Masonry Magazine April 1970 Page. 16
SOUND ISOLATION
To determine the effectiveness of wall construction as a means of sound isolation, a two room test method is generally employed. In this test a steady sound is generated and measured on one side of a wall, and the sound which passes through is measured in an adjacent room.
The measurement of sound levels is made at sixteen frequencies over a range of 125 to 4,000 cps. The difference in sound levels indicates the transmission loss of the wall. If a generated sound level of 80 db is observed in one room, and 30 db measured in an adjacent room, the reduction in sound intensity from the intervening wall is 50 db.
The wall is then referred to as having a 50 db sound transmission loss. The higher the transmission loss of a wall, the better it functions as a barrier to the passage of sound.
Arithmetic averages of sound transmission loss at selected frequencies were extensively used in the past to rate the effectiveness of walls. The classification method was sometimes unreliable, however, because a good average could be ascribed to a wall that performed poorly at an important frequency.
Concrete masonry walls perform well over a wide range of frequencies, and were satisfactorily classified by the transmission loss average. In the recent past it became clear that this method did not accurately classify some less massive, non masonry partition systems being marketed.
Accordingly, the American Society for Testing and Materials revised its test designation E-90-66T to provide a means of rating sound reduction by a single number called sound transmission class (STC). In this method the sound isolation of a wall is measured according to predetermined standards and an STC rating is obtained by comparing a plotted curve of test results against a standard set of curves or contours.
Fig. 1, showing two typical sound transmission loss curves and a set of standard STC contours, illustrates the uniformity of sound loss-frequency curves for concrete masonry in general. This indicates that concrete masonry can be depended upon to provide good sound insulation over the entire frequency range.
By comparison, the performance of a non-masonry wall having an equal average sound transmission loss exhibits a deficiency in insulating noise near the 500 cps frequency range. Failure of sound insulation at certain frequencies is often called an "acoustical hole".
A detailed explanation of STC ratings and methods of estimating STC values of concrete masonry walls are published in NCMA TEK 9.
The Federal Housing Administration has established minimum acceptable standards of sound transmission limitations for multifamily housing. These are listed in FHA Publication #2600.
The most recently developed guides from the Department of Housing and Urban Development are outlined in Table 2. As listed in the table, the STC values range from a low of 40 to a high of 55. Under certain applications, however, such as in high rental apartments where extreme tenant sensitivity to noise is anticipated, an increase of 5 db over the listed value should be utilized.
Partitions which the table shows to require the highest level of soundproofing are those located next to public space and service areas, such as, corridors and stairwells, and areas of high noise including boiler rooms, mechanical equipment rooms, elevator shafts, garages, etc.
Determination of STC requirements from Table 2 depends on background noise level as well as location of the partitions. Background noise is important in all noise control situations because it raises the threshold of audibility.
The higher the level of background noise, the more transmitted noise can be tolerated. An intruding noise might go completely unnoticed in an apartment located on a busy street where the continuous hum from traffic masks out sound without seeming unpleasant in itself.
The same intruding sound might clearly be heard in the room of a house located in a quiet area.
For design purposes a detailed noise survey by a competent acoustical engineer will provide data to determine the outside noise level in an area and the anticipated background noise level in completed living units. Consideration of future increases in noise levels for the area should be included in the engineer's evaluation.
As a general guide, Table 3 provides a scale of representative noise levels and the STC requirements for desired levels of privacy.