Sound Reduction with Concrete Masonry Walls

Sound absorption involves reducing the sound emanating from a source within a room by diminishing the sound level and changing its characteristics. Sound is absorbed through dissipation of the sound wave energy. The effectiveness of the absorption method is dependent on the ability of the room surfaces to absorb the noise rather than reflect it. Sound Absorption Coefficient (SAC) is an indication of the sound absorbing efficiency of a surface. The Noise Reduction Coefficient (NRC) is the average SAC taken at different frequencies. NRC values depend on the porosity of the material and the surface. An open rough textured surface will have a higher NRC value. This means that a more porous block such as a Splitface or Fluted block will have a higher NRC rating. Also a medium weight block, because of its porosity will perform better from an absorption point of view, than a normal weight block. If the NRC = 1, then no sound is reflected back. The percentage that is reflected back is fractionalized, subtracted from 1 and this figure is the NRC value. Sound transmission is concerned with sound traveling through barriers from one space into another. To prevent transmission the walls just have enough density to stop the energy waves. Sound Transmission Loss is the total amount of airborne sound lost at a given frequency, as it travels through a partition. The STL, which is measured in decibels, is measured at 16 frequencies and the loss at these frequencies is used to plot a curve, which is used to determine the Sound Transmission Class (STC). The STC of a wall is determined by comparing its sound transmission loss curve with a set of standard curves or contours. There is a definite correlation between Sound Transmission and the weight of the wall. If a wall is heavier and more dense then the Sound Transmission Coefficient will increase. For concrete masonry units this means that a Normal Weight Block would have a higher STC rating because of the mass of that block. Porosity of the units is also an important aspect, as the tighter a texture on the surface, the greater the resistance to sound penetration. Therefore a painted surface will increase the STC, but will decrease the NRC. If a sound of 100 decibels is generated on one side of a wall and 40 decibels is measured on the other side, then the reduction in sound intensity is 60 decibels. The wall then has a 60 decibel rating.