Page 196 - The Master Handbook Of Acoustics

171
CONTROL OF INTERFERING NOISE

at a single number that would give a reasonably accurate indication
of the sound transmission loss characteristics of a wall. This has
been done in a procedure specified by the American Society for Test-
ing and Materials in which the measured graph of a wall would be
placed in a certain Sound Transmission Class (STC) by comparison
to a reference graph (STC contour). The details of this procedure are
beyond the scope of this book, but the results of such classification
have been applied to walls of various types to be described for ready
comparison. An STC rating of 50 dB for a wall would mean that it is
better in insulating against sound than a wall of STC 40 dB. It is not
proper to call STC ratings “averages” but the whole procedure is to
escape the pitfalls of averaging dB transmission losses at various fre-
quencies. 1

Comparison of Wall Structures

Figure 8-4 gives the measured performance of a 4 in concrete block wall
as a sound barrier. It is interesting to note that plastering both sides
increases the transmission loss of the wall from STC 40 to 48. Figure 8-
5 shows a considerable improvement in doubling the thickness of the
concrete block wall. In this case the STC 45 is improved 11 dB by plas-
tering both sides. In Fig. 8-6 is illustrated the very common 2 × 4 frame
construction with ⁄8-in gypsum board covering. The STC 34 without
5
fiberglass between is improved only 2 dB by filling the cavity with
fiberglass material, a meager improvement that would probably not jus-
tify the added cost.
Figure 8-7 describes a very useful and inexpensive type of wall of
staggered stud construction. Here the inherently low coupling
between the two independent wall diaphragms is further reduced by
filling the space with fiberglass building material. Attaining the full
STC 52 rating would require careful construction to ensure that the
two wall surfaces are truly independent and not “shorted out” by elec-
trical conduits, outlet boxes, etc.
The last wall structure to be described is the double wall construc-
tion of Fig. 8-8. The two walls are entirely separate, each having its
2
own 2 × 4 plate. Without fiberglass this wall is only 1 dB better than
the staggered stud wall of Fig. 8-7 but by filling the inner space with
building insulation, STC ratings up to 58 dB are possible.