Noise Reduction Coefficient
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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.
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.
Specifications for materials used in sound absorption commonly include an NRC for simplicity, in addition to more detailed frequency vs amplitude charts.
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 difraction effects.
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