If I mount the rigid fiber board (like 703 or the JM 815) on a thin piece of wood for support, then space it away from the wall, will I still get the benefit of absorbing lower freqencies?
It depends on lots of things. I'd try to stick with one of the known and tested designs -- otherwise you're just hoping and who knows what you'll end up with.
If you have
- porous absorbtion, then wood, then a 2" air gap, then your wall (gypsum/drywall)
worst case you'll get a resonance that will reduce your soundproofing.
Generally speaking, wood is a reflector.
If you have wood in a sealed enclosure, it can be a membrane trap.
FRK (basically paper or foil) on the outside of fiberglass certainly reflects, but is light enough that it lets some sound through.
Ignoring what you might get for absorbtion -- why would you cover the entire back with plywood and then space it from the wall? Are you trying to hold up the fiberglass?
I'm trying to figure out how and why you're mounting the fiberglass to this piece of floating plywood. Why not just have the four corners. And if you're gluing the 703 to the wood, could you fill the wood with holes (i.e. 80% holes, 20% wood). Why not just six spacers (2"x4x"4") and use impailing clips on them to hold up the fiberglass. (BTW, I've never heard of anyone glueing fiberglass to wood. Usually they are some kind of nailed/clipped, or held by gravity, or a frame, or spring ties. Mine were friction fit into an exoskeliton frame)