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,
sashcosealants.com)
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.