This is what I'm putting on the rear wall of my control room. It is an absorber outlined in Philip Newell's book, he says that it will absorb below 40hz.
We start out with a standard stud wall with rockwool insulation.
External image, not preserved — original: http://dl.dropbox.com/u/1049914/Rear%20Wall%20Detail/rear%20wall%20detail%209.png
Next we have two layers of 5/8" drywall with roofing felt (he uses what he calls deadsheet) sandwiched in between.
External image, not preserved — original: http://dl.dropbox.com/u/1049914/Rear%20Wall%20Detail/rear%20wall%20detail%208.png
Then another stud frame wall.
External image, not preserved — original: http://dl.dropbox.com/u/1049914/Rear%20Wall%20Detail/rear%20wall%20detail%207.png
The next layer is a layer of roofing felt sandwiched between 2" of rockwool
External image, not preserved — original: http://dl.dropbox.com/u/1049914/Rear%20Wall%20Detail/rear%20wall%20detail%206.png
External image, not preserved — original: http://dl.dropbox.com/u/1049914/Rear%20Wall%20Detail/rear%20wall%20detail%205.png
Next we have hangers parallel to the wall.
External image, not preserved — original: http://dl.dropbox.com/u/1049914/Rear%20Wall%20Detail/rear%20wall%20detail%204.png
Here is a close up of the hanger. Rockwool on roofing felt on soundboard on rockwool.
External image, not preserved — original: http://dl.dropbox.com/u/1049914/Rear%20Wall%20Detail/Hanger%20detail%202.png
Here we have angled hangers.
External image, not preserved — original: http://dl.dropbox.com/u/1049914/Rear%20Wall%20Detail/rear%20wall%20detail%203.png
Then a frame with fabric covering
External image, not preserved — original: http://dl.dropbox.com/u/1049914/Rear%20Wall%20Detail/rear%20wall%20detail%202.png
External image, not preserved — original: http://dl.dropbox.com/u/1049914/Rear%20Wall%20Detail/rear%20wall%20detail%201.png