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SOUND ABSORPTION

Sound absorption control involves reduction of sound emanating from a source within a room.The extent of control depends on the efficiency of the room surfaces in absorbing rather than reflecting sound waves. Sound is absorbed through dissipation of the energy of the sound wavein the form of heat. The sound absorption coefficient is an indication of the sound absorbing efficiency of a surface. A surface which could theoretically absorb 100% of impinging sound would have a sound absorption coefficient of 1. Similarly, a surface absorbing 45% of incident sound would have a coefficient of 0.45. Another designator, termed the noise reduction coefficient (NRC) is calculated by taking a mathematical average of the sound absorption coefficients obtained at frequencies of 250, 500, 1,000 and 2,000 cps. The noise reduction coefficient of a concrete block surface is, to a large degree, dependent on the porosity of the block and the texture on the surface. Lightweight concrete block offers a strong material with countless minute voids. These voids appear on the surface of the unit, thereby permitting sound waves to enter the unit and be dissipated within the material, thus giving block good sound absorbing properties.

Painting will tend to seal the surface. Tests indicate that the extent of sealing depends upon the type and body of paint and method of application. This ofcourse, will decrease the sound absorption, however, it will increase the sound transmission value. Table 1 lists the approximate NRC values of both light and heavy concrete masonry.

Approximate Noise Reduction Coefficients

Material Surface Texture Approximate NRC
Lightweight Aggregate
Block, Unpainted
Coarse
Medium
Fine
0.40
0.45
0.50
Heavy Aggregate
Block, Unpainted
Coarse
Medium
Fine
0.26
0.27
0.28

Deduct grom above for painting

Paint Application One Coat Two Coats
All
Oil
Latex
Cement
Spray
Brush
Brush
Brush
10%
20%
30%
60%
20%
55%
55%
90%

For additional information on Noise control refer to the latest edition of your "Concrete Masonry Design Manual"...

 
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