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. A product brochure probably will mention how effectively an acoustic block absorbs, blocks and/or diffuses sound waves, depending on the product's specialty. This information could be presented only as an STC or an NRC value. Acoustic block differ from standard CMU in design. Some have slots that open up into cavities on one side of the block. Others have cores within the block that can be filled with other material, and porosity varies greatly.