The noise reduction coefficient (commonly abbreviated NRC) is a scalar representation of the amount of sound energy absorbed upon striking a particular surface. An NRC of 0 indicates perfect reflection; an NRC of 1 indicates perfect absorption.
Building materials, especially interior finishes, through standard testing earn ratings of STC (Sound Transmission Class) and NRC based on their sound characteristics. NRC generally applies to a single material such as on the surface of a wall which determines the reverb or liveliness of a room. STC generally applies to the assembly that composes the wall, including framing, insulation, sheet products (gypsum wallboard for example) and a surface material (such as vinyl wallcovering) which limits the amount of sound that travels through the wall assembly into an adjacent space.
NRC (Noise Reduction Coefficient) is an arithmetic value average of sound absorption coefficients at frequencies of 250, 500, 1000 and 2000 Hz indicating a material's ability to absorb sound.
In particular, it is the average of four sound absorption coefficients of the particular surface at frequencies of 125, 250 Hz, 500 Hz, 1000 Hz, 2000, and 4000 Hz. These frequencies encompass the fundamental frequencies and first few overtones of typical human speech, and, therefore, the NRC provides a decent and simple quantification of how well the particular surface will absorb the human voice. A more broad frequency range should be considered for applications such as music or controlling mechanical noise.
Specifications for materials used in sound absorption commonly include an NRC for simplicity, in addition to more detailed frequency vs amplitude charts.
