The noise reduction coefficient (commonly abbreviated NRC) is a single number value ranging from 0.0 to 1.0 that describes the average sound absorption performance of a material. An NRC of 0.0 indicates the object does not attenuate mid-frequency sounds, but rather reflects sound energy. This is more conceptual than physically achievable: even very thick concrete walls will attenuate sound and may have an NRC of 0.05. Conversely, an NRC of 1.0 indicates that the material provides an acoustic surface area (in units sabin) that is equivalent to its physical, two-dimensional surface area. This rating is common of thicker, porous sound absorptive materials such as 2"-thick fabric-wrapped fiberglass panel. Materials can achieve NRC values greater than 1.00. This is a shortcoming of the test procedure and a limitation of how acousticians define a square unit of absorption, and not a characteristic of the material itself. Technical definition The noise reduction coefficient 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 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 (typically in an configuration) and appropriate mounting. The NRC is a logarithmic representation of the decay rate (dB/s) due to a panel or object with a defined surface area absorbing energy compared to the decay rate in a standard reverberant room without the panel or object History Wallace Clement Sabine was the first scientist to study the sound-absorbing characteristics of materials in a scientifically rigorous manner. Paul Sabine, a distant cousin of Wallace, studied the repeatability of sound absorption coefficient measurements in reverberation chambers. Paul Sabine's work in the 1920s–1930s laid the groundwork for the ASTM C423 test methodology still used today. Prior to the development of a standard procedure for material testing or reverberation chamber construction, data at low frequencies was highly unreliable and differed significantly from manufacturer to manufacturer. This is one of the primary reasons why the noise reduction coefficient historically