Source: Masonry Construction
Publication date: September 1, 1998
By Elizabeth Keating
Acoustic block, more porous than standard CMU, are designed to absorb, block and/or diffuse sound waves.The type of acoustic block that should be used depends on the type of noise and your goal. You may want to block a sound, ensure good acoustics in a room, or absorb, block and diffuse all at once. You may need to supplement an acoustic block with other acoustic materials.Highway sound-barriers, school concert halls, lecture rooms -- all require specific sound control. Sounds at low frequencies are some of the most annoying to endure and some of the most difficult to reduce.An STC rating measures a wall's resistance to sound transmission. ASTM E 90 "Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions" stipulates a way to measure sound transmission loss. The National Concrete Masonry Association (NCMA) has a simple formula for estimating STC.The NRC (Noise Reduction Coefficient) is the average of the sound absorption coefficients at four frequencies. A sound absorption coefficient (SAC) is the percentage of sound waves absorbed by the block. NRC is calculated from the SACs at only four frequencies: 250, 500, 1,000 and 2,000 hertz. A table or graph giving sound absorption coefficients at all measured frequencies is much more informative.
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