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The Noise Reduction Coefficient (NRC) is a 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.
In particular, it is the average of four sound absorption coefficients of the particular surface at frequencies of 250 Hz, 500 Hz, 1000 Hz, and 2000 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.
Acoustical materials manufacturers often report NRC values higher than 1.0 due to the way the number is calculated in a laboratory. A test material's area does not include the sides of the panel (which are exposed to the test chamber) which vary due to its thickness. A certain percentage of the sound will be absorbed by the side of the panel due to diffraction effects.
NRC: .50 = 50% absorption
NRC: .90 = 90% absorption
NRC: 1.15 = ??? This represents an average higher than 1 which generally means that this is a thicker panel and has better performance in the lower frequencies - because the sound wave completes more of a full cycle when it is in the panel.
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