acoustical wall

How Installation and Testing Affects NRC Ratings

Noise Reduction Coefficient is a value between 0.0 and 1.0 that describes the sound-absorbing performance of a material. NRC ratings are primarily used to evaluate the sound absorption of a given material to reduce reverberation and increase speech intelligibility in a space.

How Does a Material Get an NRC Rating?

Testing is done in a reverberation chamber with different mounting standards to measure NRC at different frequencies. Most tests include multiple mounting standards to provide a comparison.

How any material is applied to a surface or installed can increase or reduce the NRC rating. For example, PET felt has an NRC rating of .75. However, if the material is applied directly to the wall (ie: if it’s glued) the NRC rating drops to .30.

A hollow cavity between the material and the surface increases the NRC rating of this material to .55-.65, depending on the size of the cavity. If the material is suspended or mounted as a traditional baffle, the material achieves its full potential, like PET with an NRC rating of .75.

Geometry Matters

Given the testing limitations, it’s important to consider both the material as well as the geometry of the product. A highly rated material is a must-have. To achieve optimal sound absorption, an acoustical application with geometry that adds more square footage of that material in a given volume of space will provide added benefit.

For example, a wall treatment that is three-dimensional with a hollow cavity provides both the hollow cavity for improved sound absorption as well as more square footage of sound-absorbing material compared to a flat application.

NRC Ratings Higher than 1.0

Often, an acoustical application can achieve an NRC rating of higher than 1.0. How does that happen, if a rating of 1.0 is perfectly absorbing? It’s a testing limitation. Testing does not account for 3-dimensional objects, objects of significant depth or objects with a pronounced hollow cavity; the testing assumes the objects are flat. Therefore, NRC testing can yield ratings higher than 1 and should be approached with this consideration.

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