Page 195 - The Master Handbook Of Acoustics

170 CHAPTER EIGHT

in contrast to the transmission loss for solid, rigid walls, which is
approximately 5 dB for each doubling of the thickness.

Sound Transmission Classification (STC)

The solid line of Fig. 8-3 is simply a replotting of data from the mass
law graphs of Fig. 8-2 for a wall weight of 10 lb per sq ft. If the mass
law were perfectly followed, we would expect the transmission loss of
a practical wall of this density to vary with frequency, as shown by the
solid line. Unfortunately, things are not this simple and actual mea-
surements of transmission loss on this wall might be more like the bro-
ken line of Fig. 8-3. These deviations reflect resonance and other
effects in the wall panel, which are not included in the simple mass
law concept.
Because of such commonly occurring irregularities, it would be of
great practical value to agree on some arbitrary procedure of arriving

60

50 Mass law
Transmission loss - dB 40 Actual

30

20
100 200 500 1,000 2,000 5,000
Frequency - Hz
FIGURE 8-3
Actual measurements of transmission loss in walls often deviate considerably from
the mass law (Fig. 8-2) because of resonances and other effects.