Music room

Hey all, i've been following this board for some time, and decided it is time to ask a question. Originally, I was planning to build a studio in the new house we were going to build next year, but we have postphoned the build for 5 years. So now i have a room available (3m x 2m and 2m height ) for my guitarescapades. Now, this room is completly finished, so i cannot break walls, replace windows and other stuff. Target: I play guitar at a reasonable quiet volume (Normal radio or TV volume) but i would still like to diminish the sound for my neighbours. I have a shared wall with the neighbours (3m wide, 2m height) ... What can i do to this wall to keep a bit of sound from going through? Thx Dave