I did what you are intending to do about 7 years ago. I have a large double garage and built a room in half of it. As it was single skin I (yes I did all the work
) put Synthaprufe on the external walls (lovely job!) then battened it all. A carpenter friend sourced all the materials for me which included fibreglass insulation that he said was not only for heat insulation, but had acoustic deadening properties as well, Cellotex sheets and acoustic grade plasterboard (a 1200x2400 sheet weighed about 80lbs) Two of us tried to put the plasterboard sheets up for the ceiling.......
even with supports we failed and ended up with one "holed" board. it took three of us to do the ceiling.
My room is about 9.5ft x 19ft. Not an ideal shape but it is as it is. I get an LF boom prob around 200Hz, but with clever tuning it is not too much of a problem. What it does give me is :-
A virtually sound-proofed room
Somewhere I can listen with no hassle
It is a shit-hole most of the time
Mine is a concrete floor. I painted it, and put 2 layers of underfelt and an old carpet that we just renewed in our bedroom and it is very good.