Airborne sound can only travel through air pathways. Since
walls and floors are continuous, once you plug keyholes, cracks
around windows and doors, and various penetrations for
mechanical and electrical systems, you effectively block those
Blocking Airborne Sound
The weatherstripping that seals outside windows against
drafts also seals out airborne sound, at least while the
windows are closed. You can take the same tack with an interior
door, weatherstripping it just as you would a door to the
outside, complete with a threshold or sweep. If you're adding a
door as part of a remodel, use a solid-core door rather than a
hollow-core for better sound control.
Electrical outlets in open-cavity stud walls
are another easy sound path between rooms. These can be plugged
by removing the outlet from the wall, stuffing some mineral
fiber insulation behind the box, sealing the holes in the box
with silicone caulk, and re-installing the box.
Ductwork. Cutting sound transmission through
ductwork is not as simple. The problem shows up in cases where
the duct outlet that serves a "quiet" room shares a common duct
with an outlet in an adjacent noisy room. A lot of sound can be
transmitted if the outlets are close together, less if they are
distant from each other. The best fix is to separate the
takeoff points as much as possible. If you can't do this,
you'll help things somewhat by lining the inside of the duct
with acoustic insulation. Be sure to size the duct to
compensate for the added insulation.
In many cases,
noise transmitted directly through walls or floors may be a
much greater nuisance than airborne noise. These sounds are
caused either by noise in one room that vibrates the
intervening floors or walls or by physical impact, such as
doors slamming and footfalls.
To get a feel for how this works, think of the
drywall surface on one side of a 2x4 stud wall as a diaphragm
that vibrates in response to sound pressure waves in the air or
a door slamming against it. The studs, rigidly attached, pass
these vibrations right through to the drywall on the other
side, which passes the noise to the next room.
The key to stemming this noise route is to
isolate the elements of the wall from one another. An easy way
to do this to an existing wall is to add a second layer of
drywall on the far side, attached with resilient channels (see
Figure 1.To improve the
sound-dampening ability of a existing stud wall top) or ceiling
(bottom), add a second layer of drywall supported on resilient
channel. The channel helps isolate the new wall of ceiling
surface from any vibrational noise that might be carried
through the original framing and drywall, reducing sound by
The channels act like springs to absorb the
vibrations from the main wall, cutting the structure-borne
sound by about 40%. (You can use a similar approach for the
ceiling of a room with living space above.)
An even better solution is to build a separate
new wall (Figure 2), which will give you a cavity for new
wiring and a surface for the receptacles.
sound-dampening option is to build a new wall alongside an
existing wall, separated by 1/2 inch to break the sound path.
Adding batt insulation will additionally cut down on airborne
sound between the rooms.
Adding R-11 batts helps absorb airborne
Jerry Germer is an architect in Marlborough,