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...

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