Noise reduction coefficient
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.
Technical definition 
It is the arithmetic average, rounded to the nearest multiple of 0.05, of the absorption coefficients for a specific material and mounting condition determined at the one octave band center frequencies of 250, 500, 1000 and 2000 Hz. The absorption coefficients of materials are commonly determined through use of standardized testing procedures, such as ASTM C423 that is used to evaluate the absorption of materials in eighteen one-third octave frequency bands with center frequencies ranging from 100 Hz to 5000 Hz. Absorption coefficients used to calculate NRC are commonly determined in reverberation rooms of qualified acoustical laboratory test facilities using samples of the particular materials of specified size and appropriate mounting.
NRC is most commonly used to rate general acoustical properties of acoustic ceiling tiles, baffles, and banners, office screens, and acoustic wall panels. It is occasionally used to rate floor coverings and construction materials.
NRC is intended to be a simplified acoustical rating of room construction and finish materials when the acoustical objectives of the space are less than sensitive. In certain applications, such as designs of music rehearsal rooms, performance spaces, and rooms employed for critical speech, it is usually more appropriate to consider the sound absorption coefficients at the individual one-third octave band frequencies, including those above and below the bands used to compute NRC.
New standards 
NRC is being replaced by the Sound Absorption Average (SAA), which is described in the current ASTM C423-09a. The SAA is a single-number rating of sound absorption properties of a material similar to NRC, except that the sound absorption values employed in the averaging are taken at the twelve one-third octave bands from 200 Hz to 2500 Hz, inclusive, and rounding is to the nearest multiple of 0.01.