Masonry Magazine August 1967 Page. 31
SOUND TRANSMISSION CLASS (STC) IN DECIBELS
FIG. 5 RELATIONSHIP OF SOUND TRANSMISSION CLASS (STC) TO WALL WEIGHT FOR SINGLE-WYTHE PAINTED CONCRETE MASONRY WALLS
Restrictions are imposed at those frequencies where noise reduction is more critical. Since its introduction in 1961, STC has received considerable acceptance by designers, and in fact, many acoustical consultants today consider Sound Transmission Class the only acceptable rating scheme. FHA Standards now include minimum STC requirements for multi-family housing.
Figure 3 shows a typical sound transmission loss curve for a concrete masonry wall, along with the sound transmission class contour which establishes its STC. Similarity in the shape of the two curves, the sound loss curve of the concrete masonry wall and the standard sound contour, is noteworthy and might have been expected by the fact that concrete masonry walls do have a fairly uniform sound loss-frequency curve. The similarity in shape suggests there is little difference in value between the average sound transmission loss and the sound transmission class (STC) for this material and, indeed, is the case here. Figure 4 compares the average nine-frequency sound transmission loss of 55 concrete masonry walls with their sound transmission class (STC). The STC values are, in the majority of tests, equal to or greater than the average STL values. With this relationship established it is evident that the STC of a concrete masonry wall can be assumed to equal its average STL in the absence of TL versus Frequency data.
STC Values
When walls of concrete masonry are plastered, painted, or otherwise sealed so that sound transmission through them is due to vibration rather than passage of airborne sounds, their STC may be predicted from the wall weight with reasonable accuracy. The relationship between wall weight per square foot and Sound Transmission Class (STC) is illustrated in Fig. 5. All walls were painted with one or two coats of cement-base or resin-emulsion paint. The scattering of data is attributed to paint systems not always sealing wall surfaces sufficiently, therefore, sound could travel through pore openings. Wall surfaces must be effectively sealed for this relationship between weight and STC to be valid.
Unpainted concrete masonry walls tend to follow this relationship between weight and STC to the extent that their surfaces are free of openings and pores. Units of well compacted concrete, possessing closed or fine textured surfaces, behave much the same as painted walls. Their Sound Transmission Class (STC) can also be predicted from the weight of the wall. Walls of such units are not greatly improved as sound insulators by the application of plaster or paint. On the other hand, unpainted, open-textured units will generally have lower STC values than would be expected on the basis of wall weight. Figure 6 shows results of research that demonstrate this relationship. The curve shown was developed by measuring the increase in average sound transmission loss due to sealing the surface of concrete masonry units of different porosities; porosity being measured by resistance to air flow through the units. As may be