Colin; Ranier is righty about metal studs flexing. For only a some more wood cost you can get much less resonance and transmission by building "studio walls". Use 2x6 top and bottom plates, 2x4 studs positioned alternating between front and back edges of the plates, cross bracing not needed. This allows much more damping insulation material between sheets-- reducing res & trans. Dupont Hollowfill is better at damping lows than fiberglass. Isolation is further improved by having no studs connecting the two sheets. You should be satisfied with single 5/8 sheeting this way. Martin W.