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?
Best Answer
The reason the floor materials aren't as effective is because this problem is best solved with an air-gap and a flexible connection spanning that air-gap. But you have a hard time supporting a floor with that air-gap. Therefore, solving this problem via the ceiling below this floor is better, using resilient or sound attenuation channel there.
That said, on your floor, carpeting over Homasote, QuietWood, or some other material will be as good as you'll get on the floor. Since I don't have experience with MLV or GG, I'll defer to others on that. However, the cement backer board seems like the weak link here since that will transmit sound rather than absorb it.