Sound proofing floor

Emailist,
I had that in mind- using the mass loaded vinyl on the floor.
Larryi,
I plan on using that vinyl on the walls as well.
My challenge is this: I will build the floor out of plywood and it will be supported at both ends. The span is about 11'(that's the width of the room), so I'll have a joint some where in the middle which will be staggered, and will be forced to use some lumber 2x4 or other. If I stand the 2x4 on edge to keep the floor from springing to low in the middle my floor gets thicker, something I don't want. So I was thinking towards the middle of the span installing srings of a sort? Foam? Rubber? Sorbothane?
But what will it feel like to walk on? And if I am sitting in the middle of the floor and the raised floor sinks to the spring material then it defeats the purpose of the raised floor which is suppose to be decoupled. Any suggestions. How much air do I need between the raised and existing floors to stop the sound from passing through?
In the end I think I will be installing the raised floor joists on edge.

Psacanli,
Thank you!
One other concern of mines(I know, it never ends) is the outgassing from the foam. I am not a health nut, actually a quite balanced health oriented individual. The outgassing is an important concern though, and in the winter when the stack effect forces air and vapors to move upwards it'll just end up saturating my living space. So I am hesitant with not just the foam but most materials that are being considered. I think the vinyl will not present as big an issue as foam would. The outgassing concern is what has kept me from going forward with this project as it has been on my mind for a while. Any suggestions Psacanli?

In addition to what I just posted: Something I was considering was cotton batting to avoid the outgassing issue.

Thank you Psacanli, you know you make a good point the vinyl should do a good job of slowing down the outgassing! I was also concerned about the vinyl itself, I remember in my research of outgassing that foam was one of the greater culprits and that plastics too, the softer the plastic the greater the outgassing. I don't expect the plywood to slow gassing too much but if vinyl proves to be not so harmful then I will go with your suggestion! Thanks!

I concur entirely with getting consul from an acoustical engineer, I needed to hear that from members, thank you.