Acoustical Noise Control 77- Steel stud partitions usually have an STC from two
to ten points higher than the equivalent wood stud
partition. The flange of the common C-shaped steel
stud is relatively flexible and transmits somewhat
less sound energy from face to face. - If multiple layers of gypsum board are used,
mounting the second layer with adhesive rather
than screws can affect an STC increase by as much
as six points. This is especially helpful with higher
density walls. - A fiberglass cavity filler (such as R-7) may
increase STC by five to eight points. It is more
effective in multilayer partitions if the second layer
is attached with adhesive. - A slight increase in STC results from increasing
stud spacing from 16 inches to 24 inches on center. - Increasing stud size from 2 inches to 3 inches does
not significantly increase either transmission loss or
STC in steel-stud partitions with filler in the cavity. - Additional layers of gypsum wallboard increase
STC and TL, but the greatest improvement is with
lighter walls. Adding layers increases stiffness,
which tends to shift the coincidence dip to a lower
frequency. - Attaching the first wallboard layer to studs with
adhesive actually reduces STC.
4.3.3 Concrete Block WallsConcrete block walls behave much like solid walls of
the same surface weight and bending stiffness. In Table
4-7, wall system 9 is a lightweight, hollow, concrete
block wall with both sides sealed with latex paint. In
Fig. 4-15 we see that the performance of this specific
wall falls close to pure mass operation. The STC-46 is
matched or exceeded by many frame walls listed before
it in Table 4-7. Wall system 10 in Table 4-7 is the same
as 9, except wall system 10 has a new leaf, is furred out,
and has mineral fiber added to one side in the cavity.
These additions increase the STC from 46 to 57. It
should be noted that there are less expensive frame
structures that perform just as well. The performance of
concrete walls can be improved by increasing the thick-
ness of the wall, by plastering one or both faces, or by
filling the voids with sand or well-rodded concrete, all
of which increase wall mass. The STC performance of
such walls can be estimated from Fig. 4-15 when the
pounds-per-square-foot surface density is calculated. To
further improve the performance one must add a
furred-out facing (such as 10) or adding a second block
wall with an air space.
4.3.4 Concrete WallsThe empirical mass law line in Fig. 4-15 goes to
100 lb/ft^2 (488 kg/m^2 ), just far enough to describe an
8 inch concrete wall of 150 lb/ft^3 density (surface
density 100 lb/ft^2 or 732 kg/m^2 ). This wall gives a rating
close to STC-54. By extending the line we would find
that a 12 inch wall would give STC-57, and a concrete
wall 24 inches thick, about STC-61. The conclusion is
inescapable. This brute-force approach to sound TL is
not the cheapest solution. High TL concrete walls can
be improved by introducing air space—e.g., two 8 inch
walls spaced a foot or so apart. Such a wall requires
specialized engineering talent to study damping of the
individual leaves of the double wall, the coupling of the
two leaves by the air cavity, the critical frequencies
involved, the resonances of the air cavity, and so on.4.3.5 Wall CaulkingThere is continual movement of all building compo-
nents due to wind, temperature expansion and contrac-
tion, hygroscopic changes, and deflections due to creep
and loading. These movements can open up tiny cracks
that are anything but tiny in their ability to negate the
effects of a high-loss partition. An acoustical sealant is
required to caulk all joints of a partition if the highest
TL is to be attained. This type of sealant is a specialty
product with nonstaining, nonhardening properties that
provides a good seal for many years. Fig. 4-16 calls
attention to the importance of bedding steel runners and
wood plates in caulking to defeat the irregularities
always present on concrete surfaces. A bead of sealant
should also be run under the inner layer of gypsum
board. The need for such sealing is as important at the
juncture of wall-to-wall and wall-to-ceiling as it is at the
floor line. The idea is to seal the room hermetically. Fig.
4-17 is a nomograph that illustrates what happens if
there is leakage in a partition. The X axis represents a
partition that is not compromised by any leaks. The
family of curves are gaps or holes expressed as percent-
ages of the whole surface area of the partition. This
nomograph shows that a partition rated at a TL of 45
with no penetrations would perform as a TL-30 wall if
only 0.1% of the wall were open. Consider what this
means in real terms. A partition has a surface area of
10 m^2 , 0.1% of 10 m^2 amounts to an opening with an
area of a square centimeter (cm^2 ). This could be a gap in
the wall/ floor junction where the caulking was omitted,
or it could be the area left open by the installation of an
electrical box in a partition. This small gap will reduce
the performance of the wall by a significant amount. All