Early reflections cause confused sound
There are two important things to consider in monitoring. Firstly, that you can hear exactly what's on the recording, secondly that you can judge what your listeners will eventually hear.
Both of these necessities are made more difficult by early reflections. The brain is programmed to accept reflections that arrive within about 30 to 40 milliseconds of the original direct sound as part of the sound source. Reflections that occur after that are processed separately.
This means that it is acceptable to have a certain amount of reverberation in a recording studio control room, but the levels of early reflections should be minimized.
So where do these early reflections come from? Well in a typical home or project studio, the control room is quite small compared to a properly set up commercial studio. Therefore early reflections come from surfaces all around. But there are key places where these reflections will be worse, and therefore significant improvement can be made if these are dealt with.
So, sit in your monitoring position in front of your loudspeakers and look around you. You'll see a ceiling above and two walls to either side. Now imagine that your monitors are firing out billiard balls rather than sound waves. Imagine where they would bounce after they struck the ceiling and walls.
This is where the sound waves are bouncing too. If you trace the path between loudspeaker, wall and your ears, the point where they strike the wall is where your acoustic treatment should be placed. You can either absorb the sound waves using porous material, or panel absorbers if you want to be more thorough. Or you can diffuse these reflections with irregular surfaces, so the reflections are split up and less energy comes in your direction.
You could make a serious improvement with a square meter of treatment on each wall and perhaps two square meters on the ceiling, because of the width of the sound image.
But there is another reflecting surface too - your mixing console. A mixing console is a large, flat surface that is very good at reflecting sound. It would be nice if the console could be covered by an absorber too. It really would make a difference.
But if you mix with a computer and a mouse, then you don't have the hang-up of that large, flat surface. The computer, monitor and keyboard are - to the sound waves - irregular in shape and will provide useful diffusion.
Reducing the effects of early reflection is easy and inexpensive. It will make a significant improvement to the quality of your monitoring.