exceptions to the DON'TS... MLV and 3 leaf

I think I might have a use for MLV.... it's not going to be expensive for what I need it for. The metal duct vents on my basement ceiling (to provide HVAC for the first floor). This is the definate weak spot for soundproofing the basement. My ceiling isn't up yet, but I need to do all that I can for this now since I won't be able to do anything later. So far, I tried puddy over the metal. It got me a few db of isolation, but i'm afraid that it won't be enough once i've finished. I'm thinking this would be a place where MLV is actually a good option. I'm afraid to put drywall around it because the HVAC and the metal could cause sweating. I also don't want to mess with the vents (don't want to remove them to put something else there). So I can buy MLV (by the running foot) with adhesive on the back. And use that around the vents. If it's just on the bottom of the vent, it actually should still only be 2 leaf when the vents are open (which is all the time). I was thinking of doing 1/4" layer... ANY reason why I should not do this?? Second exception... I realize that 3 leaf is bad for low frequencies, but what about for vocal booths? Because of all the stuff on my ceiling (pipes and stuff), my outer/upper leaf (the upstairs floor) is not going to be so great. So would I be better off putting something between the joists beneath the pipes? Yes, this would create a 3-leaf affect, but would it be OK for vocal frequencies? Or would I still be better off with 2-leaf?

If you can give actual dim's, materials and thicknesses of each mass and each air gap I'll give it a "SWAG"... Steve

The MLV would be 1/4" thick stuck right to the vent (around it and then stuck a little bit to the flooring above). The vocal booth ceiling... The first leaf would be the subflooring on the first floor. It's tough to tell, but I think it's around 3/4" thick. This has pipes going up through it into the kitchen. I would use acoustic caulk around them in case of air gaps there. Then 6 inches down (or lower if it makes sense) from the subfloor would be where I'd add the middle leaf (2 layers 5/8" drywall in between joists). Then I'd use RSIC on the bottom of the joists which would create an airspace of 5 inches before the next 2 layers of 5/8" drywall. Is that what you need to make a scientific wild ass guess? :lol: