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.

Building materials, especially interior finishes, through standard testing earn ratings of STC 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 (Sound Transmission Class) 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 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 diffraction effects.

There are several factors on why a manufacturer may post NRC higher than 1.0.

They include:

- An Absorption coefficient is not a percentage
- There are four theoretical factors that explain NRC exceeding 1.0 A)Specimen Shape, B)Specimen Size C) Diffraction/Bending Wave Effects D) Edge Effects
- The most important is the ORGIN of the absorption coefficient. Simple history, aka in 1930 Dr. Sabine and Chrisler used 1" thick hair felt and had no access to fiber glass since it had not been invented yet.