The noise reduction coefficient (abbreviated NRC) is a single number value ranging from 0.0 to 1.0 that describes the average sound absorption performance of a material. It is a measure of a material’s ability to reduce or absorb sound. An NRC of 0.0 indicates that the object does not attenuate mid-frequency sounds but rather reflects sound energy. This is more theoretical than physically achievable: even very thick concrete walls will attenuate sound and may have an NRC of 0.05. It is commonly used in architectural and interior design to evaluate and select appropriate materials for soundproofing or noise control because it quantifies the acoustic properties of a surface or material.
A material’s NRC rating ranges from 0 to 1, with higher values indicating better sound absorption. An NRC of 0 indicates that the material reflects all sound, whereas an NRC of 1 indicates that the material absorbs all sound. The NRC ratings of the most commonly used sound-absorbing materials, such as acoustic panels, ceiling tiles, or wall coverings, range from 0.5 to 1.
An NRC of 1.0, on the other hand, indicates that the material has an acoustic surface area (in sabin) that is equal to its physical, two-dimensional surface area. This rating is typical of thicker, porous sound-absorbing materials, such as a 2-inch-thick fabric-wrapped fibreglass panel. Materials with NRC values greater than 1.00 are possible. This is a limitation of the test procedure and how acousticians define a square unit of absorption, not a property of the material itself.
To determine the NRC of a material, it is typically tested in a laboratory using standardised procedures such as ASTM C423 or ISO 354. These tests involve exposing the material to various frequencies of sound and measuring the amount of sound energy absorbed by the material. The NRC is then calculated using the average absorption coefficients at the specified frequency bands.
The NRC is useful for selecting appropriate materials to control the acoustic properties of a space. Materials with high NRC values, for example, are frequently chosen for environments where sound absorption is desired, such as concert halls, recording studios, or open-plan offices. Conversely, materials with low NRC values may be suitable for spaces where sound reflection is desired, such as lecture halls or theaters.
