Mineral wool absorption coefficient
Mineral wool — also sold as stone wool, rock wool or slag wool — is the workhorse fill for DIY acoustic panels and bass traps. Here are its sourced octave-band coefficients (α) and NRC by thickness and with an air gap — pulled straight from our absorption database — and a straight answer to the question that trips up most first-time panel builders: how much density actually matters. We don't sell panels or wool.
Mineral wool absorption coefficients by octave band
Random-incidence coefficients measured per ISO 354 / ASTM C423. α is the fraction of incident sound energy absorbed (1.00 = fully absorbed). NRC is the average of the 250 / 500 / 1000 / 2000 Hz values. The last row is a 50 mm slab spaced off the wall by a 50 mm air gap.
| Mineral wool | 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, 100 mm, on wall | 0.45 | 0.95 | 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 |
| 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. See every value and its source in the full database or download the CSV (CC BY 4.0).
Thickness and an air gap beat density
The most common shopping mistake is chasing high-density slabs believing denser wool absorbs more. It doesn't work that way. A porous absorber turns sound into heat through the friction of air moving inside the fibres, and that mechanism is governed by airflow resistivity — which has a broad sweet spot. Ordinary acoustic mineral wool (roughly 30–70 kg/m³) already sits in it. Push the density much higher and the surface starts to reflect rather than admit sound; go too light and sound passes straight through. Within the normal range, density is a minor variable.
What actually moves low-frequency absorption is depth. A porous absorber only becomes efficient once its thickness approaches a quarter of the wavelength, and bass wavelengths are metres long. The table above shows it directly: at 125 Hz the same wool climbs from α 0.15 at 50 mm to 0.45 at 100 mm to 0.65 at 150 mm.
Mineral wool vs. rigid fiberglass vs. foam
At the same 50 mm thickness, mineral wool and rigid fiberglass (Owens Corning 703) perform almost identically and both leave open-cell foam behind in the low mids — the region where a small room's problems usually live. Foam only catches up in the treble.
| 50 mm material | 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 |
| Rigid fiberglass 703, 50 mm | 0.17 | 0.86 | 1.00 | 1.00 | 1.00 | 1.00 | 0.95 |
| Open-cell acoustic foam, 50 mm | 0.08 | 0.25 | 0.60 | 0.90 | 0.95 | 0.98 | 0.70 |
Sources: Knauf / Rockwool mineral-wool acoustic board published datasheets. Owens Corning 700-series rigid fiberglass board published datasheet. Cox, T. J. & D’Antonio, P. — Acoustic Absorbers and Diffusers (3rd ed., CRC Press). Compare against acoustic foam in detail.
Rockwool, rock wool, stone wool, slag wool — what the names mean
The single most confusing thing about buying this material is that it is sold under at least five names, and shoppers reasonably assume the names describe different products. Mostly they don't.
- Mineral wool is the material class: fibres spun from molten mineral.
- Stone wool and rock wool are the same thing as each other — mineral wool spun from rock. Slag wool is spun from smelter slag instead. Both are mineral wool.
- ROCKWOOL and Knauf are manufacturers, not material types. "Rockwool" is frequently used generically in DIY discussions to mean any stone-wool board. The coefficients on this page come from exactly that kind of source: Knauf / Rockwool mineral-wool acoustic board published datasheets.
- Glass wool, usually sold as rigid fiberglass (the 703 row above), is a genuinely different fibre — but it is the same kind of porous absorber and obeys the same physics.
So does the fibre actually change the result? At 125 Hz, where small rooms have their real problems, mineral wool and rigid fiberglass 703 differ by α 0.02 at the same 50 mm — while simply going from 50 mm to 100 mm of the same wool changes it by α 0.30, about 15× more. In the bass, the thickness on the label matters and the name on the label essentially doesn't.
There is one honest exception, and it is worth knowing before you buy: at 250 Hz the two separate by α 0.31 — and there Rigid fiberglass 703 is the one ahead. That is the single band where the choice of fibre shows up rather than the thickness.
What mineral wool is good for
- Broadband wall panels at first reflection points — 50–100 mm covers mids and highs fully.
- Bass traps in corners, where thickness and boundary loading do the most for low frequencies.
- Whole-room RT60 control, giving far more absorption per m² than foam of the same thickness.
What to keep in mind
- Absorption is not soundproofing. Wool tames reflections inside a room; it does not stop sound reaching the next room, which needs mass and decoupling.
- Wrap it. Loose fibres shed — a breathable fabric cover keeps performance while containing the wool.
- Don't over-compress it. Squeezing wool to raise density past the normal range gains nothing acoustically and can reduce it.
Plan mineral wool into your room
To size the coverage your room needs and see where it goes, use the acoustic panel calculator, then check the result against a reverberation target with the RT60 calculator — it lets you pick mineral wool (or any material in the database) as a surface and see the per-band effect. For corner bass traps, confirm your room's problem frequencies first with the room mode calculator.
Frequently asked questions
What is the absorption coefficient of mineral wool?
A 50 mm mineral-wool slab on a wall has published octave-band coefficients rising from about α 0.15 at 125 Hz to 0.90 at 500 Hz and 1.00 at 1 kHz — an NRC near 0.85. Values are random-incidence figures measured per ISO 354 / ASTM C423; a 100 mm slab reaches α 0.45 at 125 Hz.
What density of mineral wool should I use for acoustic panels?
Absorption depends on airflow resistivity, which has a broad optimum — this is why common acoustic mineral wool sits around 30–70 kg/m³ (e.g. semi-rigid slabs). Going much denser does not absorb more and can start to reflect; going too light lets sound pass through. Once you are in that normal range, thickness and an air gap change low-frequency absorption far more than density does.
Is thicker mineral wool better for bass?
Yes, and it is the single biggest lever. At 125 Hz the same material goes from α 0.15 at 50 mm to 0.45 at 100 mm to 0.65 at 150 mm — roughly quadrupling low-frequency absorption — because a porous absorber only works once its depth approaches a quarter of the wavelength.
Does leaving an air gap behind mineral wool help?
A lot, for free. Mounting a 50 mm slab with a 50 mm air gap raises its 125 Hz coefficient from α 0.15 to 0.40 — close to a solid 100 mm slab (α 0.45) using half the material. The gap places the absorber where air is moving fastest for low frequencies.
Is Rockwool the same as mineral wool?
Effectively, yes, for planning purposes. ROCKWOOL is a manufacturer of stone wool, and stone wool (also called rock wool) is a type of mineral wool — so a "Rockwool" board in a DIY thread is normally just stone-wool mineral board. Slag wool, spun from smelter slag rather than rock, is the other common form of mineral wool. What changes the absorption you get is the thickness, the air gap behind it and the airflow resistivity — not which of these names is printed on the packaging.
Is mineral wool the same as fiberglass?
No — mineral wool is spun from rock or slag, while rigid fiberglass such as Owens Corning 703 is glass wool — but both are porous absorbers governed by the same physics, and at 50 mm they behave very similarly. At 125 Hz they differ by only α 0.02, whereas going from 50 mm to 100 mm of the same mineral wool changes 125 Hz by α 0.30. The clearest difference between them is at 250 Hz, where they separate by α 0.31.
Mineral wool vs rigid fiberglass (703) vs acoustic foam — which absorbs more?
At 50 mm, mineral wool (NRC 0.85) and rigid fiberglass 703 (NRC 0.95) are close and both clearly beat open-cell foam (NRC 0.70), especially in the low mids: α 0.55 and 0.86 versus 0.25 at 250 Hz. Mineral wool and 703 are the standard DIY panel fills; foam is a high-frequency-only material.
Related material guides
- Acoustic foam absorption coefficient — NRC by thickness and foam type — and why it barely touches bass.
- 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 101 · Browse all material absorption data · See our methodology & sources.