I would wrap the pipes with Armaflex, which is that rubberized pipe insulation that you find at HD.
Click on the link below to see a sample.
Additionally, you can fill the stud bay with fiberglass insulation.

Here is a link that might be useful: Armaflex

This post was edited by willtv on Sun, May 19, 13 at 10:26

Thanks!

I actually went to HD and they didn't have the Armaflex pipe wrap (as shown in your link). They just had the Armaflex tape/wrap. I did get some fiberglass insulation. I will see if I can find the Armaflex somewhere else, maybe a plumbing supply store?

I thought of this issue also when adding on our new masterbath with shower pipe running through the wall adjoining the head of the master bed and another plumbing stack adjacent to little DD's crib. Upon our request, the insulation crew (whole house reno with lots of new insulation) put in some kind of thick, wool-like insulation they said would help with the noise. Indeed, when I stand in the incomplete room it is eerily quiet.

I used the rockwool sound rate stuff between my living room and master bedroom. I also used QuietRock instead of regular sheetrock, both there and between the bathroom and bedroom.

There is some sort of heavy wrap for pipes -- I think it works out to about 5 dollars per foot of drain pipe. Regular fiberglass is nearly worthless for sound.

Rock wool. That's it. That's what we used. DH and I were just saying how we wished we'd have used it in more places to help block sound.

If you really want to go all the way, you could replace that PVC section of drain pipe with cast iron. Then you would not hear it at all. (Probably not gonna happen, I imagine!)

Thanks for the input!

I will not be replacing it with cast iron :). Only the top part of the drywall is out -- where the soffit was.

I went and got the "rock wool" from Menards (mineral wool insulation, brand Thermafiber) -- hopefully that's right!! So happy they had it since when I called this morning, they had no clue what I was talking about. I decided to run down there and by chance found it at the end of an aisle - I'm very happy about that! I figure I at least need to make it as quiet as it was with the soffit up there (it was a sturdy plywood box so must have offered a pretty good sound barrier).

Thanks again! This is why I love GW!!