Acoustical treatment

Hi all! I'm really impressed of this forum, there are very good ideas and suggestions for acoustics. After reading for some time now I need your help :) I need a suggestion for the acoustical treatment of an almost square room (around 5.25m x 5.25m). Walls have already been insulated with brick + rockwool + lead + mdf and soffit also with 2" pyramids. The room should be used for group practice and drum/guitar recording. Here is the sketchup file with walls. Thank you in advance :)

because its square and it sounds like you built it pretty solid, you're very likely to have strong room modes from around 120-300hz. so adding some slat resonators will help. if you could construct another wall inside to alter the room shape - might help but may become a third leaf situation which could result in less isolation than desired. you should take some room measurements to see where you stand in regards to problem frequencies. once you know how your room is actually behaving you will likely get some better advice.

maybe move this one to the Acoustics part of this forum

Done.