NRC (noise reduction coefficient), explained
NRC is a single number: the average sound absorption coefficient (α) of a material at 250, 500, 1000 and 2000 Hz. It is handy for a quick comparison — but because it leaves out 125 Hz, a material with a high NRC can still absorb almost no bass. Here's how NRC is calculated, what a "good" value is, and how to read past it — all backed by our sourced absorption database. We don't sell panels.
The short version
- NRC = one number, the average of α at 250 / 500 / 1000 / 2000 Hz, rounded to the nearest 0.05, on a 0–1 scale.
- It excludes 125 Hz, so it says nothing about bass. Foam 50 mm has NRC ≈ 0.70 but only α 0.08 at 125 Hz.
- For bass, read the 125 Hz column, not the NRC. Two materials with the same NRC can differ by 3–4× at 125 Hz.
- Use α (the full per-band curve) to design; use NRC only for a fast, rough comparison.
How NRC is calculated
NRC is defined as the arithmetic mean of the absorption coefficients at the four mid octave bands — 250, 500, 1000 and 2000 Hz — rounded to the nearest 0.05. The 125 Hz and 4000 Hz bands are simply not part of the formula. Worked through one real row from the database, fabric-wrapped panel, 50 mm:
| Band | 250 Hz | 500 Hz | 1 kHz | 2 kHz | Mean | NRC |
|---|---|---|---|---|---|---|
| α | 0.55 | 0.95 | 1.00 | 1.00 | 0.88 | 0.90 |
( 0.55 + 0.95 + 1.00 + 1.00 ) ÷ 4 = 0.88, rounded to 0.90. Source: Knauf / Rockwool mineral-wool acoustic board published datasheets.
Why NRC hides bass — the same NRC, very different low end
Because NRC ignores 125 Hz, it cannot distinguish a genuine low-frequency absorber from a material that only works in the mids and highs. Below are database materials that all reach a strong NRC (≥ 0.85), sorted by their 125 Hz coefficient. Notice the NRC column barely moves while the 125 Hz column ranges from α 0.12 to 0.65 — a 5× spread the NRC completely hides.
| Material (NRC ≥ 0.85) | 125 Hz | 250 Hz | 500 Hz | 1k Hz | 2k Hz | 4k Hz | NRC |
|---|---|---|---|---|---|---|---|
| Polyester acoustic batt, 50 mm | 0.12 | 0.45 | 0.85 | 1.00 | 1.00 | 1.00 | 0.85 |
| Mineral wool, 50 mm, on wall | 0.15 | 0.55 | 0.90 | 1.00 | 1.00 | 1.00 | 0.85 |
| Fabric-wrapped panel, 50 mm | 0.16 | 0.55 | 0.95 | 1.00 | 1.00 | 0.98 | 0.90 |
| Rigid fiberglass 703, 50 mm | 0.17 | 0.86 | 1.00 | 1.00 | 1.00 | 1.00 | 0.95 |
| Mineral wool, 50 mm, 50 mm air gap | 0.40 | 0.85 | 1.00 | 1.00 | 1.00 | 1.00 | 0.95 |
| Mineral wool, 100 mm, on wall | 0.45 | 0.95 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
| Fabric panel, 50 mm + 100 mm gap | 0.45 | 0.90 | 1.00 | 1.00 | 1.00 | 0.98 | 1.00 |
| Cylindrical tube trap (corner, ~280 mm) | 0.45 | 0.70 | 0.85 | 0.90 | 0.85 | 0.80 | 0.85 |
| Broadband corner trap (300 mm) | 0.50 | 0.85 | 1.00 | 1.00 | 1.00 | 1.00 | 0.95 |
| Fabric-wrapped panel, 100 mm | 0.52 | 0.96 | 1.00 | 1.00 | 1.00 | 0.99 | 1.00 |
| Superchunk corner (mineral wool) | 0.60 | 0.95 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
| Rigid fiberglass 703, 100 mm | 0.65 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
| Mineral wool, 150 mm, on wall | 0.65 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
The 125 Hz column is highlighted. Every value is rendered from the absorption database; download the CSV (CC BY 4.0) to sort it yourself.
Look up an NRC value: what each rating hides
The table above only covers strong absorbers (NRC ≥ 0.85), so it cannot show what a low NRC conceals. This one covers every NRC value that actually occurs in the database — 19 distinct ratings across 60 materials — with the range of 125 Hz absorption found inside each one. If you arrived looking up a specific rating, find your number in the first column.
| NRC | Materials | α at 125 Hz | Spread | What that rating does not tell you |
|---|---|---|---|---|
| 1.00 | 6 | 0.45 – 0.65 | — | Rigid fiberglass 703, 100 mm (best bass) … Fabric panel, 50 mm + 100 mm gap (worst) |
| 0.95 | 3 | 0.17 – 0.50 | 2.9× | Broadband corner trap (300 mm) (best bass) … Rigid fiberglass 703, 50 mm (worst) |
| 0.90 | 1 | 0.16 | — | Fabric-wrapped panel, 50 mm |
| 0.85 | 3 | 0.12 – 0.45 | 3.8× | Cylindrical tube trap (corner, ~280 mm) (best bass) … Polyester acoustic batt, 50 mm (worst) |
| 0.80 | 1 | 0.39 | — | Audience, occupied upholstered seats (per m²) |
| 0.75 | 1 | 0.55 | — | Suspended acoustic tile + plenum |
| 0.70 | 4 | 0.08 – 0.34 | 4.3× | Mineral-fiber ceiling tile (15 mm) (best bass) … Open-cell acoustic foam, 50 mm (worst) |
| 0.65 | 1 | 0.22 | — | Mattress, bare |
| 0.60 | 4 | 0.05 – 0.17 | 3.4× | Acoustic plaster, sprayed (~25 mm) (best bass) … PET felt panel, 24 mm (double layer) (worst) |
| 0.55 | 3 | 0.05 – 0.19 | 3.8× | Upholstered seat, fabric, empty (best bass) … Convoluted egg-crate foam, 50 mm (worst) |
| 0.50 | 1 | 0.05 | — | Open-cell acoustic foam, 25 mm |
| 0.45 | 5 | 0.03 – 0.18 | — | Adult, standing (per person, sabins) (best bass) … PET felt panel, 12 mm (worst) |
| 0.35 | 4 | 0.04 – 0.36 | — | Concrete masonry block, coarse/unpainted (best bass) … Area rug, medium pile, on hard floor (worst) |
| 0.30 | 1 | 0.02 | — | Carpet, thin, on concrete |
| 0.20 | 2 | 0.30 – 0.75 | 2.5× | Tuned membrane trap (~60 Hz) (best bass) … Wood paneling on battens (worst) |
| 0.15 | 3 | 0.25 – 0.35 | — | Ordinary window glass (best bass) … OSB / chipboard panel (10 mm, air gap) (worst) |
| 0.10 | 3 | 0.14 – 0.20 | — | Hardboard (3 mm) on battens, air gap (best bass) … Solid wood door (worst) |
| 0.05 | 12 | 0.01 – 0.29 | — | Gypsum board (12 mm on studs) (best bass) … Plaster on masonry (worst) |
| 0.00 | 2 | 0.01 | — | Marble or glazed tile (best bass) … Concrete or terrazzo floor (worst) |
Every row is computed from the absorption database at build time. "Spread" is simply the highest 125 Hz coefficient in that row divided by the lowest, so you can check it against the two numbers next to it. It is shown only where it reaches 1.5× and where the lower value is at least α 0.05 — below that, a ratio is an artefact of rounding a near-zero coefficient rather than a real difference. The widest honest ratio is at NRC 0.70: α 0.08 to 0.34 at 125 Hz, a 4.3× difference between materials that retailers and spec sheets will all describe with the same single number. Reading down the first column also answers the question the rating cannot: the lowest-rated material here that still absorbs real bass is gypsum board (12 mm on studs), at NRC 0.05 but α 0.29 at 125 Hz.
Every material's NRC, and whether it helps with bass
The full lookup, ranked by NRC and then by 125 Hz absorption. The last column applies one fixed rule to the 125 Hz coefficient — nothing more — so you can ignore it and read the α value yourself.
| Material | Category | NRC | α 125 Hz | Useful for bass? |
|---|---|---|---|---|
| Rigid fiberglass 703, 100 mm | Porous absorbers | 1.00 | 0.65 | Yes |
| Mineral wool, 150 mm, on wall | Porous absorbers | 1.00 | 0.65 | Yes |
| Superchunk corner (mineral wool) | Bass traps | 1.00 | 0.60 | Yes |
| Fabric-wrapped panel, 100 mm | Acoustic panels | 1.00 | 0.52 | Yes |
| Mineral wool, 100 mm, on wall | Porous absorbers | 1.00 | 0.45 | Yes |
| Fabric panel, 50 mm + 100 mm gap | Acoustic panels | 1.00 | 0.45 | Yes |
| Broadband corner trap (300 mm) | Bass traps | 0.95 | 0.50 | Yes |
| Mineral wool, 50 mm, 50 mm air gap | Porous absorbers | 0.95 | 0.40 | Yes |
| Rigid fiberglass 703, 50 mm | Porous absorbers | 0.95 | 0.17 | No |
| Fabric-wrapped panel, 50 mm | Acoustic panels | 0.90 | 0.16 | No |
| Cylindrical tube trap (corner, ~280 mm) | Bass traps | 0.85 | 0.45 | Yes |
| Mineral wool, 50 mm, on wall | Porous absorbers | 0.85 | 0.15 | No |
| Polyester acoustic batt, 50 mm | Porous absorbers | 0.85 | 0.12 | No |
| Audience, occupied upholstered seats (per m²) | Soft furnishings | 0.80 | 0.39 | Partly |
| Suspended acoustic tile + plenum | Ceiling | 0.75 | 0.55 | Yes |
| Mineral-fiber ceiling tile (15 mm) | Ceiling | 0.70 | 0.34 | Partly |
| Perforated wood panel + backing | Acoustic panels | 0.70 | 0.25 | Partly |
| Rigid fiberglass 703, 25 mm | Porous absorbers | 0.70 | 0.11 | No |
| Open-cell acoustic foam, 50 mm | Porous absorbers | 0.70 | 0.08 | No |
| Mattress, bare | Soft furnishings | 0.65 | 0.22 | Partly |
| Acoustic plaster, sprayed (~25 mm) | Porous absorbers | 0.60 | 0.17 | No |
| Person, seated (per m² of audience) | People | 0.60 | 0.16 | No |
| Curtains, heavy, deep folds | Soft furnishings | 0.60 | 0.14 | No |
| PET felt panel, 24 mm (double layer) | Acoustic panels | 0.60 | 0.05 | No |
| Upholstered seat, fabric, empty | Soft furnishings | 0.55 | 0.19 | No |
| Carpet, heavy, on foam pad | Floors | 0.55 | 0.08 | No |
| Convoluted egg-crate foam, 50 mm | Porous absorbers | 0.55 | 0.05 | No |
| Open-cell acoustic foam, 25 mm | Porous absorbers | 0.50 | 0.05 | No |
| Adult, standing (per person, sabins) | People | 0.45 | 0.18 | No |
| Wood-wool cement board, 25 mm | Porous absorbers | 0.45 | 0.10 | No |
| Carpet, medium, on felt underlay | Floors | 0.45 | 0.05 | No |
| Curtains, lightweight, draped | Soft furnishings | 0.45 | 0.05 | No |
| PET felt panel, 12 mm | Acoustic panels | 0.45 | 0.03 | No |
| Concrete masonry block, coarse/unpainted | Hard surfaces | 0.35 | 0.36 | Partly |
| Filled bookshelf | Soft furnishings | 0.35 | 0.30 | Partly |
| Upholstered seat, leather, empty | Soft furnishings | 0.35 | 0.15 | No |
| Area rug, medium pile, on hard floor | Floors | 0.35 | 0.04 | No |
| Carpet, thin, on concrete | Floors | 0.30 | 0.02 | No |
| Tuned membrane trap (~60 Hz) | Bass traps | 0.20 | 0.75 | Yes |
| Wood paneling on battens | Glass & wood | 0.20 | 0.30 | Partly |
| Ordinary window glass | Glass & wood | 0.15 | 0.35 | Partly |
| Plywood panel (10 mm, air gap) | Glass & wood | 0.15 | 0.28 | Partly |
| OSB / chipboard panel (10 mm, air gap) | Glass & wood | 0.15 | 0.25 | Partly |
| Hardboard (3 mm) on battens, air gap | Glass & wood | 0.10 | 0.20 | Partly |
| Wood floor on joists | Floors | 0.10 | 0.15 | No |
| Solid wood door | Glass & wood | 0.10 | 0.14 | No |
| Gypsum board (12 mm on studs) | Hard surfaces | 0.05 | 0.29 | Partly |
| Heavy plate glass | Glass & wood | 0.05 | 0.18 | No |
| Gypsum board ceiling | Ceiling | 0.05 | 0.15 | No |
| Concrete masonry block, painted/sealed | Hard surfaces | 0.05 | 0.10 | No |
| Parquet on concrete | Floors | 0.05 | 0.04 | No |
| Brick, unglazed | Hard surfaces | 0.05 | 0.03 | No |
| Cork tiles, 25 mm, on solid backing | Hard surfaces | 0.05 | 0.02 | No |
| Vinyl / linoleum on concrete | Floors | 0.05 | 0.02 | No |
| Rubber / PVC sports floor tile | Floors | 0.05 | 0.02 | No |
| Wooden chairs, unoccupied (per m²) | Soft furnishings | 0.05 | 0.02 | No |
| Plaster ceiling | Ceiling | 0.05 | 0.02 | No |
| Plaster on masonry | Hard surfaces | 0.05 | 0.01 | No |
| Marble or glazed tile | Hard surfaces | 0.00 | 0.01 | No |
| Concrete or terrazzo floor | Floors | 0.00 | 0.01 | No |
The bass rule: Yes = α ≥ 0.40 at 125 Hz, Partly = 0.20–0.39, No = below 0.20. It is a reading of one number, not a product recommendation. For the full octave-band curve of any row — 125 Hz through 4 kHz, with its source — see the absorption coefficient database or download the CSV (CC BY 4.0). Values are nominal published figures for planning, not a specific product's test report.
How to actually use NRC
- Quick shortlisting: NRC is fine for a first pass — an NRC below ~0.5 means a material is a weak absorber whatever the band.
- Broadband / speech rooms: for taming echo and improving intelligibility, an NRC around 0.85–1.00 is a strong absorber and NRC is a reasonable guide.
- Bass and room modes: ignore NRC. Read the 125 Hz α, and prefer thick porous slabs, corner traps (superchunk corner (mineral wool) reaches α 0.60 at 125 Hz) or panels on an air gap.
- Designing treatment: feed the full per-band α curve — not the NRC — into an RT60 calculation, so each frequency is handled on its own.
Turn coefficients into a room plan
NRC is a comparison shortcut; a room plan needs the per-band numbers. To see how much absorption your room actually needs and where it goes, use the acoustic panel calculator, then check the result against a reverberation-time target with the RT60 calculator, which uses each material's full α curve — not its NRC — so the bass is handled honestly. If your problem is boom rather than echo, start with the room mode calculator.
Frequently asked questions
What is NRC (noise reduction coefficient)?
NRC is a single number that summarises how absorptive a material is: the average of its sound absorption coefficients (α) at 250, 500, 1000 and 2000 Hz, rounded to the nearest 0.05, on a 0–1 scale. An NRC of 0.00 reflects everything; 1.00 is nominally full absorption. It is a quick comparison figure, not a full description of a material's behaviour.
Does a high NRC mean a material absorbs bass?
No — and this is the most common NRC mistake. NRC only averages the 250–2000 Hz bands, so it says nothing about 125 Hz or below. Open-cell acoustic foam 50 mm thick has an NRC around 0.70, yet it absorbs only about α 0.08 at 125 Hz. A high NRC tells you a material tames echo and brightness; it does not tell you whether it will fix boomy, uneven bass.
What counts as a good NRC?
For broadband problems — speech intelligibility, a bright room, mixing at the mid/high end — an NRC of about 0.85–1.00 marks a strong absorber. But "good" depends on the frequency you need to treat: if your problem is bass or room modes, ignore the NRC and read the 125 Hz column instead, because two materials with the same NRC can differ by 3–4× at 125 Hz.
Which materials have an NRC of 0.95?
In our 60-material database, 3 rows carry an NRC of 0.95: mineral wool, 50 mm, 50 mm air gap (α 0.40 at 125 Hz), rigid fiberglass 703, 50 mm (α 0.17 at 125 Hz), broadband corner trap (300 mm) (α 0.50 at 125 Hz). They are equal on the NRC scale and roughly 1× apart at 125 Hz, which is the clearest possible demonstration that an NRC of 0.95 tells you a material is a strong broadband absorber but not whether it does anything for bass. Read the 125 Hz column before choosing between them.
What is the NRC of carpet?
It depends entirely on the build-up, and the range is wide: carpet, thin, on concrete has an NRC of 0.30, carpet, medium, on felt underlay has an NRC of 0.45, carpet, heavy, on foam pad has an NRC of 0.55. Thickness and a soft underlay do most of the work. Note that even the best carpet in the database absorbs only α 0.08 at 125 Hz — carpet is a mid/high-frequency absorber and is not a bass treatment.
What is the NRC of gypsum board (drywall)?
About 0.05 — near the bottom of the scale, because gypsum board reflects almost everything above 250 Hz. But its NRC badly understates one thing it does do: mounted on studs it absorbs α 0.29 at 125 Hz, because the wall flexes and the cavity behind it behaves as a panel resonator. That is more low-frequency absorption than 50 mm acoustic foam provides (α 0.08), despite foam's much higher NRC of 0.70. It is the single clearest example of NRC hiding the bass.
Can a material with a low NRC absorb bass better than one with a high NRC?
Yes, and our database contains a direct example. The strongest low-frequency absorber in all 60 rows is the tuned membrane trap (~60 Hz) at α 0.75 at 125 Hz — and its NRC is only 0.20. The best 125 Hz figure among the materials scoring a perfect NRC of 1.00 is α 0.65 (rigid fiberglass 703, 100 mm). A tuned membrane trap is built to absorb one narrow low band and deliberately does little in the mids, which is exactly where NRC is measured — so the rating that retailers headline ranks it near the bottom while it outperforms everything else at the frequency it was designed for.
What is the difference between NRC and the absorption coefficient (α)?
α is the full picture: the fraction of energy absorbed measured separately in each octave band, so it shows how absorption changes with frequency. NRC compresses four of those mid bands into one averaged number. Use α (the whole row) when you are designing treatment — especially for bass — and NRC only for a fast, rough comparison between materials.
NRC vs SAA — are they the same?
They are close but not identical. SAA (Sound Absorption Average, defined in ASTM C423) averages twelve one-third-octave bands from 200 to 2500 Hz and is not rounded, whereas NRC averages four octave bands (250–2000 Hz) and is rounded to the nearest 0.05. In practice they usually agree within about 0.05. Both are single-number summaries that, like NRC, exclude the deep bass.
Why do retailers headline the NRC?
Because it is one flattering number. A material can post a high NRC while doing almost nothing at 125 Hz, so quoting NRC alone makes thin, cheap absorbers look better than they are for bass control. We don't sell panels, so the honest advice is to read the per-band α curve — and specifically the 125 Hz value — before you buy.
What the numbers mean: what is the sound absorption coefficient? · New here? Start with acoustic treatment for a small room · Browse all material absorption data · See why thickness and an air gap beat density · Read our methodology & sources.