Masonry Magazine April 1972 Page. 18
Sound Absorption
If all the sound striking a given surface in a room were absorbed, the room would have a sound absorption coefficient of 1. If only 40 percent of it were absorbed, the coefficient would be 0.40. A commonly used measure of sound absorption is the noise reduction coefficient (NRC). It is determined by measuring the sound absorption coefficients at sound frequencies of 250, 500, 1000, and 2000 cycles per second (cps). The NRC is the average of these four measured coefficients. Noise reduction coefficients for concrete block and some other materials are given in Table 1. To obtain the sound absorption of a room the designer multiplies the noise reduction coefficient of each material by its area, adds the results and divides by the total area. For noise reduction comfort the sound absorption for the room as a whole should be between 0.20 and 0.50.
Where there is need to obtain exceptionally high absorption of low frequency sound the designer may wish to use a special concrete masonry unit with patented features that is now available.
Isolating Noise
The effectiveness of a wall in isolating sound is measured by a two-room test method in which sound generated in a specially designed room is passed through the test wall and measured in the adjacent test room. The test is made at sixteen frequencies from 125 to 4000 cps, and sound intensities transmitted are measured in decibels (db). The difference in intensity of the generated sound and the received sound, in decibels, is the transmission loss.
In Fig. 1 are sound transmission loss (STL) curves for a 4-inch hollow concrete masonry wall without any surface treatment and for a nonmasonry wall. The transmission loss varies with the frequency.
The lighter lines are sound transmission class contours that are used in establishing the sound transmission class (STC) of a wall by Method E 90-66T of the American Society for Testing and Materials.
Both the masonry wall and the nonmasonry wall in Fig. 1 have average sound transmission losses of 38 db, but this average does not reveal the poorer performance of the nonmasonry wall at the important frequency of 500 cps, where it has an "acoustical hole" through which much sound is transmitted. Accordingly, walls are rated in the ASTM method by comparing the curves to standard sound transmission class contours. The STC rating of a wall is equal to the sound transmission loss at 500 cps of its equivalent contour. By the ASTM method the STC of the masonry wall would be about 39, and of the nonmasonry wall about 30.
Sound Isolation Requirements
The minimum acceptable standards of limitations on sound transmission set by the Federal Housing Administration for multifamily housing are given in FHA Publication No. 2600. The current guides from the Department of Housing and Urban Development are outlined in Table 2. STC values required range from a low of 40 to a high of 50. Whenever extreme sensitivity to noise is anticipated, however, as in high-rent apartments, the values given should be increased by 5. It can be deduced from Table 2 that the partitions that require the most soundproofing are those next to public space and service areas, such as corridors and stairwells, and those that separate such high noise areas as boiler and mechanical equipment rooms, elevator shafts, and garages.
Sound isolation requirements are lower in the city than in rural neighborhoods, for the reason that high background noise raises the threshold of audibility. An intruding noise of an intensity that is clearly heard in a quiet neighborhood might go completely unnoticed in an apartment on a street where the continuous hum of traffic masks the sound without seeming unpleasant itself. The effects of background noise on the minimum STC levels required to achieve various degrees of privacy are shown in Table 3.
Selecting Walls
Structural adequacy of walls does not necessarily guarantee acoustical adequacy. The type of masonry and its thickness should be chosen to meet STC requirements for the various kinds of walls and partitions in multifamily housing. Both mass per unit wall area and porosity of concrete masonry units are factors to consider. In Fig. 2 the STC values of concrete masonry partitions are shown in terms of wall weight per square foot of single-wythe painted or plastered walls. They may be seen to increase with wall weight.
Block of equal weight but different porosity have STC values that vary inversely with porosity when measured in terms of air permeability. Air permeability is a particularly apt measurement for this purpose because it eliminates that portion of porosity that includes dead spaces or partially enclosed pores. Porosity may be reduced (and STC values thereby increased) by sealing the wall surface. The STC value may be increased approximately 8 percent by a layer of gypsum board, 10 percent by two coats of paint or plaster, and 15 percent by two layers of gypsum board.
Two points should be emphasized in this respect. The first is that painting, plastering or covering two sides of the wall has little more beneficial effect on STC rating than sealing only one side.
The second point is that a sealed surface not only decreases sound transmission, which is desirable, but also decreases sound absorption, which may be undesirable. This effect was shown in the bottom part of Table 1. The best combination of objectives is to leave the surfaces unsealed in noisy areas like stair wells or machine rooms, but to seal them in living areas. It is also possible to seal an interior surface of a cavity wall by back plastering one of the wythes.
* This contour is the one from which the sum of the Wall's STI. differences at %-octave intervals, is not more than 32 db and from which any one STL difference is not greater than 8 db.