Sound
Noise Reduction Coefficient
NRC
A single number describing how much sound a surface absorbs, averaged across four octave bands from 250 to 2000 Hz. An absorption rating, which is a different property from isolation.
In more detail
NRC describes how much sound a surface absorbs, not how much it blocks. A specimen is measured in a reverberation room under ASTM C423, and the absorption coefficients at 250, 500, 1000 and 2000 Hz are averaged and rounded to the nearest 0.05. An NRC of 0.85 means roughly 85 percent of incident energy in that range is not reflected back.
The four-band average is the limitation. It ignores everything below 250 Hz, where thin absorbers do very little, and everything above 2000 Hz. Two materials at the same NRC can behave quite differently across the spectrum, and low-frequency absorption depends mostly on thickness and air gap — which no single number conveys.
Values above 1.0 appear in published data and are not errors. They arise from edge diffraction in the test, which makes the specimen behave as though it were slightly larger than its measured area. SAA, averaged over twelve bands, is the more detailed figure where it is published.
Why it matters
Absorption is what makes a hard, glassy room habitable — it shortens reverberation, lowers the effort of conversation, and takes the edge off kitchens and stairwells. It is also the acoustic tool most often reached for when the real problem is isolation. Panels change how a room sounds to the people inside it. They do not meaningfully change what the neighbors hear.
The common misunderstanding
That absorption and isolation are the same thing. Foam on a wall does not stop sound passing through it; a dense, sealed, decoupled wall does, and it may absorb almost nothing. Rooms needing both need two separate interventions — and the panels are the cheaper and more visible of the two, which is why they get installed for the wrong problem.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.