NRC (Noise Reduction Coefficient)

The Noise Reduction Coefficient (NRC) is a single-number rating of a material’s sound absorption, calculated as the arithmetic average of the absorption coefficients at 250, 500, 1000, and 2000 Hz, rounded to the nearest 0.05. Defined in ASTM C423, NRC provides a quick comparison metric for specifying acoustic materials. Values range from 0.00 (fully reflective, like polished marble) to 1.00 (fully absorptive, like thick mineral wool). While convenient, NRC has limitations: it ignores low-frequency performance below 250 Hz and high-frequency performance above 2000 Hz. Two materials with identical NRC values can have very different frequency responses. For critical applications, designers should review the full octave-band absorption data rather than relying solely on NRC. The SAA metric (ASTM C423) provides a slightly more precise alternative.