their 5.25" on the side walls & 7.3" on the front/back (you don’t have to do all absorption, these are just recommendations if you are using absorption, which is a good way to start). If you do want to add diffusion, just be sure to check how far away the treatment needs to be installed, as you need to be a certain distance away from diffusors for them to work properly (& can actually harm the sound if you’re too close), its fairly common to need to be 5-6’ away. If the distance is much lower then this, I’d be cautious of how much of a diffusor it actually is. If you can’t get the required distance & don’t want strictly absorption, adding scatter plates to the absorber can be another option.
Contact GIK, they have resource outside of the Uk but they are all helpful.
I sent a few pics and video of my room and just did the basics I.e. first reflection points and corners.
Ideally they will want REW measurements but I never went that far as good results doing the basics.
I did find symmetry was important and it was the sum of the parts. They do them on sale or return basis but you have to suck up the cost of returning them if they don’t work out.
Thanks and these are the next steps to take. There is a lot of potential with this room due to the very solid construction and a great scratch render with a lack of smooth and flexible reflective surfaces, but in the end there are limitations with the low ceiling at 2.2 m and one dimension half another. I need to do some analysis using the suggested tool after trying to find the best speaker positions using the ears. If I fire across the room, the speakers are quite far from side walls. I will try initially some natural treatments like CD shelves (corners on same wall as speakers) and LP shelves (behind listening position) but will for sure need some treatment to reduce the worst effect probably due to distance multiples. That’s where the living room above that room with the higher ceiling is better behaved in that respect except for the large smooth reflective surfaces (glazing and plasterboard)
Yes , GIK UK , contact Lukas for advice. He has given me good advice over the years.
Also i found a lot of useful information from online articles and videos from Dennis Foley [Acoustic Fields] and Ethan Winer [Real Traps].
As has been mentioned do not use any large quantity of thin panels. This is very likely to cause a dead sound. I used GIK Monster bass traps with range limiters on all walls. Also do not forget the ceiling , it makes a very real difference , as its usually the nearest surface to the listening position. I started with 2 panels on the ceiling first reflection points but ended with 12 panels on the ceiling, its that important.