Acoustic foam absorption coefficient
Acoustic foam absorbs high and mid frequencies well and bass barely at all. Here are the sourced octave-band coefficients (α) and NRC for common foam — pulled straight from our absorption database — plus a straight answer on what foam can and can't do. We don't sell foam.
Foam absorption coefficients by octave band
Random-incidence coefficients measured per ISO 354 / ASTM C423. α = fraction of incident sound energy absorbed (1.00 = fully absorbed). NRC is the average of the 250 / 500 / 1000 / 2000 Hz values.
| Foam | 125 Hz | 250 Hz | 500 Hz | 1k Hz | 2k Hz | 4k Hz | NRC |
|---|---|---|---|---|---|---|---|
| Open-cell acoustic foam, 50 mm | 0.08 | 0.25 | 0.60 | 0.90 | 0.95 | 0.98 | 0.70 |
| Open-cell acoustic foam, 25 mm | 0.05 | 0.12 | 0.30 | 0.65 | 0.85 | 0.95 | 0.50 |
| Convoluted egg-crate foam, 50 mm | 0.05 | 0.18 | 0.45 | 0.70 | 0.85 | 0.90 | 0.55 |
Source: Cox, T. J. & D’Antonio, P. — Acoustic Absorbers and Diffusers (3rd ed., CRC Press). See every value and its source in the full database or download the CSV (CC BY 4.0).
Why foam is weak at bass frequencies
Foam is a porous absorber: it converts sound to heat through friction as air moves inside the open cells. A porous absorber only works well once its thickness approaches about a quarter of the sound's wavelength. High frequencies have short wavelengths, so a few centimetres of foam is plenty — hence α 0.95 at 2 kHz. Bass wavelengths are metres long: 125 Hz is roughly a 2.7 m wavelength, so you'd need on the order of 0.7 m of foam to absorb it. That's why even 50 mm foam manages only α 0.08 at 125 Hz.
How thick would foam have to be? The quarter-wavelength table
That quarter-wavelength rule is arithmetic, not a rule of thumb, so it is worth seeing in full. Wavelength is the speed of sound divided by frequency (λ = c ÷ f, with c = 343 m/s), and a porous absorber wants a depth on the order of λ ÷ 4. The last two columns are the point of the table: how much of that required depth 50 mm of foam actually supplies, and what the dataset measures as a result.
| Band | Wavelength | Depth wanted (λ/4) | 50 mm foam supplies | Measured α |
|---|---|---|---|---|
| 125 Hz | 2.74 m | 686 mm | 7% | 0.08 |
| 250 Hz | 1.37 m | 343 mm | 15% | 0.25 |
| 500 Hz | 0.69 m | 172 mm | 29% | 0.60 |
| 1k Hz | 0.34 m | 86 mm | 58% | 0.90 |
| 2k Hz | 0.17 m | 43 mm | 117% | 0.95 |
| 4k Hz | 0.09 m | 21 mm | 233% | 0.98 |
The two right-hand columns move together until absorption saturates, and that is the whole story of this material. Where 50 mm of foam supplies most of the depth the physics wants, α is high; once it supplies more than the quarter-wavelength — the 2k and 4k Hz rows, at 117% and 233% — α is already near 1.00 and the extra depth buys nothing. At the other end it supplies 7%, and α is 0.08.
Turned around: for foam to be as effective at 125 Hz as it already is at 1 kHz, it would need the same 58% of that band's quarter-wavelength — about 400 mm, or 8 times the thickness of an ordinary tile, on every treated surface. This is not a quality problem that a better brand of foam solves; it is a dimension problem.
NRC 0.70 hides exactly the band that matters
Foam is usually sold on a single NRC figure, and NRC is defined as the average of the 250, 500, 1k, 2k Hz coefficients — nothing else — rounded to the nearest 0.05. For 50 mm foam those four values average 0.675, which the rounding step publishes as NRC 0.70. That reads as a respectable broadband absorber. The bands NRC leaves out are:
- 125 Hz — α 0.08 — the band where small rooms actually have problems.
- 4k Hz — α 0.98 — well above anything a room complains about.
So the one excluded band at the bottom is the one a small-room owner is usually trying to fix, and the averaging window quietly removes it from the headline number. NRC is not a dishonest metric — it was designed for speech-frequency office work, where it is entirely appropriate — but it answers a different question from the one most people buying foam are asking. Read the octave-band row, not the average. Our guide to NRC covers what it does and does not measure.
"Acoustic foam", "studio foam", "soundproofing foam" — what the labels mean
Foam is sold under a tangle of names, only some of which describe anything physical. What actually changes the measured numbers is the material, the thickness and the profile — not the marketing term.
- Acoustic / studio / auralex-style tile foam — normally open-cell polyurethane. These are the same product category, and the coefficients in the table above describe them. The name on the box is not an acoustic property.
- Melamine foam — a different open-cell foam, generally the more capable of the two acoustically, but bound by the same quarter-wavelength ceiling. We publish no coefficients for it because we hold no sourced measurement for it.
- Convoluted / "egg-crate" foam — profiled rather than flat. Because the profile averages to less than its peak height, 50 mm convoluted foam behaves like thinner flat foam: NRC 0.55 against 0.70. The peaks and valleys are often sold as an acoustic advantage; in the measured data they are a thickness penalty.
- Wedge and pyramid profiles — the same trade-off as egg-crate, for the same reason. Any shaped surface that removes material removes absorption with it.
- "Soundproofing foam" — a misnomer, and the most expensive misunderstanding in this whole category. No open-cell foam blocks sound transmission; blocking needs mass and decoupling, which foam has neither of. If your goal is that the neighbours hear less, nothing on this page will help.
- Closed-cell foam (camping mat, packing foam) — absorbs very little, because sound cannot move air through a closed cell. It is not an acoustic absorber at all.
An air gap buys more bass than more foam
A porous absorber works on air velocity, and velocity is at its minimum right against a rigid wall and at its maximum a quarter-wavelength away from it. Foam mounted flat on the wall therefore spends its first centimetres in the least useful place available. Moving the same material off the wall shifts its useful absorption downward in frequency for no extra material at all.
Our dataset documents this for mineral wool rather than foam — the same 50 mm slab, measured on the wall and with a 50 mm gap behind it:
| Mounting | 125 Hz | 250 Hz | 500 Hz | 1k Hz | 2k Hz | 4k Hz | NRC |
|---|---|---|---|---|---|---|---|
| Mineral wool, 50 mm, on wall | 0.15 | 0.55 | 0.90 | 1.00 | 1.00 | 1.00 | 0.85 |
| Mineral wool, 50 mm, 50 mm air gap | 0.40 | 0.85 | 1.00 | 1.00 | 1.00 | 1.00 | 0.95 |
Source: Knauf / Rockwool mineral-wool acoustic board published datasheets.
At 125 Hz the gap multiplies absorption by 2.7× — a larger low-frequency gain than doubling the material's thickness delivers. We are deliberately not restating this as a foam figure, because we have no sourced foam-with-gap measurement and will not publish an unmeasured number. The mechanism is a property of porous absorbers as a class, so the direction applies to foam; the magnitude for foam specifically is not something this site can currently evidence.
How much foam replaces one panel?
Absorption is area multiplied by coefficient, measured in sabins, so materials with different coefficients compare directly by area. "How much foam equals one proper panel" therefore has an exact answer per octave band — it is simply αpanel ÷ αfoam, here against a 100 mm fabric-wrapped panel:
| Octave band | 50 mm foam α | 100 mm panel α | m² of foam per 1 m² of panel |
|---|---|---|---|
| 125 Hz | 0.08 | 0.52 | 6.5× |
| 250 Hz | 0.25 | 0.96 | 3.8× |
| 500 Hz | 0.60 | 1.00 | 1.7× |
| 1k Hz | 0.90 | 1.00 | 1.1× |
| 2k Hz | 0.95 | 1.00 | 1.1× |
| 4k Hz | 0.98 | 0.99 | 1.0× |
From 1 kHz upward the ratio is between 1.0× and 1.1× — foam is very nearly the equal of a proper panel up there, and a perfectly reasonable buy. At 500 Hz it is 1.7×, at 250 Hz 3.8×, and at 125 Hz 6.5×. That progression is the honest summary of foam: competitive where rooms rarely have problems, and increasingly outclassed as you descend into the range where they do. Put in room terms, using the same reference room as our panel calculator (4 × 3 × 2.4 m):
Foam vs. a proper broadband absorber
At the same 50 mm thickness, mineral wool and fabric-wrapped mineral-wool panels absorb far more in the low-mids than foam — the region where a small room's problems usually live. Foam's advantage is only price and convenience, not performance.
| 50 mm material | 125 Hz | 250 Hz | 500 Hz | 1k Hz | 2k Hz | 4k Hz | NRC |
|---|---|---|---|---|---|---|---|
| Open-cell acoustic foam, 50 mm | 0.08 | 0.25 | 0.60 | 0.90 | 0.95 | 0.98 | 0.70 |
| 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 |
Sources: Knauf / Rockwool mineral-wool acoustic board published datasheets. Knauf / Rockwool mineral-wool acoustic board published datasheets.
What acoustic foam is good for
- Flutter echo between parallel hard walls — a few well-placed tiles kill it.
- Early high-frequency reflections at a desk or mic position, cheaply.
- Taming a bright, splashy room where the problem is treble, not bass.
What it is not for
- Soundproofing. Absorption ≠ isolation. Foam won't stop sound reaching the next room.
- Bass and room modes. Use thick porous traps in corners instead.
- Whole-room RT60 control on a budget — thick fabric panels give far more absorption per m².
Plan foam into your room
Foam counts as treatment area in the same way any absorber does. To see how much coverage your room actually needs — and where it goes — use the acoustic panel calculator, then confirm the result against a reverberation target with the RT60 calculator, which lets you pick foam (or any material in the database) as a surface and see the per-band effect.
Frequently asked questions
What is the absorption coefficient of acoustic foam?
Open-cell acoustic foam 50 mm thick has published octave-band coefficients of roughly α 0.60 at 500 Hz rising to 0.95 at 2 kHz, but only about 0.08 at 125 Hz — an NRC near 0.70. Values are random-incidence figures measured per ISO 354 / ASTM C423.
Does acoustic foam absorb bass?
Not meaningfully. At 125 Hz even 50 mm foam absorbs only about α 0.08, because a porous absorber needs a thickness on the order of a quarter-wavelength to work — and a 125 Hz quarter-wavelength is roughly 0.7 m, far more than any wall foam. Bass control needs thick porous corner traps or membrane/panel resonators, not foam.
Is thicker acoustic foam better?
Yes, up to a point. 50 mm foam outperforms 25 mm foam across every band — e.g. α 0.60 vs 0.30 at 500 Hz (NRC 0.70 vs 0.50). Doubling thickness shifts useful absorption lower in frequency, but foam still runs out below ~250 Hz.
Does egg-crate (convoluted) foam work as well as flat foam?
No. Because its profile averages to less than its peak thickness, 50 mm convoluted "egg-crate" foam behaves like thinner flat foam — NRC around 0.55 versus 0.70 for flat 50 mm foam. It is fine for taming flutter echo, not for broadband treatment.
Is acoustic foam the same as soundproofing?
No. Foam absorbs reflections inside a room; it does not stop sound passing between rooms. Isolation (blocking transmission) needs mass, decoupling and air-tight sealing — foam on a wall barely changes how much your neighbour hears.
How thick does acoustic foam need to be?
It depends entirely on the frequency you want to absorb, and the arithmetic is unforgiving. A porous absorber wants roughly a quarter of the wavelength: at 1 kHz that is 86 mm, so 50 mm foam is comfortably thick enough and reaches α 0.90. At 125 Hz the quarter-wavelength is 686 mm — 50 mm of foam is about 7% of the depth required, which is why α collapses to 0.08. No practical wall foam is thick enough for bass.
Does an air gap behind acoustic foam help?
Yes, and it is the cheapest low-frequency upgrade available. Mounting a porous absorber away from the wall places more of it in the region where air is actually moving, which shifts useful absorption downward in frequency. Our dataset documents this for mineral wool: the same 50 mm slab goes from α 0.15 to 0.40 at 125 Hz — a 2.7× gain — purely by leaving a 50 mm gap behind it. We do not publish a measured foam-with-gap row because we have no sourced measurement for one, but the mechanism is a property of porous absorbers generally, not of mineral wool specifically.
What is the NRC of acoustic foam?
About 0.70 for 50 mm foam and 0.50 for 25 mm. Treat that headline figure with care: NRC is the average of the 250, 500, 1000 and 2000 Hz coefficients only, so it excludes the 125 Hz band where foam measures just α 0.08. A single NRC number cannot tell you that a material fails at bass, and for foam that is the single most important thing to know about it.
Is melamine foam better than the studio foam sold in tiles?
Melamine and polyurethane are different open-cell foams, and melamine is generally the more capable acoustic material of the two, but the physics ceiling is the same: both are porous absorbers, so both are governed by thickness relative to wavelength and both run out at low frequencies. We do not publish coefficients for melamine because we have no sourced measurement for it — every value in our database is traceable to a cited source, and we would rather show nothing than an invented number.
Related material guides
- Mineral wool absorption coefficient — Thickness and an air gap beat density — the panel-builder’s material.
- Carpet absorption coefficient — Tames echo, not bass — and why it is not soundproofing.
- Curtains absorption coefficient — Fullness and an air gap are the levers — “soundproof” curtains do not block noise.
- Gypsum board (drywall) absorption coefficient — The one surface in the room that absorbs bass rather than treble — and NRC 0.05 hides it.
- Cork absorption coefficient — Rated the same as bare brick — sold as treatment, measured as a hard surface.
- Concrete absorption coefficient — Four different materials share the name — and painting the block costs you most of it.
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 our methodology & sources.