Controlling Sound Transmission through Concrete Block Walls

C o n s t r u c t i o n Te c h n o l o g y U p d a t e N o . 1 3
Controlling Sound
Transmission through
Concrete Block Walls
by A.C.C. Warnock
This Update discusses the various factors that affect sound transmission
through different types of concrete block walls, including single-leaf walls,
double-leaf walls and walls with gypsum board attached. Knowledge of
these factors will assist construction practitioners to design and build walls
with high levels of acoustic performance economically.
Concrete block walls are commonly used
to separate dwelling units from each other
and to enclose mechanical equipment
rooms in both residential and office buildings because of their inherent mass and
stiffness. The bending stiffness of the material making up the wall is also important.
Neither of these fundamental properties
(mass and stiffness) can be altered by users.
Some Basic Concepts —
Transmission Loss (TL) and
Sound Transmission Class (STC)
For significant noise reduction between
two rooms, the wall (or floor) separating
them must transmit only a small fraction of
the sound energy that strikes it. The ratio
of the sound energy striking the wall to
the transmitted sound energy, expressed
in decibels (dB), is called the transmission
loss (TL). The less sound energy transmitted, the higher the transmission loss.
In other words, the greater the TL, the better
the wall is at reducing noise.
Sound transmission class (STC) is a singlenumber rating that summarizes transmission loss data. It is obtained by fitting a
standard reference contour to the data (see
Figures 2 and 3).
However, there are additional factors that
need to be considered in building highquality walls.
Single-Leaf Concrete Block Walls
Mass per Unit Area
For single-leaf walls, the most important
determinant of sound transmission class
(STC) is the mass per unit area: the higher
it is, the better. A single-leaf concrete block
wall is heavy enough to provide STC ratings
of about 45 to 55 when the block surface is
sealed with paint or plaster.
Figure 1 shows measured STC ratings
for single-leaf concrete block walls from a
number of published sources. The considerable scatter demonstrates that, while
important, block weight is not the only
factor that determines the STC for this type
of wall. In the absence of measured data,
the regression line in Figure 1 can be used
to estimate the STC from the block weight.
Alternatively, Table 1 provides representative STC values for 50% solid blocks that
have been sealed on at least one side.
It shows the relatively modest effects
of significant increases in wall thickness
on STC.
Adding materials such as sand or grout
to the cores of the blocks simply increases
the weight; the increase in STC can be
65
60
STC
55
50
45
40
35
0
10
20
30
40
Block weight, kg
Figure 1. Effect of block weight on STC for single-layer
concrete block walls
estimated from Figure 1. Adding soundabsorbing materials to the cores is not
effective because the sound bypasses the
insulation, taking the more direct path
through the block.
Figure 1 also shows that using heavier
block to get an STC rating of much more than
50 leads to impracticably heavy constructions.
High STC ratings are more easily obtained
by adding layers of gypsum board mounted
on studs or resilient (flexible) furring that
do not have a solid connection to the
concrete block.
Table 1. STC ratings for 50% solid normal-weight
and lightweight block walls sealed on at least
one side
Wall
Thickness,
mm
90
140
190
240
290
2
Lightweight
block
STC
43
44
46
47
49
Normal
weight
block
STC
44
46
48
49
51
Effect of Porosity
When the concrete block is porous, sealing
the surface with plaster or block sealer
significantly improves the sound insulation; the more porous the block, the greater
the improvement. Improvements of 5 to
10 STC points, or even more, are not
uncommon for some lightweight block
walls after sealing. Conversely, normalweight blocks usually show little or no
improvement after sealing. This improvement in STC in lightweight blocks is related
to the increased airflow resistivity of these
blocks. The leakage of sound through the
material of the block is different only in
degree from leakage of sound through
inadequate mortar joints. To ensure good
sound insulation, mortar joints must always
be properly finished — that is, free from
obvious penetrations.
Gypsum board attached directly to
blocks by means of glue or screws does not
seal the surface as well as plaster or paint
since the gypsum board does not bond
completely to the blocks and behaves as a
separate layer. Sound radiated from the
gypsum board continues to leak through the
blocks. Table 2 provides examples of two
very different types of block and their relative STC ratings. In each case, the plaster
allows the sound insulation of the wall to
reach its full potential; the gypsum board
does not. The effect of the plaster on the
very porous (wood-aggregate) blocks, which
have been included for purposes of comparison, is particularly dramatic. Block sealer
would yield the same result as plaster.
Table 2. STC for very porous wood-aggregate
blocks and lightweight concrete blocks sealed with
gypsum board or plaster
Concrete
90-mm wood190-mm
block
aggregate
lightweight
treatment
blocks
concrete blocks
Unsealed
14
42
Gypsum
29
46
board
on one face
Plastered
43
49
Double-Leaf Concrete Block Walls
In principle, double-leaf masonry walls can
provide excellent sound insulation. They
appear to meet the prescription for an ideal
double wall: two independent, heavy layers
separated by an air space. In practice,
constructing two block walls that are not
solidly connected somewhere is very
difficult. There is always some transmission of sound energy along the wire ties,
the floor, the ceiling and the walls abutting
the periphery of the double-leaf wall, and
through other parts of the structure. This
transmitted energy, known as flanking
transmission, seriously impairs the effectiveness of the sound insulation. Flexible
ties and physical breaks in the floor, ceiling
and abutting walls are needed to reduce it.
Even if such measures are considered in
the design, mortar droppings or other debris
can bridge the gap between the layers and
increase sound transmission. Such errors
are usually concealed and impossible to
rectify after the wall has been built. In one
test conducted by IRC researchers, a double-leaf wall that was expected to attain an
STC of more than 70 provided an STC of
only 60 because of mortar droppings that
connected the two leaves of the wall.
80
Bare blocks
13-mm resilient channels
75-mm resilient Z-bars
60
C
57
51
ST
C
50
ST
Transmission loss, dB
70
C
50
ST
40
30
Mass-air-mass resonance
20
63
125
250
500 1000 2000 4000
Frequency, Hz
Figure 2. Sound Transmission Loss (STC) through a 190-mm
concrete block wall with 15.9-mm gypsum board attached to
one side using a) 13-mm resilient metal channels, and
b) 75-mm resilient Z-bars
Construction Technology Update No. 13
In other tests carried out at IRC, a number of double-leaf cavity block walls, in
which the layers were structurally isolated
from each other, achieved STC ratings in
the range of 70 to 80. The constructions
provided the least amount of structural
vibration transmission that can be achieved
in the laboratory, which is better than what
can be achieved in normal practice. In
practical situations, careful design, meticulous workmanship and skilled supervision
are needed for such walls to achieve their
full potential.
Concrete Block Walls with
Gypsum Board Added
Adding gypsum board supported on furring
or studs at a distance from the surface of
the block wall can greatly improve the
sound insulation of the wall assembly.
Three factors that govern the degree of
improvement are:
1. The method of support — ideally, the
gypsum board should not be solidly
connected to the block;
2. The depth of the cavity (the distance
between facing surfaces of the block and
the gypsum board);
3. The use of sound-absorbing material in
the cavity between the gypsum board
and the surface of the block.
If the furring supporting the gypsum board
is rigid, sound can travel directly through it
from the gypsum board to the blocks.
However, if the furring is sufficiently flexible,
the sound will be attenuated. But the best
method of support for the gypsum board is
the use of independent studs that have no
direct connection to the block. Resilient
metal furring may be used on its own or in
combination with wood furring.
Figure 2 provides two sets of transmission
loss (TL) data for walls with unfilled cavities
of two different depths, as well as data for
an unfinished block wall. The effect of the
added gypsum board on acoustic performance
is striking — at high frequencies, the performance of the walls with the gypsum board is
better than that of the bare block wall; at
low frequencies, it is worse. There is a
decrease in TL at low frequencies and an
increase in TL at high frequencies in both
walls relative to the unfinished block wall.
3
In general, the greater the cavity depth and
the greater the mass, the lower the massair-mass resonance frequency, which usually
means an increase in STC. There is an
exception to this trend, however, which
occurs at frequencies around 100 Hz, where
the STC of the cavity walls decreases relative
to that of the unfinished block wall. While
voices are not a source of low-frequency
noise, modern stereos can be, as can
mechanical equipment, meaning that walls
around machine rooms should be designed
with great care if these rooms are adjacent
or close to living or working spaces.
Mass-air-mass resonance
frequency
This is the specific frequency at which
two layers of material separated by an air
space resonate, somewhat like the skin of
a drum. In the case of a block wall with
gypsum board added, vibrational energy
transfers from the gypsum board through
the air in the cavity to the block wall more
effectively than it does through the bare
block wall.
90
Bare blocks
One side only
Both sides
Transmission loss, dB
80
70
9
C4
ST
60
C
ST
54
0
C5
ST
50
40
30
20
Mass-air-mass resonance
10
63
125
250
500 1000 2000 4000
Frequency, Hz
Figure 3. Sound Transmission Loss (STC) for a 190-mm
concrete block wall with glass fibre batts in the cavity and with
15.9-mm gypsum board attached on 13-mm resilient metal
channels to a) one side and b) both sides of the wall
4
Sound-absorbing material is commonly
added to the cavity in a wall or floor to
improve the sound insulation. The addition of this material:
• lowers the mass-air-mass frequency and
usually improves the STC, and
• reduces the detrimental effects of other
cavity resonances at higher frequencies.
Fibrous materials, such as cellulose fibre,
glass fibre or rock wool insulation, are
good materials for this purpose; closed-cell
materials, such as expanded polystyrene,
are not, as they do not significantly absorb
sound.
Figure 3 shows the effect of adding glass
fibre to the cavity of the wall with 13-mm
resilient metal channels. The mass-airmass resonance frequency drops to around
100 Hz and the STC increases to 54.
Figure 3 also shows that when gypsum
board is applied to both sides of the blocks,
the TL increases to a greater degree at
higher frequencies, but decreases dramatically around the mass-air-mass resonance.
The STC for the wall with gypsum board
on both sides is one point less than that for
the wall that does not have gypsum board
added. More importantly, in the case of
the wall with gypsum board added to both
sides, the effectiveness of the sound insulation around 100 Hz is significantly reduced
by the added material. This diminished
performance occurs because the cavity
depth is too small, which leads to a massair-mass resonance that lowers the STC.
To maximize the improvement in sound
insulation, the cavity depth should be as
large as is practical. As a general guideline, the product of the mass per unit area
of the gypsum board (in kg/m2) and the
cavity depth (in mm) should exceed 425
for a cavity filled with sound-absorbing
material, and 720 for an unfilled cavity.
Estimating STC for Concrete
Block Walls with Gypsum
Board Added
Normal-weight, non-porous block
Data from an extensive series of measurements on normal-weight block walls were
used to develop an empirical method for
predicting STC for block walls with gypsum
Construction Technology Update No. 13
board supported on resilient furring or
independent studs. The simplified version
of this method, provided in Table 3, uses
only STC ratings but still provides reasonable accuracy. The equations give the STC
increment (∆STC) that can be achieved,
with or without sound-absorbing material,
when gypsum board is added to one or
both sides of a bare block wall.
For example, if 15.9-mm gypsum board
is added to both sides of a block wall filled
with sound-absorbing material, using 65mm steel studs, ∆STC is 0.44x65 - 7.37 =
21.2. Thus the STC rating of the bare wall
improves from 46 to about 46 + 21 = 67.
Note that for small values of d (i.e., small
cavities), ∆STC can be negative — in other
words, the effectiveness of the wall in controlling sound is reduced. Predictions
made using this procedure have an uncertainty of about ±2dB, but can be used with
some degree of reliability for 12.7- or
15.9-mm thick gypsum board.
Table 3. Equations for estimating increments in STC using
the cavity depth d (in mm) and assuming a single layer of
gypsum board
No sound-absorbing
material in the cavity
Cavity filled with
sound-absorbing
material
∆STC = 0.11xd - 1.22 one side
only
∆STC = 0.14xd - 2.78 both sides
∆STC = 0.12xd + 1.87 one side
only
∆STC = 0.44xd - 7.37 both sides
• On the second side of the wall, use
resilient metal furring or independent
studs to support a layer of gypsum
board. As with non-porous block, the
larger the cavity depth, the better.
• Add sound-absorbing material to the
cavity.
Porous block walls finished in this
manner can provide STC ratings as good as
those provided by non-porous, heavier
blocks. Currently, it is not possible to
predict the STC for composite walls such
as these. Approximate estimates may be
obtained using the procedures for non-porous
blocks. In critical situations, measurement
of the sound transmission loss (TL) values
may be needed.
Summary
Because of their weight, with little effort on
the part of the designer, concrete block
walls provide good sound insulation. In
general, the greater the
• mass of the wall
• the depth of the cavity
• the amount of sound-absorbing material
the better the performance of the block
wall, although there are some exceptions to
this approach. However, by applying the
principles described in this Update, it is
possible to construct high-quality walls
that can meet the most acoustically
demanding situations.
References
Porous block walls
The leakage of sound through a porous
concrete block wall can be used to advantage when gypsum board is added to finish
the wall. Because sound penetrates the
block, the effective depth of the cavity is
greater than the actual distance between
the inner faces of the gypsum board and
the block. The effective depth depends
on the porosity of the block. The best way
to finish a leaky concrete block wall is as
follows:
• Plaster or paint one side only. If a
gypsum board finish is required on this
side, it should be glued or screwed on
directly as this has the least detrimental
effect on STC.
1. A.C.C. Warnock and D.W. Monk. Sound
transmission loss of masonry walls: tests
on 90, 140, 190, 240 and 290 mm concrete block walls with various surface
finishes. Division of Building Research,
National Research Council, Building
Research Note 217, June 1984.
2. T.D. Northwood and D.W. Monk. Sound
transmission loss of masonry walls: twelveinch lightweight concrete blocks —
comparison of latex and plaster sealers.
Division of Building Research, National
Research Council, Building Research
Note 93. September 1974.
5
3. T.D. Northwood and D.W. Monk. Sound
transmission loss of masonry walls:
twelve-inch lightweight concrete blocks
with various surface finishes. Division
of Building Research, National Research
Council, Building Research Note 90.
April 1974.
4. A.C.C. Warnock. Factors affecting sound
transmission loss. Institute for Research
in Construction, National Research
Council, Canadian Building Digest 239,
Ottawa, 1985.
5. A.C.C. Warnock. Sound transmission
loss measurements through 190 mm and
140 mm blocks with added drywall and
through cavity block walls. Institute for
Research in Construction, National
Research Council, Internal Report 586,
1990.
Dr. A.C.C. Warnock is a senior research officer
in the Indoor Environment Program of the
National Research Council’s Institute for
Research in Construction.
© 1998
National Research Council of Canada
May 1998
ISSN 1206-1220
“Construction Technology Updates” is a series of technical articles containing
practical information distilled from recent construction research.
For more information, contact Institute for Research in Construction,
National Research Council of Canada, Ottawa K1A 0R6
Telephone: (613) 993-2607; Facsimile: (613) 952-7673; Internet: http://irc.nrc-cnrc.gc.ca