When I formed my acoustical consulting company in 1981, I
planned to focus on common residential acoustic problems like
plumbing, equipment, and footstep noise, and on ensuring speech
However, I quickly discovered that few effective and affordable
acoustic solutions were suitable for residential settings. And
even when solutions were available, I often found that the
products we specified were poorly installed.
Therefore, I expanded the original objectives of my company to
include doing my own research, specifying the products, and
supervising their installation. By managing all aspects of the
job, I found I could finally guarantee a solution to my
clients' acoustic problems.
In this article I will present some scientific principles of
sound and some of the strategies builders and remodelers can
use to solve common acoustic problems in their homes. As modern
subdivisions push new homes closer and closer together, and as
home-theater systems gain popularity in an increasingly noisy
world, the need for these types of acoustic controls becomes
Keep in mind that while some of the products and methods shown
on these pages are demonstrated in upscale homes, the same
principles apply to more modest projects.
Typically, my company offers a "good, better, best" approach to
sound-control strategies, as follows:
• "Basic acoustic treatment" is the simplest, lowest-cost
• "Recommended acoustic separation" is used for home
theaters and other specialized needs.
• "Acoustic isolation" is used in high-end projects when
near total sound control is required.
I'll explain the specific differences a little later.
Sound — or noise — is measured in decibels (dB). As
sound increases or decreases, decibels increase or decrease
logarithmically. What this means in practical terms is that
doubling the volume of a sound shows only a 10-point increase
in dB. For example, one TV set at a normal conversational level
is about 60 dB, but 10 television sets at the same volume will
sound twice as loud and will register at about 70 dB.
Interestingly, even in what one would consider a silent room,
there is typically about 30 dB of background or ambient noise.
Sensitivity to noise is a personal response, but generally
speaking, any noise that exceeds the background noise level by
5 dB or more is perceivable and should be considered a
potential disturbance (see table).
An increase of only
10 decibels (dB) translates into sound that's twice as loud.
Any sound more than 5 dB above the background noise level can
be distracting or can interfere with sleep.
One of the more common tasks I perform is acoustically
separating media rooms from bedrooms. I start by calculating
how much sound is being produced by the TV or audio system, and
then I design wall, floor, and ceiling assemblies to handle the
decibel levels being produced.
The same concept would apply if you were building a house on a
busy street. You would start by measuring the average decibel
level produced by traffic noise, and then you'd design a wall
assembly that would reduce the outdoor noise to normal indoor
background levels, typically about 30 dB.
First, though, we need to know how good a wall, floor, or
ceiling is at reducing sound. We use a measurement called the
sound-transmission class — STC — to rate the
effectiveness with which a material or assembly prevents sound
transmission. The STC is a numeric value that quantifies the
sound reduction that occurs when airborne sound passes through
Most standard floors and walls — even with sound
insulation — have STC values so low that common airborne
sounds, such as TV or bathroom noises, are often audible in
For example, if someone is watching TV or a movie, the sound
levels within the room range from 60 dB (quiet conversations)
to 80 dB (explosions or loud music). On the other side of the
wall, the noise level in the bedroom — where someone
might be sleeping — is approximately 30 dB. By doing some
simple math, 80 dB - 30 dB = 50 dB, we know that we need a wall
with an STC of approximately 50 to reduce the TV noise to an
acceptable level in the adjacent room.
With this information and the STC ratings of various wall
configurations, we can determine what type of wall assembly
will work best (see table).
You can't find the STC rating —
which measures how effectively a material prevents sound
transmission — of a wall assembly by adding up the STC
values of individual materials. Applying a layer of drywall
with an STC of 15 to an STC-36 wall assembly, for example,
results in a wall with an STC of about 44 — not
The 80-dB maximum sound in our example is appropriate for the
viewing or listening habits of most people. If you are asked to
build a practice room for your client's rock band, you'll need
to know at what volume level they ordinarily practice.
It's important to remember that STC refers only to airborne
sound. Footsteps and other impact-produced noise that travels
through the home's structure are measured with the impact
insulation class — IIC — ratings. The lesser-known
IIC measurement refers to how well a wall or floor resists this
particular kind of sound transmission.
An IIC rating of 50 is generally considered adequate for most
residential spaces, but a higher rating may be required for
home theaters and other loud areas. Improving the IIC rating of
a wall or floor generally involves adding a soft material to
the surface or uncoupling the wall, floor, or ceiling surface
from the rest of the structure.
We'll talk more about that later, in the sections on acoustic
separation and acoustic isolation.
Basic Acoustic Treatment
Designing a wall assembly that meets the STC requirement, along
with sealing any air leaks that would allow airborne sound to
pass from room to room, is what I call basic acoustic
treatment. In our example involving the bedroom and TV room,
adding 1-pound mass-loaded vinyl to a typical 2x4 wall with
sound-control batts and 5/8-inch drywall on both sides would
meet our STC requirement of 50.
Mass-loaded vinyl is similar in composition to sheet-vinyl
flooring except that it has high-density additives that push
its weight from roughly 1/2 pound to 2 pounds per square foot,
depending on the thickness — which ranges from about 1/16
to 1/4 inch thick. I generally specify 1/8-inch-thick, 1-pound
material; a common 54-inch-by-60-foot roll weighs about 270
pounds. In addition to being used on its own, mass-loaded vinyl
is found in many of the other acoustic control products we use
(see Figure 1).
Figure 1. Mass-loaded
vinyl reduces sound transmission to adjacent spaces better than
multiple drywall layers and resilient channels do. The
54-inch-wide rolls (top left) are installed over the framing
(top right) and lapped 6 inches between studs or joists. Cuts
are made with a hook-blade utility knife (bottom left), and the
material is fastened with a pneumatic roofing nailer (bottom
People often ask me how such a thin material can work better
than multiple drywall layers and other more-traditional
sound-control techniques. Mass-loaded vinyls are effective
because they have high density and they're flexible. When
describing how they work, I find that density is easy to
explain: Dense materials do a better job of stopping sound
waves. Understanding why flexibility is desirable is a little
less intuitive, but I advise people to think back to the
paper-cup-and-string telephones we all made as kids. When the
string was taut, the sound traveled easily from one cup to
another. When the string was limp, the spoken word could not be
heard on the other end.
Because it is both dense and flexible, lead was once a common
material for sound attenuation. However, it's largely been
replaced by mass-loaded vinyl, which is easier to work with and
less expensive. One-pound loaded vinyl generally sells to
contractors for about $2 per square foot. Not all loaded vinyl
products are created equal, so it's important to review the
manufacturer's specifications — especially the rated
transmission loss, which refers to a material's effectiveness
at reducing sound — before installation.
In cases where the acoustic problem includes structure-borne
noise, I typically recommend a higher level of sound control,
which I call recommended acoustic separation. This level of
acoustic control includes all of the measures taken in basic
acoustic control plus an additional emphasis on controlling
structure-borne noise, which refers to sound traveling through
the house framing or structure. When you hear the house shaking
to the beat of a bass drum, you are experiencing low-frequency
So, is adding a mass-loaded vinyl barrier the best acoustic
solution for a media room? The answer is no, since loaded vinyl
mostly addresses airborne sound and has less effect on
If a media room will have a subwoofer (most do), it's important
to prevent the low-frequency sounds of the subwoofer from
traveling through to the framing, which you can do by using
some form of decoupling or a special loaded vinyl. My company
started manufacturing its own loaded-vinyl product for this
application, NoiseOut 2, after being unable to find anything
NoiseOut 2 is similar to 1-pound mass-loaded vinyl, but it can
reduce noise in lower frequencies by roughly 10dB more than
typical 1-pound mass-loaded vinyl can (Figure 2).
Figure 2. To control
low-frequency structure-borne noise, this subwoofer cabinet was
lined with a special version of mass-loaded vinyl called
NoiseOut 2, which is better at reducing low-frequency noise
than conventional loaded vinyl. The material can also be used
with hvac equipment and other low-frequency sound
While more effective than the basic acoustic treatment,
recommended acoustic separation will not completely eliminate
impact-generated, structure-borne sounds like those produced by
If you're trying to eliminate virtually all structure-borne
noise, you'll need to move to the next level of acoustic
treatment, acoustic isolation. Acoustic isolation is the most
effective and expensive method of sound control. It's not
uncommon for one of my residential clients to spend thousands
of dollars acoustically isolating a high-end media room.
In the past, acoustic isolation often meant using the "room in
a room" method, which involved attaching the walls to either
staggered or double studs. But this was labor-intensive, and a
lot of space was sacrificed to accommodate the extra studs and
the air space between them.
In modern construction, acoustic isolation is better achieved
by decoupling the final layer of drywall from the framing.
Depending on the space, I use several methods to accomplish
this, all of which outperform the traditional double-stud
One approach uses spring isolators, which are basically
acoustic shock absorbers. They can be mounted on the floor,
walls, or ceiling; by cushioning the impact of sound waves,
they prevent them from spreading to framing and other rigid
materials like pipe and ducts. Some spring isolators can
support hundreds (or even thousands) of pounds each, so they
can be spaced farther apart than other types of isolation
hardware (Figure 3).
Figure 3. Controlling
structure-borne noise requires acoustic isolation or decoupling
of the finished space from the framing and other rigid
materials. This can be done by using double-stud framing to
build a "room within a room" or with specialty products such as
these spring isolators (top). On the ceiling of this room,
spring isolators support heavy-gauge steel framing (bottom)
that will ultimately be covered with a layer of 3/4-inch
plywood, a layer of mass-loaded vinyl, and then drywall.
Multiple pipes and ducts didn't leave a lot of room for
regularly spaced mounting hardware in this high-end home
theater, so spring isolators — which generally have a
higher load capacity than other types of isolation hardware
— were a good choice.
While spring isolators have their place, I generally prefer
another type of isolator, with rubber pads instead of springs.
These come in wall-mounted and ceiling-mounted versions. We
commonly use the latter, with a drywall suspension system, but
we don't just attach drywall to the suspension system. We also
install a layer of 3/4-inch plywood and mass-loaded vinyl to
the assembly before the drywall layer. The final assembly is
heavy, so the isolators must be spaced accordingly.
The ceiling mount is typically available in 50- and 100-pound
versions. The other model can be wall- or ceiling-mounted and
is made to receive hat-shaped resilient channel. As with the
ceiling assembly, we usually install plywood and a layer of
mass-loaded vinyl under the drywall (Figure 4) when we use this
Figure 4. Another
method for acoustically isolating walls and ceilings uses
isolation clips with rubber mounting pads. The clips reduce
transmission of low-frequency acoustic waves that would
otherwise pass into the structure. They come in several
versions; some receive resilient channel (top and middle left),
and others provide attachment points for a dropped ceiling
(middle right and bottom).
For any type of acoustic treatment to work properly, the wall,
ceiling, and floor assemblies need to be continuous. If there
are any gaps or holes at room corners or around mechanical
penetrations, the sound will pass through and carry to the next
room and beyond. These sound leaks, called flanking paths, can
seriously degrade acoustic performance and reduce the effective
STC rating of a wall to well below the anticipated value.
So before the drywall is installed, we always thoroughly check
the following areas.
Electrical boxes. One of the most common
sound-leakage points is electrical boxes. A single box is bad
enough, but back-to-back outlets in the same stud cavity
— often found in partition walls — are even worse.
Whenever possible, electric boxes should be placed three stud
bays apart. In addition, I always wrap electrical boxes with
mass-loaded vinyl or specify site-built enclosures for them to
prevent sound from escaping (Figure 5). And once the drywall is
hung, I seal around the perimeter of each electrical box with
acoustic or silicone sealant.
Figure 5. Electrical
boxes, a common sound-leakage point, are easy to seal with
site-built enclosures (left) or acoustic barriers like
mass-loaded-vinyl. Here, a worker folds and tapes a precut
piece of mass-loaded vinyl around a single-gang box (right).
After the drywall is hung, each box should be sealed around the
perimeter with acoustic sealant. When possible, outlets on
opposite sides of a wall should be offset by at least three
Piping. Plumbing pipes and electrical
conduit present yet another problem. Since they're solid
materials, they can carry sound quite a distance. To keep pipes
from picking up sound in a media room and carrying it to
another room, and to prevent pipe noise from disturbing
conversations or media enjoyment, I always wrap pipes with
special pipe insulation made from fiberglass with a layer of
loaded vinyl inside (Figure 6).
Figure 6. All piping
— including water-supply and waste lines, gas lines, and
electrical conduits — should be wrapped (top left) and
the joints sealed with silicone (top right). The author prefers
special fiberglass acoustic pipe insulation with a layer of
mass-loaded vinyl inside (bottom). The insulation prevents
noise from traveling between rooms via pipes and minimizes the
sound of running water.
Ductwork. Perhaps the most
troublesome spot in any acoustically treated room is the
heating and air-conditioning ductwork.
Because their smooth, hard interior tends to amplify sound
waves, and because the large openings at either end make it
easy for sound to get in at one room and out into the adjacent
one, ducts are an ideal conduit for sound.
To reduce sound transmission, I wrap the exterior of ducts with
an insulation that's similar to the product I use on pipes. And
I install fire-rated acoustic foam or fiberglass duct liner on
their interior (Figure 7).
Figure 7. Preventing
ducts from spreading sound throughout the house is a two-step
process. First, fire-rated acoustic foam or fiberglass duct
liner is installed on the ducts' interior to deaden any sound
that gets inside (top); next, the ducts' exterior is wrapped
(bottom), which ensures that very little sound will penetrate
in the first place. Both steps are especially important in
basement home theaters, which often have many ducts in the
Built-in speakers. In-wall or in-ceiling
speakers, common in today's home theaters, should always be
backed with an effective, high-density sound barrier. We use
quilted fiberglass specifically designed for this purpose. Not
only does lining the stud cavity behind the speakers increase
the STC of the speaker cavity, but it also prevents
low-frequency impact speaker sounds from transferring to the
structure and traveling to adjacent rooms (Figure 8).
Figure 8. Most dedicated
home theaters have built-in speakers. To contain noise and
low-frequency vibrations, the author often lines the cavity
behind the speakers with a special quilted fiberglass
insula-tion that contains a high-density loaded-vinyl
Doors. Depending on the level of
noise reduction required, the acoustic treatment of doors is
critical. At a minimum, any door connecting an area that
contains a speaker system to another area should have a solid
core with a retractable acoustic door sweep (Figure 9).
Figure 9. Without a
proper door, home theaters can be a source of irritation to
family members trying to study or sleep. Any door separating a
media room from the rest of the house should have a solid core,
an acoustic sweep, and good weather stripping. An acoustically
rated model is preferable.
Recessed lights. Like in-ceiling speakers,
recessed lights allow noises to travel easily between floors. I
use an acoustic muffler above each recessed light in rooms
below acoustically critical rooms — especially in TV
rooms and in kitchens below bedrooms. Acoustic mufflers should
be used only with can lights approved for insulation
Sealing the Holes
Properly sealing joints, junctures, and cracks is an essential
part of acoustically treating a room. Gaps are often covered by
molding, but it's still vital to caulk these invisible
openings; the most troublesome leaks occur at the wall/floor
Acoustic sealants must be durable and flexible enough to
withstand settlement — along with the expansion and
contraction of materials — as the building ages. Acoustic
sealants are excellent because they remain flexible forever;
dense, high-quality silicone-based sealants are even
The importance of sealing cracks can't be overstated. A
1/16-inch-wide by 16-inch-long crack along the bottom plate of
an acoustically treated STC-52 wall effectively reduces its STC
rating to 40.
Finally, there are several additional steps I take to ensure
that mechanical rooms are acoustically isolated from the rest
of the living space. They include using flexible connections
where the ductwork meets the furnace plenum; installing proper
machine-base decouplers below any hvac equipment; and
installing a Class-A fire-rated noise barrier and noise
absorber on the perimeter walls and ceiling.
The wall and ceiling barriers inhibit low-frequency machinery
noises from traveling through the foundation walls and ceiling
into living areas (Figure 10).
Figure 10. With
basements increasingly used for living space, heating and
air-conditioning noise is a growing problem. Adding acoustic
controls is a good idea, but the products installed in
mechanical rooms must be Class-A fire-rated.
Once the installation is finished and all the areas have been
treated, how can one be certain there haven't been any
oversights or mistakes?
Two simple techniques can help guarantee a successful
installation. The first is to shine a bright light on one side
of the high STC-rated wall or floor to see if any light passes
through. This is a good test, because if light can travel
through an acoustic wall or floor, then sound can too, just as
easily. Electrical penetrations are generally the biggest
The second technique, and the only way to test the
effectiveness of acoustic treatments for hvac ducts and
plumbing, is to use a tone generator and either listen for the
noise or measure the decibel level with a special device
Figure 11. Acoustic
treatments should always be tested before drywall is installed.
The author uses a tone generator and graphic sound analyzer to
measure the decibel level at specific frequencies. If the
required STC is not met, she takes additional
This article discussed only the first part of the acoustic
equation: making certain that the sound is controlled or kept
in the space where it was intended to be heard.
The second part of acoustic design is making certain that the
room has the correct reverberation time so the sound is clear
and easily understood. We do this kind of work as well, but it
is a subject for another article.
Bonnie Schnittais the owner of
Sound Sense LLC, a full-service acoustic consulting firm and
manufacturer of specialty sound-control products.