sound_isolation and noise control in home theatres

© 2018 Taylor & Francis
Sound Isolation and Noise
Control in Home Theaters
by
Floyd E. Toole
This is a supplement to Sound Reproduction, 3rd Edition, Focal Press, 2018

Home theaters vs. listening rooms
• Quality listening experiences in multichannel
movies or stereo music have the same
fundamental needs in terms of sound isolation
and noise control.
• The cost and difficulty of achieving high levels of
isolation and low levels of background noise are
reasons for serious thought before embarking on
a project.
• If you are an installer/contractor you need to
discuss this with the customer, determining what
is and is not likely to be acceptable. Some things
cannot be changed after the fact.

Sound travels both ways: in and out
• Noises from outdoors or other parts of the house
are distractions while listening to music or
watching movies or TV.
• Constant background hums, rumbles and hisses
from HVAC are annoying.
• Rumbles, booms and crashes emanating from the
theater can be disruptive to someone elsewhere
in the house who is trying to work, relax or sleep.
• The main problem: The bass sounds that are so
impressive in movies and music are very difficult
to isolate.

Sound isolation is a hidden virtue
• Sound isolation in a home theater or listening
room needs to be designed in from the
beginning; retrofitting is much more difficult and
often less satisfactory.
• If it is well done, nobody is likely to notice − a
good thing!
• If it is inadequate, listeners are repeatedly
reminded of the fact. Clattering dishes in the
kitchen, toilet flushes, door slams, and screaming
children at play can break the carefully crafted
“spell” of music and movies. That is not good!

Measurements vs. perceptions
• Sound level measurements are related to human
reaction to sounds, but it is a very imperfect
correlation. One reason is that we react to more
than simple loudness, especially if the sound has
information content. Annoyance can be very
personal and situation-dependent. For example:
• Voices or some kinds of music when you are trying to
concentrate. Merely hearing these sounds faintly in the
background can be distracting to some people.
• Sounds of violence, as in many movies, are more
distressing than neutral sounds at the same sound level;
loud bass sounds can be particularly bothersome to
some.

Limitations of measurements
• Given the complexity of sounds in movies and
music, it is no surprise that a single-number
rating is imperfect, even for simple loudness.
• Nevertheless, single-number ratings exist for
sound levels, sound absorption and sound
transmission loss. Their usefulness is limited
because of their simplicity, and because some of
them were created only to address matters
related to speech intelligibility and privacy.
• The frequency range of speech does not include
the abundant, easily transmitted, and annoying
low bass sounds in movies and music.

Factors in home theater location
Most situations offer few or no options for the
location of the listening room, but if there is a
choice, here are some factors to consider:
• Door closing thumps, garage door rumbles, toilet flushes
that travel through frame construction,
• HVAC rumbles (fan vibration and combustion noise) and
hisses (turbulence at the vents),
• TV and games in nearby rooms, footfalls upstairs,
• Traffic, aircraft, etc.

• Identify those areas of the home that need to be
protected from sounds leaking from the theater:
• bedrooms,
• study and office/working areas.
• Can the theater space be isolated by sound
attenuating walls, ceiling and floor, doors, etc.?
• If not, can a better location for the theater be
found?
• If not, is the customer willing to accept the
noise?

• Noises that interfere with enjoying movies and
music:
• sources within the theater space—cooling fans
on the projector and other equipment, hard
drives, transformer hum, HVAC vent noise, etc.
• sources outside the theater space—traffic,
aircraft flyovers, household noises such as
toilets, kitchen appliances, vacuums, furnace
fan and combustion noise, garage doors, etc.

• Beware of flanking paths—sound that leaks in or
out through acoustical or mechanical
connections:
• HVAC ducts, plumbing and electrical services
• Structural coupling to sources of vibration,
e.g. garage door openers, roof- or attic-
mounted AC, furnace fan, drainage and sewage
lift pumps, etc.
• Sound communicated through a shared attic,
basement or crawl space.

Access to the theater, toilet, refreshments, etc.
• Is there a convenient
gathering/talking/socializing space close by? Is
one necessary?
• Is there a need for handicapped access?
• Any stairs or ramps need to be lighted, and
elderly or unsteady people will benefit from hand
rails—the interior decorator will simply have to
consider it an additional challenge!

Light
• In multipurpose rooms, are there light sources
that will wash out a projected image or be
reflected from the glass of a direct view display?
• Can they be controlled with moveable or motorized
curtains, shades or screens?
• Can the display be positioned to minimize the effect?
Projection screens with gain discriminate against light
arriving from the sides.
• In dedicated theaters, will an open door spill light
on the screen?

Windows
• From the perspective of interior room acoustics,
double-glazed windows are very similar to a single
layer of drywall.
• If windows are to be preserved, sound attenuation can
be much improved by the addition of a second
window, physically separated by several inches.
Special products exist with damped multiple
laminations.
• If they are to be closed off, use an opaque coating or
closed shutters to preserve a good outside appearance
and cover the interior surface with a wall.

We need some acoustical terminology
1. To describe background sound levels that are
acceptable for the activities in the room.
2. To describe the amount by which sounds are
reduced in travelling through various materials,
wall, ceiling and floor constructions.
3. To describe the proportion of incident sound
that is absorbed by acoustical materials.

What happens when sound hits a wall?
• Some of the sound is
reflected.
• Some of the sound is
absorbed by the
surface or what is on
the surface.
• Some of the sound
travels through the
wall to the adjacent
space.

Sound absorption
• The absorption
coefficient describes the
percentage of sound
energy that is reflected
back into the room.
• 100% absorption
suggests that nothing
should be reflected back
into the room.
Problem: this
specification assumes a
diffuse sound field. In
small listening rooms
the sound field is not
diffuse. The spec is
misleading.

Sound transmission loss
• Transmission loss
defines the amount of
attenuation experienced
by sound as it passes
through a room
boundary.
• The amount of loss is
very frequency
dependent. In home
theaters attenuation of
bass frequencies is very
important—and difficult
to achieve.

Sound absorption vs. transmission loss
• Materials that provide effective absorption often
provide little or no sound transmission loss.
• They are very different mechanisms.
• The acoustical performances of materials and
structural methods used in home theater
construction need to be specified as a function
of frequency so that we can absorb or isolate
the right sounds.
• At present, there is no single-number rating
that is reliably useful for our purposes.

Sound absorption specifications:
 Sound absorption coefficient—measured at
several frequencies in a diffuse sound
field—useful in large reverberant spaces,
but less reliable in small acoustically well-
damped rooms. Directional absorption data
is needed to address individual reflections.
 Noise Reduction Coefficient (NRC)—a single
number applying to speech frequencies
only—use only as a rough guide

Sound attenuation specifications
Sound attenuation through floors and walls:
 Impact Insulation Class (IIC)—floors only—a
measure of impact noise transmission from the
floor above. Useful for “high heels on
hardwood,” but useless for anything else.
 Sound Transmission Loss—real
measurements at many frequencies—this
is the “truth.”
 Sound Transmission Class (STC)—speech
frequencies only—use only as a rough guide!
 Weighted sound reduction index (Rw)—speech
frequencies only—use only as a rough guide!

Background noise specification
 Noise Criterion curves (NC)—the “traditional”
measure created for evaluation of speech
interference.
 There is no evaluation of the quality of the
background noise. A background noise that
perfectly fits an NC curve will sound boomy
and hissy.
 The “adjacency” rating method allows
background noises that can be irritating.
Some judgment is required to ensure that
there are no prominent spectral peaks.

The new NC contours
NC-70
NC-65
NC-60
NC-55
NC-50
NC-45
NC-40
NC-35
NC-30
NC-25
NC-20
NC-15
Sound
Pressure
Level
(dB)
Frequency (Hz)
90
100
80
70
60
50
40
30
20
10
0
16 31.5 63 125 250 500 1k 2k 4k 8k 16k Figure 4.8 in Sound Reproduction, 3rd ed. 2018
Reprinted from ANSI/ASA S12.2:2008 American National Standard on
Criteria for Evaluating Room Noise, © 2008, with the permission of the
Acoustical Society of America, 1305 Walt Whitman Road, Suite 300,
Melville, NY 11747.
The new NC contours
showing a measured
spectrum registering NC-51
according to the tangency
criterion. The NC rating of a
spectrum is designated as
the value of the highest NC
curve “touched” by the
measured octave-band
spectrum.

Some examples of NC ratings:
• Recording & broadcast studios:
• distant microphone pickup NC-5 to NC-10
• close microphone pickup NC-15 to NC-25
• Concert halls and other live performance spaces NC-15 to NC-20
• Home theaters, suburban homes NC-15 to NC-25
• Movie theaters NC-30 to NC 35
• Urban residences and apartments NC-30 to NC-40

A practical example

Observations:

More observations:

How much sound transmission loss is
needed for different room boundaries?
• Standard construction (STC=34): OK for non-critical
situations. Examples: garage, storage room, hallway.
• Level 1 (STC=52): noise outside HT is moderate and/or
the adjacent space is moderately noise sensitive.
Examples: kitchen, casual dining area.
• Level 2 (STC=60): significant noise outside HT and/or
the adjacent space is significantly noise sensitive.
Examples: living room, dining room, office, library.
• Level 3 (STC=70): high noise levels outside HT and/or
the adjacent space is very noise sensitive. Examples:
bedrooms, noisy areas like gaming room, playroom, and
outdoor noises from heavy traffic, aircraft flyovers, trains,
etc.

The idealized acoustical goal

Being practical:

More details

Understanding sound transmission
through walls

Increasing transmission loss:

Fundamentals of sound isolation
Poor at all
frequencies
Better at
mid & high
frequencies.
Small air
volume is
stiff at low
frequencies
though.
Slightly
better, but
the
mechanical
connection
dominates
Even better at
mid & high
frequencies.
Some isolation
at low
frequencies
now.
Little change
at mid & high
frequencies.
More isolation
at low
frequencies.
Fibrous material: fiberglass, mineral wool, etc.

• Adding more layers of drywall (mass) improves
everything.
• Adding damping (visco-elastic compound, sheets or
pads) between the layers, or using prefabricated
multilayer products improves things even more,
including low frequencies, which is good.

No caulking:
STC 14
Caulking only under
floor plate:
STC 30
Caulking at edges of
floor plate and at
edges of gypsum
board:
STC 50
Caulking eliminates a sound leak

Soft pads eliminate a sound leak
e.g. ISO-SILL™ from acoustiguard.com or GenieMat FIS® from pliteq.com

Practical Problems with Retrofits:
• When adding a theater to an existing
building, there is much less flexibility:
• Existing walls usually must stay.
• During construction new interior walls may only be
accessible from one side only.
• There may be space or weight limitations. E.g.
adding thickness to a wall outside a theater may
violate a corridor width regulation. Many layers of
drywall may exceed the load rating for building
structure—seek advice from a structural engineer.
• Flanking paths often cannot be treated in the most
effective ways.

Techniques for retrofits or where space
is limited
Plan
view
Elevation
view
Multiple layers of gypsum board mounted on
Resilient Channel or Resilient Sound Isolation
Clips (RSICs) plus fiberglass make huge
mid/high frequency improvements. Resilient
mounting also reduces structure-borne
flanking path leakage.
Adding more damping compound or sheets
between layers can be beneficial.
Poor at all
frequencies

Resilient Channel-1 RC-1
Plan
view
Elevation
view
Intended for ceilings or walls
Caution: drywall screws
must NOT contact the
studs, short-circuiting
the mechanical isolation!

Resilient Channel-2 RC-2
Intended for ceilings
Caution: drywall screws
must NOT contact the
studs, short-circuiting
the mechanical isolation!

Resilient sound isolation clips
Resilient
sound
isolation
clip RSIC-
1
Drywall furring
“hat” channel,
or “hat track”
Wall Ceiling
Illustrations from: www.pac-intel.com
Supports up to 2 layers of 5/8” drywall
for substantial gains in sound isolation

Resilient Sound Isolation Clips
RSIC-1-SI (spring isolation)
Illustrations from: www.pac-intel.com

Sound transmission losses of some
real walls

How much transmission loss do we need?
Assume that the sound
source is pop music at
an average listening
level of about 85 dB—a
good “foreground”
listening level. These
are the sound levels
that would appear on
the other side of various
walls.
• The standard wall
fails.
• A Level 1 wall is OK,
except at bass
frequencies.

Crescendo sound levels are problems!
If the sound inside the
HT is music or a movie
at peak crescendo level
of about 105 dB:
• Even a Level 3 wall
will have problems with
bass, but all others are
in serious trouble.
•Lesson: even elaborate
walls cannot isolate all
sounds.

Sound leakage and structure-borne
sound
• Sound propagates through structures in parallel—
flanking—paths compromising the performance of
even the best walls.
• In some instances it can be true that an
overdesigned wall is a waste of space and money.
• A total design must consider all possible paths
sound can take on its way from the source to the
listener’s ears.

Flanking paths
This is why ground
floors and
basements are
attractive locations
for noisy home
theaters!
And, why the most
expensive and
acoustically
effective wall, floor
or ceiling may not
completely solve a
problem.

Flanking paths reduce effectiveness of walls
Different
construction styles
of the floor
connecting two
spaces makes a
difference.

All of this and more is in:
The National Research Council of Canada
“Guide for Sound Insulation in Wood Frame
Construction,” by Quirt, Nightingale and King,
2006. Publication RR219
Available at no cost at:
http://nparc.cisti-icist.nrc-
cnrc.gc.ca/eng/search/?q=rr219&m=1

Concrete Floor on Grade - Simple
The risk? That the weight of the walls
will cause the floor to sink and break.

Concrete Floor on Grade—Better
This may require removing part of the floor
and splicing in new concrete with footings

Some sample walls
• It is important to remember that in all of the
following examples significant variations can
occur, depending on the details of materials and
construction.
• Note also that equivalent acoustical performance
is possible using a number of different designs.
• NO allowance for flanking sound has been made!

Single-stud walls
STC 34
STC 57
Manufactured laminated
internally damped panels

Walls used in
some of the
examples
Manufactured laminated
internally damped panels

Other variations

Concrete options

Some retrofit ideas
Level 2 example
for +

Room-within-a-room options:
A basic add-on interior wall is good enough
for many situations.

Simple ways to push the acoustical isolation of
a retrofit wall to a much higher level.

A serious assault on sound isolation in a retrofit situation. The
resilient mounting adds wall isolation and attacks flanking path
vibration.

Ceilings

Floors
There are two considerations
• Preventing sound from escaping to a lower floor.
• Allowing the floor surface to vibrate in response
to the bass, giving listeners a tactile sense of
bass energy. In this case the floor must be very
well mechanically damped to avoid embarrassing
“one note” vibrations.

Floors, continued
• For maximum isolation use a poured concrete slab
for the floating floor.
• Alternative: layers of gypsum board sandwiched
between ¾” plywood sheets, joints staggered.
• If floor vibrations are desired for aesthetic effect,
add a decorative floor on compliant sleepers (e.g.
strips of rigid fiberglass board or iso mounts) above
the floating floor.

Floors, continued
• The kind and distribution density of the isolation
mounts depends on the total mass they must support.
Consult a manufacturer (e.g. www.kineticsnoise.com).
• If sound isolation is not an issue—e.g. the structural
floor is a ground level concrete slab—and some bass
“feel” in the floor is desired, just use the decorative
floor.

Putting it all together ...
• Individual walls, the floor and ceiling can only be
effective if they are carefully mated at the corner
junctions.
• Interior and exterior wall structures must be kept
mechanically separate from each other, cracks
must be filled with acoustical (i.e. non-hardening)
caulk, or strips of compliant material.
• This requires a knowledgeable builder and/or very
careful supervision.

Doors and windows
• These are usually the “weak links” in any well-designed
home theater.
• Acoustical upgrades to conventional solid-core doors
(added mass, weatherstripping, drop seals) improve
things. Inner and outer doors are a good idea.
• Sound rated doors are very effective, but expensive.
• Eliminate windows if possible. But there are ways to
upgrade the acoustical performance when they must
remain.

Improving the transmission loss of windows
Data from:
www.soundproofwindows.com
Original single or double pane: STC 26-32
With spaced sound-attenuating glass added: STC 48-54

How soundproof doors work
Sold as a pre-hung
system.
Demanding circumstances
may require and inner and
outer doors in separated
frames or, better,
separated by a small
chamber or room.
It is possible to create
functional do-it-yourself
versions, but acoustical
performance is not
assured.

Commercial sound-rated doors
• There are numerous manufacturers of custom
doors accessible through the Internet—e.g.
search “soundproof doors”.
• There are also several guides for do-it-yourself
improvements to readily available doors and
companies selling some of the components shown
in the previous slide.
• Always check carefully for properly conducted
sound attenuation measurements.

Isolating projector noise
Ceiling
Remotely located
quiet (centrifugal) fan
¾” plywood box
with 2” fiberglass
Ensure that air flow
is in the same direction
as that of the projector

Reducing HVAC noise in ducts
Force air through 90º or 180º turns
in a duct lined with not less than 2”
(50 mm) fiberglass duct liner (coated
to prevent fiber loss).
Manufactured duct silencers are
available.
These devices and long runs of lined
duct are useful to isolate middle and
high frequency sounds. Bass is not
effectively isolated so a dedicated
HVAC system for the home theater is
recommended.

Reducing HVAC noise at the vents
Calculate the air flow necessary to support
human life, and to remove heat generated in
the HT. Use industry charts to determine
appropriate duct size and diffuser—keeping the
face velocity low to prevent turbulence.
The ASHRAE handbook, for example, has such
information.

Quieting garage door openers
Vibration isolating the opener motor . . . . . . and the track
So that sound is not communicated by the house frame.
Illustrations from: www.pac-intl.com

A simple A/C isolation
Outdoor
A/C unit
Rubber puck or block
Concrete slab on grade
When the supporting surface
or structure weighs much
more than the vibrating
unit, isolation is simple.
Energy that is transferred to
the structure generates very
little motion, and little or no
noise.

Complicated rubber “pucks”
These are much
more effective than
the traditional solid
rubber pucks and
blocks. They are
designed to support
specific load ranges,
so be sure to pick
the right one for
your needs.
From: www.mason-industries.com

A more complicated situation:
Outdoor
A/C unit
Spring with the right amount of stiffness
for the supported mass. The
manufacturer of the isolator has this
information
Typical roof construction on a frame building
When the supporting surface or
structure weighs less than the
vibrating unit, isolation is more
difficult. The optimum amount
of compliance (springiness) is
necessary to minimize vibration
transfer. It is a more
complicated “tuned” system.

Attic installations
Indoor
A/C/furnace/fan
unit
Another situation in
which the supporting
surface or structure
weighs less than the
vibrating unit.
Maximum isolation
requires that the
springs be matched
to the load being
supported

Sophisticated hangers
Rubber bushings reduce
high frequency vibrations
while springs take care of
low-frequency rumbles
and shakes. Units are
designed to tolerate a
certain amount of
movement, but seismic
restraints are necessary.
Be sure that they do not
couple vibrations to the
structure of the house.
Available for specific load
ranges, pick the right
one!
From: www.mason-industries.com

Recommended reading
An excellent comprehensive textbook on room acoustics
and noise control:
Architectural Acoustics, Marshall Long,
Elsevier Academic Press, 2006.
A good introductory book:
Master Handbook of Acoustics, 5th Edition, Everest and
Pohlmann, McGraw-Hill, 2009.

Recommended reading
Sound Transmission Loss measurements on many wall and floor
constructions, and papers on fundamental research can be found at:
http://nparc.cisti-icist.nrc-
cnrc.gc.ca/eng/search/?q=sound+transmission+loss&s=sc&ps=25&m=
1
The following are more specific:
http://nparc.cisti-icist.nrc-cnrc.gc.ca/eng/view/fulltext/?id=57dbff41-
7b93-4621-bd17-ae2c6ab6c7e0
http://nparc.cisti-icist.nrc-cnrc.gc.ca/eng/view/object/?id=04ac8069-
a5d2-4038-8787-da064b073e7f
No cost.

Recommended reading—Internet
Searches on specific topics yield many sources of
information.
Good information on vibration and sound isolation
products and techniques at:
www.mason-ind.com
www.pac-intl.com
www.kineticsnoise.com
www.asc-soundproof.com
And others ...

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