NRC: Looking Beyond Face Value
Wouldn't it be nice if products came with a numeric value that expressed their level of performance in acoustic applications? That way, you would know what to expect when you specify and install a particular product. Is the Noise Reduction Coefficient (NRC) the answer? Possibly, but not at face value. Although NRC provides a valuable gauge of a product's performance level, you cannot rely on it in every case. It is crucial to understand the nuances of NRC before depending on it for a project's success.
A Little Background
NRC is a standard, not a regulation or code, that represents the lab test for determining a product's ability to absorb sound. This industry standard ranges from zero (perfectly reflective) to 1* (perfectly absorptive). It is always expressed as a decimal rounded to the nearest 5%. While NRC is widely used and accepted in the architecture and design communities, some become too dependant on the numeric value provided by the manufacturer, thereby disregarding the circumstances from which that value was recorded.
*(Based on the testing methodology, and depending upon the material's shape or surface area, some products can test at an NRC above 1.)
Information from: Concepts in Architectural Acoustics
by M. David Egan.
NRC can be a valuable tool, as long as you understand what the value actually represents.
A responsible design professional will not rely solely on information provided by manufacturers. While some do provide accurate information, it is advisable to seek an un-biased third-party for product verification.
- 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.
- Communication of product ratings by manufacturers can be misleading and sometimes deceitful for the following reasons:
- The information provided is based on lab tests. Because the lab is a 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.
- Some manufacturers will quote absorption at the more-desirable higher frequencies without clearly explaining the testing conditions. NRC is based only on absorptive characteristics (250, 500, 1000, 2000 Hz). Make sure the product data you're reviewing is of these frequencies.
- Make sure the mounting procedure used in the tests is consistent with your intended installation if you expect the same results. For example, a manufacturer of a wall carpet product provides an NRC rating of .80, which is extremely good. But, if you know how to read the fine print (if there is any), you'll see 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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