The noise reduction coefficient, or NRC, is a single number that tells you how much sound a material absorbs rather than bouncing back into a room. It runs on a scale from 0 to 1, where 0 means the material reflects all sound and 1 means it absorbs all sound. The rating is calculated by averaging a material’s absorption performance at four specific frequencies: 250, 500, 1,000, and 2,000 Hz. Those frequencies cover the range of most human speech and everyday noise.
How NRC Is Calculated
To get an NRC rating, a material sample is placed in a specially designed reverberation room, and testers measure how quickly sound decays at each of four octave bands. The test follows ASTM C423, a standardized method maintained by ASTM International. At each frequency, the material receives an absorption coefficient between 0 and 1. The NRC is simply the arithmetic average of those four coefficients, rounded to the nearest 0.05.
So if a material absorbs 40% of sound at 250 Hz, 60% at 500 Hz, 90% at 1,000 Hz, and 90% at 2,000 Hz, its absorption coefficients are 0.40, 0.60, 0.90, and 0.90. Average those and you get 0.70. That’s the NRC. You may occasionally see NRC values above 1.0 on product spec sheets. That doesn’t mean the material absorbs more than 100% of sound. It’s a quirk of how reverberation rooms work, caused by edge diffraction and other measurement effects.
NRC Ratings for Common Materials
The rating gives you a quick way to compare how different surfaces will behave in a room. Here are some typical ranges:
- Smooth concrete, glass, and metal: close to 0.00. These are near-perfect reflectors, bouncing almost all sound back.
- Drywall: 0.05 to 0.15. Standard painted walls absorb very little.
- Carpet: 0.20 to 0.40. Helpful for reducing footstep noise and some mid-range reflections, but far from a full acoustic treatment.
- Acoustic foam: 0.70 to 0.95. Purpose-built to absorb a high percentage of sound energy across the speech frequency range.
A room full of hard, reflective surfaces (tile floors, glass walls, concrete ceilings) will have heavy echo and reverberation. Replacing or covering even one surface with a material that has an NRC of 0.70 or higher makes a noticeable difference in how the room sounds.
What NRC Actually Controls
NRC measures absorption, not blocking. This is the most common point of confusion. A material with a high NRC reduces echo and reverberation inside a room. It does not stop sound from traveling through a wall into the next room. That’s a completely different property, measured by the Sound Transmission Class (STC) rating. STC rates how well a wall, door, or window prevents sound from passing through it. NRC rates how well a surface soaks up sound within the space it faces.
If your goal is to make a conference room less echoey and improve speech clarity, you want materials with high NRC. If your goal is to keep your neighbor from hearing your music, you need materials with high STC. Many acoustic projects require both, but they solve different problems and use different materials in different ways.
Why Two Materials With the Same NRC Can Sound Different
Because NRC is an average of four frequencies, it can hide important differences. Consider two materials that both rate 0.70. Material A absorbs evenly: 0.70 at every tested frequency. Material B absorbs 0.40 at 250 Hz, 0.60 at 500 Hz, 0.90 at 1,000 Hz, and 0.90 at 2,000 Hz. Same NRC, very different behavior.
If your noise problem is a low-frequency hum from HVAC equipment (around 250 Hz), Material A would absorb nearly twice as much of that sound as Material B, despite the identical NRC. This is why professionals often look past the NRC and examine the full absorption coefficient table across all tested frequencies before selecting a material. The NRC is a useful shorthand for general comparison, but it can mislead you when you’re dealing with a specific noise profile.
The four test frequencies also skip everything below 250 Hz and above 2,000 Hz. Deep bass rumble and very high-pitched sounds are invisible to the NRC rating entirely. A material could perform terribly at low frequencies and still earn an impressive NRC if it absorbs well in the mid-range.
SAA: A More Detailed Alternative
The Sound Absorption Average, or SAA, was developed as a more granular version of NRC. Instead of averaging four octave bands, SAA averages twelve one-third octave bands spanning 200 to 2,500 Hz. This wider, more detailed sampling gives a slightly more accurate picture of how a material performs across a broader range. In practice, NRC and SAA values for the same material are often very close or identical, but SAA can reveal differences that NRC smooths over. You’ll find both ratings on spec sheets from most acoustic product manufacturers.
How Installation Affects the Rating
The same material can produce different NRC numbers depending on how it’s mounted during testing. Acoustic ceiling tiles, for example, are often tested in a configuration that simulates a 16-inch-deep air space behind them (called Type E400 mounting). That air gap boosts low-frequency absorption compared to mounting the same tile directly against a hard surface (Type A mounting). When you’re comparing products, check that the NRC values were tested under the same mounting conditions. A panel tested with an air gap behind it will look better on paper than one tested flat against a wall, even if the materials are identical.
This also matters when you install acoustic materials in your own space. A foam panel mounted directly on drywall will absorb less low-frequency sound than the same panel mounted with a few inches of air space behind it. The air gap acts as an additional absorber, particularly for longer sound wavelengths. If a product’s NRC was tested with an air gap and you mount it flush, expect slightly lower real-world performance than the spec sheet suggests.