What about carpet underlay between sheets of drywall,below and even between the floor joists, or on the sides of the floor joists? I considered installing it against the sub floor but there's way too many nails and screws for that (its an older house, must have had squeakier floors at one time).
I know where I could pick up 3 or 4 new unopened rolls for free.
Carpet underlay? Like the pad?
I've read that people have tried that and that the foam based stuff did nothing, zip, zero to help, as you needed to either get really thin stuff so that then you screwed down the top layer of drywall, the thickness of the foam didn't cause the screw to just pop through the outer layer of drywall as you were screwing it on, or you had to go with really dense rubber stuff, still fight that issue of screws pulling in, and really just ending up with a headache and almost zero improvement even using measurement devices.
The Green Glue isn't a glue at all. It stays sticky, never dries (ok, I am sure that in 200 years or something it would start to dry out), and being semi-fluid, it will always maintain its absorptive properties. Again, there is a reason that people use this over any other method. It IS messy, but doing other things with thicker materials like carpet pad will just cause headaches potentially during construction and worse of all, provide little to no improvement.
Look at it this way. Even the insulation in the wall cavity allows sound through it if the wall cavity is stuffed with it. That is why ideally you would want an air gap in addition to the insulation. The sound that gets past the Drywall/Green Glue/Clips/Channel/Whatever then gets absorbed by the insulation, however that is only at the frequencies that the R19 (or whatever you use) insulation absorbs. It takes that sound energy and turns it into heat due to the slight vibration of the insulation (at a microscopic level). If that slightly vibrating insulation is also touching the drywall on the other side of the wall, some of that sound vibration of the insulation transfers to that outer layer of drywall and into the next room.
My using a foam or rubber, or whatever product, you are only trapping a very small frequency range, but most of the sound is still just passing from one layer of drywall to the next because you would have to sandwich the stuff so tightly in order for the top layer of drywall to actually be secured.
You are sort of talking about doing "Wall Solution 3" from the link I put above.
Soundproofing Walls However you wouldn't get nearly as good of STC because option #3 is using Mass Loaded Vinyl. That stuff is HEAVY and is really there like a super dense 3rd layer of drywall (figuratively).
If I had to guesstimate, I would say that two layers of 5/8" with a layer of carpet underlayment between them, you would probably get just 43 STC (maybe even just 42 STC like not having anything in the middle of the two layers).
Same thing, but with Green Glue --> 52 STC and improved low frequency performance and reduced flanking.
Again, those guys don't make Green Glue, so it isn't just marketing. It really does work.
