Absorption Coefficients vs Noise Reduction Coefficients (N.R.C.)

Absorption Coefficients are values that indicate how much sound energy a material absorbs at specific frequencies. Different materials will have different absorption coefficients at different frequencies.

A Noise Reduction Coefficient (NRC) is a single-number rating that attempts to summarize a material's sound absorption abilities. It is calculated by averaging a material’s sound absorption coefficients at four mid-range frequencies: 250 Hz, 500 Hz, 1,000 Hz, and 2,000 Hz.

While convenient for quick comparisons, NRC has significant limitations and can be a misleading metric. One issue is that it omits a material's absorption capabilities below 250 Hz or above 2000 Hz. This could be important information depending on the job at hand.

Because NRC is an average of only these four frequencies, materials with drastically different absorption properties across the broader frequency spectrum can receive the same overall NRC rating. For example, half-inch drywall on studs and painted concrete block might both have an NRC of 0.05. However, their performance at individual frequencies differs substantially. At 125 Hz, drywall has an absorption coefficient of 0.29, while concrete block has an absorption coefficient of 0.10, indicating that drywall is significantly better at absorbing this lower frequency. Relying solely on the NRC would incorrectly suggest that these materials perform acoustically the same.

A far more informative approach is to examine a full absorption coefficient chart, which details a material's performance across a wider range of frequencies. This provides a comprehensive understanding that is crucial for making effective acoustical treatment decisions. The availability of an NRC rating typically indicates that detailed absorption coefficient testing was conducted, so more comprehensive results should be available.

It's also worth noting that absorption coefficients, from which NRC is derived, are not percentages of sound absorbed but are relative to a perfectly reflective surface. These coefficients can even exceed one. For instance, a material might have an absorption coefficient of 1.2 at a specific frequency, which doesn't mean it absorbs 120% of the sound. Consequently, because NRC is an average of these coefficients, a material can have an NRC value greater than 1.

Absorption coefficients are generated through measurements conducted in specialized acoustical laboratories, such as Riverbank Acoustical Labs. These labs utilize a Reverb Time Chamber to test different types of absorption materials.

The process involves filling the Reverb Time Chamber with sound energy and then measuring how the sound decays over time. This is done in a large, hard-surfaced room where sound reflects multiple times. The reverberation time of the empty chamber is first measured. Then, a sample of the material being tested is placed inside the chamber, and the reverberation time is measured again.
By comparing the reverberation time of the empty chamber to that with the test material, the absorption coefficient of the material at different frequencies can be determined.

Pictured below are our Flex-48 and FulFill Panels in the chamber at Riverbank for testing.