Understanding NRC Values

Noise Reduction Coefficient or NRC is single number value ranging from 0 to 1.  This number describes the average absorption value of an acoustic material and in no way does it relate to noise control.  The naming is ambiguous and is more accurately described as a Sound Absorption Average (SAA).

The value is derived from the mean of the absorption values at 250Hz, 500Hz, 1000Hz and 2000Hz which are measured in a reverberation chamber as specified in ASTM C423 to ISO 354.

The intent with NRC was to provide a single number.  However its application is limiting due to insufficient data to make a meaningful judgement on 2 different materials.  One material might have a higher absorption in the lower frequencies and another material at higher frequencies…..!!

There are also a number of factors that affect the NRC value.  

This way the test sample is mounted in the test chamber and is often not specified.  An example would be to introduce an air gap behind the sample and in this way the absorption can be increased.

The size of the sample tested in the reverberation chamber is not specified.  Larger samples will typically have a higher value.

Sealing the edge of the materials being tested also affects the total absorption value.

Thicker materials generally have more material to increase absorption values, especially in the lower frequencies.