The soundproof basement

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Publisher’s version / Version de l'éditeur: Building Practice Note, 1981-12-01

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The soundproof basement

Warnock, A. C. C.

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ISSN

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BUILDING

PRACTICE

NOTE

THE SOUNDPROOF

BASEMENT

A.C,C.

Warnock

**.AN *LYZED**

Division

of Building

Research, National

Research Council of Canada

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In single-family dwellfngs in Canada, the handyman/homeawner

a f t e n converts p a r t of a bare, unfinished h s e m n t into a warm,

comfortable family worn. Frequently,

h l s

nuin o b j e c t i v e

is

to build a r o o m where n o i s y a c t i v i t y 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 s u p p l e m n t t a these book.6

and

discusses only those factors that

affect

noise transmission. GENEMIL BACKGROUND

It

is

not necessary t o be an acoustical expert to construct an

a c w s tically w e l l - i s o l a t e d

room.

It

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 t h e transmission of souad between room and those used ta absorb sound i n s i d e a room.

To

reduce sound transmfssioa between r o o w , 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

o f s o l i d materials such

as gypsum board, wood or concrete. In theory,

f o r

_{a single layer}of s o l i d material, the greater t h e weight per unit area, the

l e s s

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 n o t 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

m a t e r i a l s such as acoustic t i l e s , carpets, &rapes, soft f u r n i s h i n g s , 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 m t e r i a l such a s polystyrene f a a m , although it

perfornrs

well

as a thermal insulator, does not absorb mch sound. A p p l y i n g 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.

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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 a i r t i g h t n e s s s i n c e some v e n t i l a t i o n 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 n o i s e 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 i n d e p e n d e n t , 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 f l o o r . 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 p a r t i c u l a r l y 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 p o s i t i o n 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 ucm ( 3 in.).

All

a£ the constructions

presented

in this

Note use two s o l i d layers of material n o t r i g i d l y linked (as they would be i f nailed to

wood studs connecting the two l a y e r s , for example) with t h e 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 t h e 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 p r o v i d e 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 Le to p r o v i d e these services to a room wi thdut s e r i o u s l y 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.

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If surface-mounted ceiling light fixtures are to be installed, then i t i s relatively simple t o caulk a l l t h e h o l e s and thus avoid

l e a k s . Often, however, lack of headroom in a basement requires recessed l i g h t i n g f i x t u r e s .

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 f o r 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, t h i s 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

t r a v e l 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 t h e form of f l e x i b l e or semi-rigid boards that

can

b e cut t o size, then glued i n s i d e 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,

It

m y also be necessary t o e x t e n d t h e treatment

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 t h e 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 t h e s e to be coo expensive, An

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that the frame is well s e a l e d 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 t h a t 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 e f f o r t

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 may be necessary in some c r i t i c a l cases.

AL'PLIAWCE

NOISE

The n o i s e 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 a i r t i g h t . 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 a i r is necessary for combustion, and once again

ic

may be necessary to provide ventilation, If ducting is used to

furnish

the required air, then t h e 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 must make sure that t h e

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 c a u l k i n g 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 p l a n 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 a f t e r the work has been completed.

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GLASS FIBRE BATT INSULATION ABOUT 75 mm THICK

RESILIENT ₁_{OR 2} _LAYERS

CHANNELS ENLARGED OF GYPSUM BOARD

IN

FIG. 1 (b) F I G U R E l ( a ) R E C O M M E N D E D F I N I S H F O R A B A S E M E N T C E l L I N G S T C = , 5 0

ATTACH

GYPSUM BOARD WITH SCREWS F I G U R E 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 A T 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

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a) ONE MWR EACH SIDE 4 GUTS

/FIBRE BATT INSULATION IN CAVITY STC = 46

TWO LAYERS + ONE LAYER + GFASS

FIBRE BATT INSULATION 1N CAVITY 5TC - 50

S O U N D T R A N S M I S S I ~ N C L A S S F ~ S T A G G E R E D R W O O D

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 T R E A T M E N T S

WALLBOARD

a) ONE LAYFR EACH SIDE + GLASS F1W BAYT INSULATION IN CAVITY STC = 4d

GLASS FIRR

BATrS

RE51 LlENT CHAlJNELS

b) TWO LAYERS -, ONE LAYER + GLASS FIBRE BATT INSULATION 1N CAVlTY STC = 50

F I G U L E 3

S O U N D TRANSMISSIQN CLASS FOR W A L L S W I T H W O O D S T U D S A N D R E S I L I E N T M E T A L F U R R I N G S T R I P S

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METAL

STUD

GYPSUM

WALLBOARD

GLASS

FIBRE

BATTS

FIGURE

4

a)

ONE

LAYER EACH SIDE

+ GLASS

'

FIBRE BATT

INSULATION IN CAVITY

STC

=

46 b)

TWO LAYERS

+

ONE

LAYER

+

GLASS

'FIBRE

BATT

INSULATION IN CAVITY

STC

=

51)

S O U N D

T R A N S M I S S I O N

C L A S S

F O R

9 2

_{m m (3-5/8}

_{i n . )}

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GLASS FIBRE BATT INSULATION CHANNELS F I G U R E 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 U S I N G A B O X INTERNAL L I N I N G

-

R E I D GLASS Fl BRE I NSU LATI ON

FIGURE 6

I N T E R N A L D U C T L I N E R TO REDUCE SOUND

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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 OF WALL FIGURE