Foam, insulation, or OC?

[losthighway]

I am working on a basement live room/rehearsal space build. I was disappointed to discover a massive 3.5' wide duct running the length of about 2/3's of my room. I unfortunately can't sacrifice the head room to put in a soffit, but luckily the duct is flush with the bottom of my floor joists, which gives me some options.

Some clever person here, or perhaps it was at the John Sayer's forum had the smart idea of using something to dampen the metal of the duct before I cover it with a couple layers of drywall which will be green glued together. There is already some 2x1 pieces of wood mounted to the bottom sides of the outside edges of the duct. This can work as a sort of frame for whatever dampening material I put in, as well as something to receive the screws when I put the drywall up so I don't puncture the duct.

Here is the important question: what material do I use?

To clarify this material will not be selected based on it's ability to add isolation, other features of the build should address that concern. What it is supposed to do is dampen the metal from ringing like one giant cymbal. It could be blankets, but of course that would not be fire safe.

Due to my ceiling height concerns, as well as the handy wood in place for screwing drywall (only 1" height) if I use regular old insulation it will be highly compressed.
I could buy those styrofoam insulation sheets in whatever thickness I need. But they might not have the same dampening effect as the soft tufts of fiberglass insulation.

Or I could just buy some more OC 503 (or whichever model). That would be the most expensive, but it could add some positive side effects (increased absorption) to the build. The only down side is I was hoping to blow my rigid fiberglass budget on room treatment, not construction.

[Scodiddly]

How about just a layer of the green glue on the duct? That should damp it down pretty good. You could also hit up a car audio supplier for those sticky heavy sheets of goo for damping car door panels.

[losthighway]

That could also be practical. My only concern would be how well that would come apart if I wanted to sell the house years down the line.

[goldstar]

lost

Do you have the money to replace the duct with a replacement made of duct board? HVAC contractors build ducts out of that material, which is similar to 703 (though not as dense); maybe you could have one made to replace the existing one, and it would be self-damping.

Duct board might work as a damper if you can't afford a replacement; it's usually 1" thick, I think.

Frank

[JWL]

If you have 1" of space, then I'd suggest using 1" 703, or 705 if it's cheap enough.

[losthighway]

You know I ran over the numbers in my head and covering the are of the duct with 703 is only about fifty bucks more expensive then the other materials (at absolute most).

I'm thinking: why not do it the best way possible. 703 it is. Thanks for being a sounding board fellas.