In some recording studios they go with a dead end / live end design approach. This is absorption behind the speakers and then extending it along the side walls approximately halfway down the rooms length. The last half is treated as reflective as it the rear wall. I've experimented with this design and found it killed the sound. The room became too dead and the music lost too much of it's liveliness for my taste. You however could hear all the fine detail within the music very easily which is what you would want when mixing music.
Someone up post mentioned keeping the side walls live as opposed to dampening them. If your speakers are not too close to the side walls this is a better approach than just dampening them like everyone else does. Those first early reflection points are typically treated for absorption because in most cases people have smaller listening rooms and cannot hold there speakers far enough off the side walls. If you do diffuse the side wall make sure the distance to your listening position is greater than 6 feet. The key is to stop the really early reflected sound waves. Those really early reflected sound waves are the ones that create distortion. There is a lag number that you can look up but designing for it requires special measuring equipment and programs so most everyone just use general rules of thumb picked up on forums, like this one or what can be read in the many articles on the subject or from a sales person you are thinking of buying panels from.
The basic rule I follow is to install absorption panels on the side wall right in front of the speakers, at that first reflection point, if your speakers are 36" or closer to the side wall. The front wall can be left fairly reflective if you can pull your speakers out into the room. My speakers are 53 inches out into the room so I felt absorption panels on the front wall was not necessary. The room's size of 12'Wx15'D with a 9' ceiling height. Due to this smaller size I added bass traps to the front two corners. Bass waves are huge and almost no one has a room large enough to allow them to fully develop, so in most cases the installation of bass traps is a good investment and will help. When using a small pair of two way bookshelf speakers then bass traps may not be as important as if you had a pair of full range speakers or if you use subwoofers. Bass traps alone are not the solution to great bass but are very helpful. Directly behind my speakers I placed my diffusion panels. In my opinion these panels are far enough from my listening position to not create an issue and they help make the sound more lively and create the illusion that the room is larger than it really is. The bass traps also help create the illusion of a larger space. Dead center between the speakers on the front wall there is no treatment. The speakers are far enough out into the room that treatment is not required and it helps keep the room sound lively. My listening position is pretty close to the rear wall. For that reason I was concerned about early reflection distortion so I installed three 6" thick absorption panels. Due to their thickness they absorb a wide frequency range. Thin absorption panels absorb mainly the high frequencies so be careful if you choose to use them. To the side walls in line with where I sit are CD racks. The CDs are placed at random spacing and some stick out further than others so I believe this helps with diffusion. I use a large wool area rug with thick padding between the listening spot and the speakers, again this aids in absorption.
When you walk into the room your ears feel a difference in pressure from the room you just left. If you clap your hands there is no slab echo. Some people may feel that would be a sign of an over damped room, but for me the room is lively enough and I have very clean and detailed bass
I hope my reasoning for how I treated my room gives you an idea on how you might think through how you might treat your room with both absorption and diffusion panels along with furniture and room features you have no ability to eliminate.
