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Sex ISSN 0701-5216
BUILDING
PRACTICE
NOTE
THE SOUNDPROOF BASEMENT
A.C,C. Warnock
.AN*LYZED
Division of Building Research, National Research Council of Canada
O t t a w a , Dkcember 4981
In single-family dwellfngs in Canada, the handyman/homeawner
a f t e n converts p a r t of a bare, unfinished hsemnt into a warm,
comfortable family worn. Frequently, h l s nuin objective is to build a
r o o m where n o i s y activity can be i s d a t e d from the rest of the home.
This Note pravldes guidance for those baneowners who want to soundproof
room in their basemnts*
S e v e r a l books dealing w i t h additions and alterations to homes are
available, This Mote is meant to be a supplemnt t a these book.6 and
discusses only those factors that affect noise transmission.
GENEMIL BACKGROUND
I is not necessary t o be an acoustical expert to construct an
t
t
a c w s tically well-isolated room. I helps, however, to understand the
basic p r i n c i p l e s of acoustics in order to d e a l correctly w i t h situations
not s p e c i f i c a l l y discussed here.
i
To begin with, one mst distinguish between materials used ro
reduce the transmission of souad between room and those used ta absorb
sound i n s i d e a room.
To reduce sound transmfssioa between roow, the walls, floor,
c e i l i n g , and doors must a l l be made f r o m y e r s of s o l i d materials such
-
as gypsum board, wood or concrete. In theory, f o r a single layer o f
s o l i d material, the greater t h e weight per unit area, the less the sound
transmission. The co-ri practice fa North Amrica is to uae
l i g h t w e i g h t w a l k constructed from gypsum board. The sound transmission
through these lightweight walls is greatly reduced by building the walls
as mechanically independent double structures wlch an intervening a i r
apace. Lightweight prefinfshed fibreboard panels, often used as w a l l
f i n i s h e s , are not heavy enough t o reduce sound transmission adequately
on their own and are best appl5ed over gypsum board.
To counteract the build-up of sound due to multiple reflcctiorts
to o b j e c t i o n a b l e l e v e l s inside a room, one uses cound-absorbing
materials such as acoustic t i l e s , carpets, &rapes, soft furnishings, and
ather soft, porous materials. An example af a hfghly e f f e c t i v e sourid-
absorbing material is exposed glass-fibre thermal Insulation. I n
' contrast, s aon-porous mterial such a s polystyrene f a a m , although it
perfornrs well as a thermal insulator, does not absorb mch sound.
Applying sound-absorbing materials such as acoustical tile, cork or
carpet to the surface of a wall, floor, or door does not subsean~iallg
decrease sound transmissiod from one room to another and reduces sound
reflections within the roam only.
In mny ways containing sound i s like containing water. Unless
t h e room to be isolated is almost at r t i g h t (watertight) t h e sound
(water) w i l l "leak" out into adjoifiing spaces. One nust therefore take
great care ta caulk and seal thoroughly all cracks and fissures with
non-hardening acoustical sealant. In practice, I t i s u n d e s i r a b l e t o
achieve complete airtightness since some ventilation is required, hut it
is very important to eliminate all unwanted leaks.
In the following pages the performance of p a r t i t i o n s as noise
barriers i s rated in terns a£ "sound transmission class" (STC). The
higher the STC number, the less sound is transmitted through the wall.
Plosr people, for example, are s a t i s f i e d if t h e STC for walls between
their dwellings is greater than 5 0 ,
TYPES OF WALL AND FLOOR CONSTRUCTION
The sketch in Figure 1Ca) shows the recommended method of
f i n i s h i n g a basement c e i l i n g . The r e s i l i e n t metal channels s h w n in
Figures 1 and 3 make the two layers of the floor or wall mechanically
independent, thereby reducing the transmission of sound between the t w o
layers. A further reduction in sound transmission can be obtained by
increasing the mass of the floor. This can be done by adding layers of
gypsam board or plywood t o the t o p of the floor or t o the underside
between the j o f n t s , whichever is more canventent. This is particularly
impartant where the f l o o r is composed of tongued and grooved boards
s i n c e it has the added benefit of sealing any cracks. Figures 2, 3 and
4 show some recommended w a l l types which use readily available
materials. The position of the glass f i b r e within these structures is
not important. The thickness needs t o be only a b u t 75 um ( 3 i . . c n)
All a£ the constructions presented in this Note use two s o l i d
layers of material not rigidly linked (as they would be i f nailed to
wood studs connecting the two layers, for example) with the cavity
between the layers filled with sound-absorbing material. Although this
type of construction i s not always superior t o others, it does have the
advantage of u s i n g lightweight materials and simple construction
techniques.
PROV IDIMG SERVICES
Containing eound becomes more difficult when one m s t s u p p l y
e l e c t r i c a l . pdwer and l i g h t , .air f a r heating and ventilating, and doors
to provide access t o a room. A l l these additions cause pcnet.ration of
t h e walls or c e i l i n g and therefore p o t e n t i a l sound leaks. However, it
is possib L to provide these services to a room wi thdut seriously
e
i n c r e a s i n g sound transmission,
ELECTRICAL OUTLETS.
When installing power outlets in a sound-isolating wall, the-
h o l e s around the boxes should be plugged as mch as p o s s i b l e using
caulking or plaster. The power o u t l e t s ofi each s i d e of the wall s h o u l d
be o f f s e t by about 0.5 m (19 in.) so that they are not back t o back.
If surface-mounted ceiling light fixtures are to be installed,
t o caulk a l l t h e holes and thus avoid
then i t i s relatively simple
leaks. Often, however, lack of headroom in a basement requires recessed
l i g h t i n g fixtures. In t h i s case, a box can be constructed around t h e
fixture so that an unbroken ceiling layer is preserved (Figure 5 . ) An
easier approach is to apply the sheets of s o l i d material d i r e c t l y to the
rear surfaces of the fixture; once again, all residual gaps should be
caulked, The fixtures used must, of course, be rated for t h i s kfnd of
use so that no p o t e n t i a l £Ire hazard is created. Sound leaks can be
avofded by mounting the l i g h t s on a wall surface that is n o t an
important sound barrier-
A I R DUCTS
In a house with blown-air central heating, the a i r ducts usually
present a major problem in reducing sousd transmission. Sound travels
very easily into and along air ducts and, unless care is taken, this can
ruin an otherwise acceptable acoustical construction. To begin w i t h ,
all air-duct surfaces should be enclosed by the c e i l i n g or w a l l s or
s p e c i a l l y encased. This prevents sound entering or escaping from the
duct through the mtal walls and travelling along it from ode room t o
another.
Sound can s t i l l enter the ducts through the air outlets and
travel along them. Using acoustical duct l i n e r will reduce t h i s
transmission of sound. Figure 6 shows how glass-fibre duct liner is
I n s t a l l e d on the fnside of a duct with a rectangular cross-section.
This g l a s s - f i b x e material is t y p i c a l l y about 25 mrn thick and in the form
of f l e x i b l e or semi-rigid boards that can be cut t o size, then glued
inside the duct. The cut edges are usually s e a l e d with rubber cement.
Round, lined d u c t s are also available.
Any duct with an outlet in the room to be soundproofed must be
treated in t h i s way, I m y also be necessary t o e x t e n d t h e treatment
t
i n t o the main plenum. There is na simple rule to determine how much
treatment is necessary, as this depends strongly on the duet dimensions
and the number of bends and junctions. However, as a rule of thumb, a
m i n i m m length of 3 m of duct liner should be inserted in each d u c t
entering the room. Adding sound-absorbing duct liner will, of course,
make the duct passages smaller and impede a i r f l o w to so= extent;
consequently, the duct may need t o be replaced by a larger one*
DOORS
L a s t l y , sound isolation may be decreased by the doors leading
into the room. The typical door used in hous.es is a l i g h t hollow-core
structure, w h i c h does little to prevent the passage of sound, especially
when there are large gaps around the edges of the door. The STC is
usually in the range of 10 to 15.
Commercially-produced acoustical doors are a v a i l a b l e but t h e
average homeowner may consider these to be coo expensive, An
a l t e r n a t i v e is to use a less c o s t l y , solid-core wood door and t a ensure
that the frame is well sealed by gaskets around all the edges. Even
then, t h i s type of door has an STC of only about 28. Its performance
will not equal that a£ the other structures suggested in t h i s Note
unless there is so- buffer space, such as a hallway with another s o l i d
door in it between rhe soundproofed room and the other areas. If there
is no buffer space, the door alone w i l l control the l e v e l of sound
entering or escaping £ram t h e basement. Clearly, it is important to
consider t h e layout of the house before any work i s done; other doors i n
other room map have to be replaced. If the bornowner considers the
effort worthwhile, then t w o independently hung, well-sealed s o l i d doors
in the same £ram can be used. Although t h i s could be inconvenient, it
m y be necessary in some critical cases.
a
AL'PLIAWCE NOISE
The noise from electrical appliances such as washers, dryers and
furnaces can be reduced by following the p r i n c i p l e s already outlined.
The walls of the room where the appliance is located should be
constructed as suggested in Figures 2, 3 and 4 . To reduce t h e Level of
sound in the room, sound-absorbing materials can be added t o the c e i l i n g
and p o s s i b l y a l s o to the w a l l s , Ductwork passing through the room
should be enclosed, and ducts entering the room should be acoustically
lined as shown in Figure 6 .
GENERAL CONSIDERATIONS
It is important to realize that a well-soundproofed room is o f t e a
alnmst airtight. This might create d i f f i c u l t f e s when no forced-air
heating or ventilation is used, In some cases it might be necessary to
p r o v i d e extra ventilation. Lack of ventilation could also be a problem
in a tightly sealed furnace rbom. A supply of air is necessary for
combustion, and once again ic may be necessary to provide ventilation,
If ducting is used to furnish the required air, then the p o s s i b f l i t y of
sound transmfssion v t a the ducts should be considered,
Before beginning alterations, one m s make sure that the
ut
existing waf 1s and floors are properly prepared. Any holes in t h e f l o o r
structure should be f113.ed with caulking, or with solid material i f the
h o l e is big. h e &thad for dealing with tongued and groaved
floorboards has been mentioned earlfer.
The importance of goad sealing and caulking techniques cannot be
overemphasized. Figure 7 shows some locations where caulking should be
added to ensure effective soundpranfing, It is most impartant thar the
final layer of gypsum board be well sealed around all the edges.
The competent handyman dl1 plan carefully, taktng acoustics and
o t h e r f a c t o r s i n t o account, before starting the a c t u a l work; it is
always =re difficult, irritating, and expensive to correct errors after
the work has been completed.
GLASS FIBRE BATT
INSULATION ABOUT
75 mm THICK
RESILIENT 1 O 2 LAYERS
R
CHANNELS ENLARGED OF GYPSUM BOARD
IN FIG. 1 (b)
FIGURE l ( a )
RECOMMENDED F I N I S H F O R A B A S E M E N T CEl L I N G
STC = , 5 0
ATTACH
GYPSUM BOARD
WITH SCREWS
FIGURE l ( b )
RE51 L l E N T C H A N N E L : USU.ALLY A P P L I E D
4.0 crn ( 1 4 i n . ) O N C E N T R E S A N D AT
R I G H T A N G L E S T O S T U D S OR J O I - S T 5
a) ONE MWR EACH SIDE 4 GUTS
/FIBRE BATT INSULATION IN CAVITY
STC = 46
T O LAYERS
W + ONE LAYER + GFASS
N
FIBRE BATT INSULATION 1 CAVITY
5TC - 50
SOUND T R A N S M I S S I ~ N CLASS F R
~ STAGGERED WOOD
STUD W A L L S W l f H D I F F E R E N T SURFACE A N D C A V I T Y
TREATMENTS
WALLBOARD
a) ONE LAYFR EACH SIDE + GLASS
F W BAYT INSULATION IN CAVITY
1
STC = 4d
GLASS FIRR
BATrS
RE51 LlENT CHAlJNELS
b) TWO LAYERS -, ONE LAYER + GLASS
FIBRE BATT INSULATION 1N CAVlTY
STC = 50
FIGULE 3
S O U N D TRANSMISSIQN CLASS FOR W A L L S W I T H
W O O D STUDS A N D R E S I L I E N T M E T A L FURRING S T R I P S
METAL STUD
a) ONE LAYER EACH SIDE + GLASS
GYPSUM
' BATT INSULATION
FIBRE IN CAVITY
WALLBOARD
STC = 46
GLASS FIBRE
BATTS
b) TWO LAYERS + ONE LAYER + GLASS
'FIBRE BATT INSULATION IN CAVITY
STC = 5 )
1
FIGURE 4
S O U N D T R A N S M I S S I O N CLASS F O R 9 2 m m (3-5/8 in.)
M E T A L STUD W A L L S
GLASS FIBRE BATT INSULATION
CHANNELS
FIGURE 5
A M E T H O D OF D E A L I N G W I T H R E C E S S E D L I G H T I N G
F I X T U R E USING A B O X
INTERNAL L I N I N G -
R E I D GLASS
FlBRE I NSULATI ON
FIGURE 6
I N T E R N A L DUCT L I N E R TO REDUCE SOUND
TRANSMISSION A L O N G DUCTS
HEADER PLATE
GLASS FIBRE
BATT I N S U L A T I O N
CHANNEL
GYPSUM BOARD
GYPSUM BOARD
(a) CAULKING AT EDGE OF C E I L I N G
GYPSUM BOARD
SOLE PLATE
FLOOR
@) CAULKING AT BASE O WALL
F
FIGURE 7
E X A M P L E S OF C A U L K I N G