NRC refers to a surface’s ability to reduce noise by absorbing sound.
NRC values are calculated from the average sound absorption coefficient measured at four frequencies: 250, 500, 1,000 and 2,000 Hz. NRC is the fraction of sound energy, averaged over all angles of direction and from low to high sound frequencies that is absorbed and not reflected.
A material's sound absorbing properties are expressed by the sound absorption coefficient, NRC, which ranges from 0 (total reflection) to 1.00 (total absorption).
Absorption in acoustics refer to the energy of sound waves being taken in and trapped within a material rather than being bounced off or reflected. Materials are rated in terms of their ability to absorb sounds.
Acoustic absorption refers to the process by which a material, structure, or object takes in sound energy when sound waves are encountered, as opposed to reflecting the energy. Part of the absorbed energy is transformed into heat and part is transmitted through the absorbing body.
All building materials have some acoustical properties in that they will all absorb, reflect or transmit sound striking them. Conventionally speaking, acoustical materials are those materials designed and used for the purpose of absorbing sound that might otherwise be reflected.
Sound absorption is defined, as the incident sound that strikes a material that is not reflected back. An open window is an excellent absorber since the sounds passing through the open window are not reflected back but makes a poor sound barrier. Painted concrete block is a good sound barrier but will reflect about 97% of the incident sound striking it.
When a sound wave strikes an acoustical material the sound wave causes the fibers or particle makeup of the absorbing material to vibrate. This vibration causes tiny amounts of heat due to the friction and thus sound absorption is accomplished by way of energy to heat conversion. The more fibrous a material is the better the absorption; conversely denser materials are less absorptive. The sound absorbing characteristics of acoustical materials vary significantly with frequency. In general, low-frequency sounds are very difficult to absorb because of their long wavelength. On the other hand, we are less susceptible to low-frequency sounds, which can be to our benefit in many cases.
For the vast majority of conventional acoustical materials, the material thickness has the greatest impact on the material’s sound absorbing qualities. While the inherent composition of the acoustical material determines the material’s acoustical performance, other factors can be brought to bear to improve or influence the acoustical performance. Incorporating an air space behind an acoustical ceiling or wall panel often serves to improve low-frequency performance.