I'm building a single story home with a full walk-out basement.  In the basement I have a den/family room with an adjacent bathroom.  The wall between the basement den and bathroom is where all of my upstairs plumbing drops down to catch the main sewer line under the slab.  For a portion of this wall where the vertical 4" sewer pipe drops down and where the 2" vents for the basement plumbing connect and go back up to the roof, I'm planning on a double 2x4 wall.  The plan is to install the 2x4 wall on the den side, notch out the 2x4s as required for the plumbing drains and vents, then install the bathroom side 2x4 wall with corresponding notches to fit around the other half of the pipes, assuming the pipes are ~centered in the wall.  I was originally going to go with a 2x6 wall here, but with 4" PVC pipe (~4.5" OD) and fittings (~5.0" OD) running up through the wall, there is little if any room left in a 5.5" wide stud wall and top/bottom plate.  So I decided to just go with a 2x8 wall but then decided on the double 2x4 wall to ease installation of the plumbing which I am doing myself.  I was also thinking of keeping the two stud walls separated by ~1/4" to further help dampen sound transmission from the bathroom to the den.  This isn't a load bearing wall by the way; it runs parallel to my floor trusses above.

The point of this post was to get input on my idea for sound deading this sewer stack in the basement.  Originally I was just going to have this section of wall packed with blown cellulose insulation, but I'm not sure how effective this would be and then I have to coordinate the installation of this insulation when other insulation is being installed.  Then I had the idea of filling this cavity where the sewer pipes are with sand to add mass and dampen the sound.  The way I thought of doing it was to rip some scrap 2x4s and nail inside the studs 1" back from the outside edge of the studs.  Then cut some 1/2" OSB or sheetrock 14.5" wide to fit between the studs 16" OC and nail/screw to the inner ripped pieces of 2x4.  This would leave a 1/2" air gap between this inner wall board and the finished 1/2" sheetrock on the den and bathroom sides of the wall to help dampen sound.  Then I planned on filling the stud cavity with the drain pipe with sand from the top of the wall to add mass to further dampen any sound created in the pipe.

Any thoughts or input appreciated.

Since you are planning on two 2x4 walls, if you have enough room, set the walls clear of the pipes. This will take up more space, but will be easier to install after the plumbing pipes are in, not nearly as much work as notching everything, and will lessen the possibility of sound transmission. You can also stuff 3.5" insulation in the walls for further soundproofing, if you wish.

I woulld expand on what wes said.  Make it a chase from 2X4 stock, just wide enough to work in if you ever need to do repairs. Have an access pannel so you can get at the plumbing.  Makes life much easier down the road.  Install sheetrock on both faces of the walls,  2 layers of 5/8 rock on the inside walls will work fine.  Alternatively, use cast iron for the vertical part (much less sound then pvc or abs).  Filling it with sand would work ok but what a mess when you need to do repairs.

Eric

Better yet, if the potable plumbing uses (or can use) the same chase, use a copper drainwater heat recovery heat exchanger for the vertical section, and blow cellulose around it (sulfate-free only, since sulfates would eventually corrode the copper in the event of a slow leak, or even condensation).  Blowing cellulose in this sort of app is a dead-easy DIY project, even with a beater box-store rental blower, and cellulose is denser than fiberglass batts (which would be a reasonable plan-B.) The mass of the heat exchanger with it's potable water wrap would be less than cast iron, but orders of magnitude higher than any polymer pipe.

With sand you'd have to pay close attention to static loading issues as well. Dry sand is on the order of 100lbs/ft^3, so you'd be looking at something like a half-ton/sq. ft loading for a 1 story column of sand (vs. ~25lbs/sq.ft for a column of cellulose.)

I've been gathering info on a few building subjects, and sound proofing is one. This seems interesting:

http://www.soundproofing.org/infopages/channel.htm

Thanks for everyone's input. I was reading about the use of cast iron pipe in the vertical stacks in one of my recently acquired plumbing books and that is what got me to thinking about this subject in the first place. I would prefer not to use cast iron. I'm sure it works fine, but I have a much higher confidence level that I can make a glued PVC joint that will never leak vs. cast iron with rubber and band clamp.

I had thought at one time about a heat recover pipe, but my water comes in the opposite end of the house between the basement and main floor level (no water supply pipe in the basement slab). I'm using Pex water supply lines and routing it through my open web floor joists between the floors. I'm already planning to use a geothermal heat pump with hot water generator and buffer tank. In my situation, I just don't see the benefits of a heat recovery pipe outweighing the costs and effort to install it. Plus, I want reliable - something I can install once and hopefully never have to worry about again.

I talked to a buddy of mine at work who used to be a plumbing and HVAC installer. He talked me out of the sand idea. His suggestion was to use a sound deadening board available at Lowes for my inner wall and then wrap or pour insulation around the pipe if desired. Another option we discussed for this inner wall was the use of 2'x4' acoustical ceiling tiles cut to fit since these have high sound absorbing properties and are readily available. Another relatively inexpensive sound damping option to include with the above as mentioned above is to add a second layer of sheetrock on the den side of the wall or install sound board completely over the stud walls first on the den side then a layer of sheetrock. I think I'm now leaning towards this option along with the extra inner layer of sound board / acoustical tile in the sewer pipe chase.

Some, but not all of this plumbing will be accessible through the back of the linen closet in the basement bathroom. I plan to install plywood screwed to the studs in the back of this closet so that I can remove it relatively easy to access the plumbing in the future if necessary. The main sewer stack discussed above will pass behind the bathroom vanity and I will have access to the bottom portion of it along with a 4" cleanout under the sink cabinet.

Sound insulation has a lot to do with mass. The acoustical tiles are thin, so they will absorb great for sound over 500hz, but for lower frequency sound, they don't do much of anything. One option would be to get a larger sized piece of cast iron and thread the pvc though it before installing the pvc. Alternatively, use a larger pvc pipe and fill the gap between with sand after sealing the bottom. Then, stuff the rest of the cavity with fiberglass and drywall over it. Putting a plywood box around the inside of whole cavity before stuffing with fiberglass and caulking the seams could also help out some too.

There are only 3 'practical' things to do to absorb the sound in lieu of cast iron pipe. 1- Wrap the pipe with insulation, 2- Fill the cavity with insulation & 3- add extra layers of sheet rock to the walls. Sound may also transmit over the top of the wall into the floor system above, so maybe this wall should run all the way up past your trusses to the plywood subfloor above to seal it. Chaulk all your joints/framing intersections to prevent the sound leaking out.

I wasn't aware of the Provent quiet shell pipe. It looks interesting. It also looks like something you could DIY with the right size pipe for the outer shell.

It may be quieter, but they use some pretty snake-oily language when presented as:

"A level of 70dB has 10 times  the noise intensity of a level of 60dB. "

That's true, since it's a logarithmic, not a linear measure. 

But human ears also have a logarithmic, not linear response.  A 10dB attenuation is perceived as reduction by about half, not 90%, even though it's a 90% reduction in acoustic pressure.  Most of the other measures discussed here would attenuate the noise by more than 10dB.  (Standard 2x4 studwall with 1/2" gypsum on both sides is worth 25-35dB in the mid-range frequencies.)

But every dB counts- if golden-silence is worth the gold, starting 9dB below the sound of PVC drain up front (it's only half as loud!) serves to make the rest even more effective.

GreenGlue (tm) w/double layering of the gypsum would buy you an extra 5-8dB per layer too:

Part 3 – The Effect of Just Adding Mass.

Green Glue Assembly II OL 05-1049  Drywall Assembly II OL 05-1059 (2) Sheet per Side (4 layers total)  OL 05-1058 (2) Sheets one side (3 layers total)  Drywall Assembly I OL 05-1057 1 sheet per side (2 layers total)

Arkie 6, on the den side of your 2x4 wall, install ( 2) 2x2 nailers from floor plate to top plate set flush to the pipe side studes. Install 3/4 dry wall then add R 13 .

For your drywall outside wall covering use QuietRock. www.quietsolution.com I use this production for music rooms.

I hope this helps