FAQ: How is NRC calculated?

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Frequently Asked Questions

Question:  How is NRC calculated?

Noise Reduction Coefficient (NRC) is a single number rating defined by the ASTM standard C423, the laboratory method for determining sound absorption in a diffuse field reverberation chamber2.

This is the definition according to the standard1:

3.2 In previous versions of this test method a single number
rating, called the noise reduction coefficient (NRC), was
defined as follows:

Round the average of the sound absorption coefficients
for 250, 500, 1000, and 2000 Hz to the nearest multiple
of 0.05. If the unrounded average is an exact midpoint,
round to the next higher multiple of 0.05. For example,
0.625 and 0.675 would be reported as 0.65 and 0.70, respectively.

It is a common misconception that NRC is calculated from octave band sound absorption coefficients.  However, ASTM C423 does not define octave band sound absorption coefficients, only 1/3 octave band values.  Therefore, sound absorption coefficients in the 1/3 octave bands centered around 250, 500, 1,000, and 2,000 Hz are used.  This is further clarified by the terminology standard ASTM 6342.

If the use of only four 1/3 octave band values sounds too narrow for general use, you are correct.  This is one reason why NRC was retired by ASTM many years ago.  However, the NRC rating continues to live on in the minds of architects, so labs still calculate and report it.  Sound Absorptive Average (SAA) was the official replacement1, but it failed to gain widespread use.

These single number ratings should only be used for rough “shotgun” comparisons or for selecting materials for non-critical listening spaces.  Sound absorption coefficients over the full spectrum give a more complete indication of the material’s performance.  It is not possible to derive the full 1/3 octave band performance from the NRC or SAA rating alone.

References:

1ASTM Standard C423, 2017, “Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method,” ASTM International, West Conshohocken, PA, 2017, https://www.astm.org/Standards/C423.htm

2ASTM Standard C634, 2013, “Standard Terminology Relating to Building and Environmental Acoustics,” ASTM International, West Conshohocken, PA, 2013, https://www.astm.org/Standards/C634.htm