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Noise Reduction Coefficient (NRC):
The NRC is a single-number index determined in a lab test and used for rating how absorptive a particular material is. This industry standard ranges from zero (perfectly reflective) to 1* (perfectly absorptive). It is simply the average of the mid-frequency sound absorption coefficients (250, 500, 1000 and 2000 Hertz) rounded to the nearest 5%.
*(Based on the testing methodology, and depending upon the material's shape or surface area, some products can test at an NRC above 1.)
To learn more...
What you need to know
NRC ratings of common building materials
While NRC is widely used and accepted, it can also be abused or misunderstood. Be aware of the following items before specifying a particular material based on NRC:
- The NRC rating is an average of how absorptive a material is at four frequencies (250, 500, 1000 and 2000 Hz). This rating is appropriate for assessing how well a material absorbs sound within the speech frequencies, but can be inadequate for sound generated by music, mechanical equipment or other low-frequency sounds.
- Because this rating is an average, two materials with the same rating might not perform the same in identical applications.
- The NRC is based on lab tests. Because the lab is a near perfect environment that is rarely duplicated in everyday applications, some products will not test the same in the field. Certain factors, such as installation variables, are not accounted for in the lab. A product that receives high ratings in the lab may not perform as well in the field.
- Make sure the mounting procedure used in the tests is consistent with your intended installation if you expect the same results.
- NRC does not have anything to do with the material's barrier effect (STC). Click here for our tutorial on the differences between NRC and STC.
- Communication
of NRC ratings by manufacturers can be misleading and sometimes deceitful for
the following reasons:
- Some manufacturers will quote absorption at the more-desirable higher frequencies. NRC should be based only on the absorptive characteristics at 250, 500, 1000, and 2000 Hz. Make sure the product data you're reviewing is at these frequencies.
- A manufacturer of a wall carpet product could provide an NRC rating of .80, which is extremely good. But, if there is fine print be sure to read it, you may see that this rating was achieved while the carpet was installed over fiberglass. In this installation configuration, the fiberglass, not the carpet, acts as the sound absorber. Without the acoustic material behind, the wall carpet will probably only achieve an NRC of .20.
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