Noise Reduction Coefficient (NRC) is the most common measurement for sound absorbing materials. It evaluates the capability of a material to absorb sound within the same space that sound originated in. A true NRC has been tested in a qualified laboratory and evaluated at four frequencies: 250, 500, 1000, 2000. A common standard of testing is ASTM C423.
Caution: A large number of products have NRC claims that were never tested, were not tested in qualified labs, or falsely tested at more desirable frequencies. Why? While there is an industry standard, there is no regulating body, nor repercussions for false claims.