[QUOTE=Ethan Winer;8439517]I'm not a construction expert, but I wouldn't extend the joists to be lower with heavy wood. Maybe you can make a structure from metal studs or some other light and inexpensive material?
Yeah I was thinking of mounting something "L" shaped down from the joists, (15cm long) facing each other on each studs, so that it will keep the mineral wool in place.
(Maybe 30 of them all together across the ceilling)
And then maybe attach metal studs where I can attach the fabric.
But you don't think all that metal will resonate?
Maybe I can find some "L" shaped in wood....?