A materials ability to absorb sound is measured by its sound absorption coefficient and is often expressed in terms of a Noise Reduction Coefficient (NRC) The NRC is the arithmetic average of the product’s sound absorption coefficients at 250, 500, 1000 and 2000Hz. The sound absorption coefficients of products are measured according to ISO140-3. An NRC of 0.4 means 40 % of sound is absorbed by the material. Typical building materials such as plasterboard have an NRC of approximately 0.04 which means only 4% sound will be absorbed. The NRC gives no information as to how absorptive a material is in the low and high frequencies, nor does it have anything to do with the material’s barrier effect (STC).