Porous absorber calculator
Enter what you are actually going to build — a thickness of mineral wool, fibreglass, polyester or foam, and how far off the wall you will mount it — and get the predicted absorption coefficient in every octave band, plus the frequency below which it stops working. Nothing here is for sale, so the curve is not flattered.
What the model actually does
A porous absorber works by making air move through a tortuous, resistive skeleton and turning that motion into heat. How much it absorbs at a given frequency follows from two properties of the material — its characteristic impedance and its complex wavenumber — and both are predicted here from a single measurable quantity, the static airflow resistivity σ.
The empirical fit is Miki's model (1990), a re-fit of the classic Delany–Bazley curves that stays physically realisable at low frequencies where the original misbehaves. The absorber and the air gap behind it are then stacked as a transfer-matrix chain against a rigid wall to get the surface impedance, the reflection coefficient follows from that, and the random-incidence figure is the Paris average of the angle-dependent result over a hemisphere. The same octave bands and the same NRC convention are used everywhere on this site.
- Delany, M. E. & Bazley, E. N. — Acoustical properties of fibrous absorbent materials, Applied Acoustics 3 (1970) 105–116.
- Miki, Y. — Acoustical properties of porous materials — modifications of Delany-Bazley models, J. Acoust. Soc. Jpn. (E) 11 (1990) 19–24.
- Cox, T. J. & D'Antonio, P. — Acoustic Absorbers and Diffusers (3rd ed., CRC Press), for the layer stack and the random-incidence average.
Full assumptions and the rest of the site's formulas are on the methodology page.
Why this reads lower than a datasheet
This is worth stating plainly, because it trips people up and most calculators quietly hide it. The prediction here is for an infinite layer. A published coefficient comes from a reverberation-chamber test on a finite sample, and sound diffracting around the sample's edges gets absorbed too — so the measurement behaves as though the sample were bigger than it is. That is the reason chamber figures saturate at 1.00 and sometimes exceed it, which is physically impossible for a true absorption coefficient.
Checked against the chamber values in this site's own absorption coefficient table, the model tracks the published bass figures closely and runs roughly 0.1–0.25 low through the 250 Hz–1 kHz region. Use the measured table when you are looking up a product that has been tested; use this calculator when you are deciding what to build and no test exists for it.
Designing with it: three decisions that matter
| Decision | What the model shows | Practical read |
|---|---|---|
| Thickness | Doubling thickness moves the "effective from" frequency down by roughly an octave. | The only lever that reaches the bass. Buy depth before anything else. |
| Air gap | A gap equal to the slab thickness gives most of the low-frequency benefit of doubling the slab. | Nearly free. Batten the frame off the wall unless something prevents it. |
| Airflow resistivity | Broad optimum; the curve is flat across roughly 10,000–30,000 Pa·s/m² and falls above it. | Do not pay for density. Very dense board reflects more than it absorbs. |
Once you know the coefficient of the absorber you intend to build, the panel calculator turns it into a panel count and a placement plan, and the RT60 calculator predicts what it does to the reverberation time of your actual room.
Panel (membrane) absorbers: the other kind, and when you need one
Everything above models a porous absorber — a layer that turns air movement into heat through flow resistance. A panel absorber, also called a membrane or diaphragmatic absorber, is a different device: an airtight sheet over a sealed cavity, which absorbs by resonating. Miki's model does not describe it, so this calculator will not give you a curve for one. It is worth knowing which you want, because the two fail in opposite directions.
A porous absorber is broadband but shallow-reaching: it works from its quarter-wavelength frequency upward, and reaching 60 Hz needs depth measured in hundreds of millimetres. A panel absorber is narrowband but deep-reaching: a 100 mm cavity behind a sheet of plywood can be tuned into the low bass, where no porous layer of that depth does anything. If you have a specific low-frequency problem and no room for a corner volume, the panel absorber is the device that fits.
The resonance is a mass on a spring — the sheet is the mass, the trapped air is the spring — so it follows from the sheet's surface mass density m and the cavity depth d alone:
f₀ = (c / 2π) · √( ρ₀ / (m · d) )
With this site's air constants that leading term evaluates to 59.9, which is the textbook f₀ ≈ 60/√(m·d). The table is computed from that formula at build time, not typed in.
| Sheet (surface mass) | 50 mm cavity | 100 mm cavity | 150 mm cavity | 200 mm cavity |
|---|---|---|---|---|
| 3 mm plywood (1.8 kg/m²) | 200 Hz / 167 Hz | 141 Hz / 118 Hz | 115 Hz / 96 Hz | 100 Hz / 83 Hz |
| 6 mm plywood (3.6 kg/m²) | 141 Hz / 118 Hz | 100 Hz / 83 Hz | 82 Hz / 68 Hz | 71 Hz / 59 Hz |
| 9 mm plywood (5.4 kg/m²) | 115 Hz / 96 Hz | 82 Hz / 68 Hz | 67 Hz / 56 Hz | 58 Hz / 48 Hz |
| Mass-loaded vinyl, 5 kg/m² (5 kg/m²) | 120 Hz / 100 Hz | 85 Hz / 71 Hz | 69 Hz / 58 Hz | 60 Hz / 50 Hz |
| 12.5 mm gypsum board (8.8 kg/m²) | 90 Hz / 75 Hz | 64 Hz / 53 Hz | 52 Hz / 44 Hz | 45 Hz / 38 Hz |
| 2 × 12.5 mm gypsum board (17.5 kg/m²) | 64 Hz / 53 Hz | 45 Hz / 38 Hz | 37 Hz / 31 Hz | 32 Hz / 27 Hz |
First figure: sealed, undamped cavity (f₀ = 59.9/√(m·d), derived from c and ρ₀ above). Second figure: cavity filled with a porous blanket, using the conventional design value 50/√(m·d) — a published convention, not a derivation. Fill the cavity in practice: an undamped panel absorber is a sharp, ringing resonance that can do more harm than good, and the blanket both widens the useful band and lowers the tuning.
What this table does not tell you
It gives the centre of the absorption, not its depth or width. A real panel absorber is clamped at its edges rather than limp, which stiffens it and pushes the true resonance above the prediction — commonly by 10–20% for a small, well-fixed panel. Peak absorption is typically 0.6–0.9 with the cavity damped and much less without, over roughly an octave. Treat these numbers as where to aim, then verify by measurement: unlike a porous layer, a panel absorber that is mistuned does very little.
Frequently asked questions
How thick does an acoustic panel need to be?
Thickness sets how low the panel works, and nothing else substitutes for it. A porous absorber starts losing grip roughly below the frequency where its total depth — material plus any air gap — is about a quarter of a wavelength. 50 mm gets you down to the low mids; 100 mm reaches into the upper bass; below about 150 Hz you need depth measured in hundreds of millimetres, which is why bass traps are built as corner volumes rather than thin panels. Set the thickness in the calculator and read the "effective from" figure: it is taken off the modelled curve, not from a rule of thumb.
Does an air gap really work as well as more material?
Partly, and it is close to free. Absorption depends on the air particle velocity inside the porous layer, which is zero at a rigid wall and peaks a quarter wavelength away from it. Mounting the same slab on a gap moves it into that faster-moving region and buys you real low-frequency absorption for the price of a batten. It is not identical to solid material — the calculator will show you a gap trailing an equal thickness of extra absorber at most frequencies — but for a fixed budget of material, spacing it off the wall is usually the single best decision available.
Does density matter?
Less than people think, and not in the direction usually claimed. What the physics actually depends on is airflow resistivity — how hard it is to push air through the material. Density correlates with it loosely within one product family, but the useful range is broad: anywhere around 10,000–30,000 Pa·s/m² behaves well for room treatment. Very dense, very high-resistivity board starts reflecting sound off its face instead of letting it in, so pushing density upward eventually makes the panel worse. Try it: raise the resistivity in the calculator past about 50,000 and watch the curve fall.
Why is this lower than the NRC on the manufacturer datasheet?
Because the two numbers are measured and modelled on different objects. This calculator predicts an infinite, laterally uniform layer. A datasheet figure comes from a reverberation-chamber test (ISO 354 / ASTM C423) on a finite sample, where sound also diffracts around the sample edges and is absorbed there — the sample effectively behaves as though it were larger than it is. That is why chamber results routinely reach 1.00 and can legitimately exceed it. Expect this tool to read roughly 0.1–0.25 lower than a chamber value through the 250 Hz–1 kHz region, and to agree closely in the bass.
Can I use this to design a bass trap?
For a porous corner trap, yes — model it as a thick porous layer with the equivalent depth you can actually fit, and read the low bands. What it will not model is a tuned device: a membrane, a Helmholtz resonator, or a slatted panel absorber all work by resonance rather than by flow resistance, and Miki’s model does not describe them. It also treats the layer as flat and wall-mounted, so a triangular corner fill is an approximation.
Related
- Sound absorption coefficient table — measured chamber values for real materials.
- Mineral wool absorption coefficient — published figures by thickness and air gap.
- Acoustic foam absorption coefficient — and why foam misses the bass.
- Acoustic treatment for a small room — what to fix first.