I've been researching soundproof underlayments for floors, but the selection seems to be very paltry. It appears that you're pretty much stuck with mass-loaded vinyl (MLV), QuietWood, and a few other sheet-type materials. The bad part is that these are not as effective as materials used for soundproofing walls and are certainly no good at blocking lower frequencies, so I came across an idea: What about MLV on my subfloor, followed by Green Glue (GG) then cement backer board (BB)?
I think this would be a very effective solution. Putting the MLV down first would put a protective layer over the subfloor so I don't get it all gooey with GG. Instead, the GG would go between the MVL and BB. After I seal all the BB seams with accoustical caulk and tape, I would then lay my carpet over that.
If BB is too over-the-top, what if I replaced that with simple 1/4" plywood?