Masonry Magazine August 1967 Page. 32
Observed Sound Insulation Benefit
Observed, the sound insulation benefit (increase in sound transmission loss) from painting, plastering, or otherwise sealing the surface of the masonry units varies from 0 to 15 decibels depending upon their porosity.
Estimating Sound Transmission Class (STC)
The foregoing information provides two methods for estimating Sound Transmission Class (STC) for concrete masonry walls:
(1) The STC can be assumed equal to the average STL when data are available from older tests (see Fig. 4).
(2) STC can be predicted from unit weight of the wall (see Fig. 7).
Examples
These methods are illustrated in the following examples:
# Example 1
Problem: Test data show as 6-in. painted concrete masonry wall had an average Sound Transmission Loss (STL) of 42.1 decibels. What is it's Sound Transmission Class (STC)?
From Fig. 4, the STC of wall can be assumed between 40 and 45 decibels.
# Example 2
Problem: What wall section will be required to obtain an STC of 45?
Solution:
From Fig. 7, if plastered or painted, either a 6-in. or an 8-in. wall could be used, providing it weighed 30 lb/sf or more. If wall is to receive no surface treatment, use either a heavy 8-in. unit (45-50 lb/sf) or some other wall section, i.e., cavity wall, 10-in. single wythe wall, or 12-in.
Estimated STC values by either method will be adequate for most purposes. Although test data are reported to a fraction of a decibel, a difference of three or less db is not especially significant because the human ear cannot detect a change in sounds of less than three decibels.
FIG.7 ESTIMATE STC OF CONCRETE MASONRY WALLS