Timlub - Yes, corners are the worst (I have window in one).
Damping materials work effectively to about 100Hz at 2" thickness. Further down it requires going to 4" panels (dense fiberglass) and positioning them 1/4 wave from the wall. I'm in process in building 2" panels (dense fiberglass material) but have to take care of corners as well. A lot of work and it doesn't come cheap. In addition there is WAF.
Damping materials work effectively to about 100Hz at 2" thickness. Further down it requires going to 4" panels (dense fiberglass) and positioning them 1/4 wave from the wall. I'm in process in building 2" panels (dense fiberglass material) but have to take care of corners as well. A lot of work and it doesn't come cheap. In addition there is WAF.