Masonry Magazine August 1967 Page. 30
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Building Partitions." The ASTM procedure was first issued in 1950 and has been extensively revised three times since.
Recognized over the years has been the need for a practical system of relating results of the test to useful architectural specification and design criteria. Until 1961, the most commonly used system for reporting sound transmission loss was the arithmetic average of TL measured at nine frequencies between 125 and 4000 cycles per second inclusive. Masonry walls, which have a relatively uniform TL curve are satisfactorily classified by the "averaging" method. In recent years, however, it has become evident that the nine frequency average does not properly classify many of the newer, less massive partition systems on the market today. Whereas the masonry walls could be depended upon to provide
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50
40
30
20
10
0
STC VALUE IS THE
TRANSMISSION LOSS
AT THE 500 CYCLE
INTERSECTION
1000
2000
2500
3150
4000
TEST FREQUENCY IN CYCLES PER SECOND
FIG. 2 TYPICAL CONTOURS FOR ESTABLISHING STC OF A PARTITION
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60
50
40
SOUND TRANSMISSION CLASS CONTOURS
ACTUAL SOUND LOSS
30
TEST ON A PAINTED
4-IN. CONCRETE
MASONRY WALL
20
10
0
1000
1250
1600
2000
2500
3150
4000
TEST FREQUENCY IN CYCLES PER SECOND
FIG.3 TYPICAL SOUND TRANSMISSION LOSS CURVE WITH STC CONTOUR
good sound insulation at critical frequencies, newer systems often exhibit acoustical "holes" at frequencies where good insulation is important. With the "averaging" method it was possible to have two walls with entirely different sound insulating characteristics and have the same TL rating, Fig. 1. The average sound transmission loss figure did little to identify the systems with acoustical "holes". The necessity of properly assessing these characteristics with a single figure rating led to the adoption by ASTM in 1961 of the "Sound Transmission Class" (STC).
Sound Transmission Class (STC) of a wall is determined by comparing its test values with a set of standard curves, or contours, Fig. 2, in accordance with certain specified conditions. Shape of the standard contour reflects the finding that insulation requirement varies with sound frequency, and greater re-
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FIG 4 RELATIONSHIP
OF AVERAGE STL
TO STC FOR 55
CONCRETE
MASONRY
PARTITIONS
50
40
30
20
10
0
50
60
10 20 30 40
NINE-FREQUENCY AVERAGE SOUND TRANSMISSION LOSS (STL) IN DECIBELS