Low end and Low-Mid frequencies
Acousticians will tell you that controlling the bottom end is the top priority when acoustically treating a room. The smaller the room, the more absorption it will need. Foam is just not up to the job, It does nothing to control the buildup of unwanted bass frequencies. Controlling the bottom end of a room is the priority, otherwise you risk the chance that the room is exaggerating certain bass frequencies, which in turn makes it very, very hard to tell how the bass is actually in your track.
The tried and tested solution is to either make your own bass traps out of high density rock wool (you’ll find all the info you need on making panels here) or buy ready made ones from a company like Gik Acoustics.
Bass does tend to get reinforced in the corners of a room, so that’s the first place you should put treatment. The corners to the back of your mix position should be the first corners treated, followed by the two behind your monitors. Just doing those four will make a night and day difference.
As you start to add more bass trap treatment, you’ll hear the bass and kick drum get more defined and distinct. The room will start to sound tighter and you will wonder how you ever mixed without it!!