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Q. Is there a way to dampen the sound of a PVC waste pipe, other than tearing out the plastic and installing cast iron?

A.Bonnie Schnitta, owner of SoundSense, an acoustic consulting firm and maker of sound-control products in East Hampton, N.Y., responds: Water running through a plastic pipe is more than twice as loud as water running through cast iron. That is mostly because PVC has less mass than cast iron and therefore a lower STC (sound-transmission class, a rating of a material’s ability to resist airborne sound transfer). Replacing the pipe with denser cast iron would result in about a 10- to 15-decibel (dB) sound reduction, but you can get the same results by increasing the plastic pipe assembly’s STC. Adding insulation around the pipe would help a little, but pipe insulation typically absorbs only 2 to 3 dB of noise, a barely perceptible change. And even if there were room for extra insulation, it takes 4 inches of fiberglass insulation to absorb just 3 to 5 dB of noise.

To achieve a 15-dB transmission loss in the plastic pipe — comparable to the results you’d get if you replaced it with cast iron — you’ll need to wrap it with a material that has a minimum STC of 29 (or, if there is already insulation in the cavity, an STC of 26). My company wraps plumbing and hvac ducts in a 1 1/8-inch-thick loaded vinyl barrier with a scrim-faced fiberglass quilt; it has an STC of 29. If the pipes are already insulated, we use 1/8-inch-thick loaded vinyl with an STC of 27. We make sure there’s an overlap of at least one inch (4 inches is preferred) when wrapping and use zip ties to hold the acoustic barrier in place. To help contain noise, we caulk the overlaps with a flexible sealant called Big Stretch (800/289-7290,

Like water, airborne sound can leak out of tiny openings: Holes as small as one inch in diameter can reduce the effectiveness of soundproofing by roughly 5 to 10 dB. So, in addition to caulking the vinyl barrier, we also seal any openings where the drainpipe passes through the framing. The best way to do this is by wrapping the pipe in advance with the vinyl barrier before passing it through the framing, since this helps create a perfect seal and prevent structural coupling. But when this isn’t possible, we cut a flange on site from a piece of the vinyl barrier, making a collar that fits around the pipe and covers the opening. Another option is to fill the gap between the pipe and the framing with a flexible acoustic caulk.

To minimize structure-borne noise, we try to avoid rigid connections between drainpipes and the framing. For example, we use neoprene pads where metal hangers that support piping and ductwork are fastened to joists or studs. For more on sound-control techniques, see “Innovations in Sound Control,” 3/06.