Sound Absorption Coefficient Table

Octave-band absorption coefficients (α) and NRC for 60 common materials and treatments. Search it, sort any column, and read every value against its source. Free to download and reuse (CC BY 4.0).

A sound absorption coefficient (α) is the fraction of incident sound energy a surface absorbs instead of reflecting, on a 0–1 scale: 0.00 is perfectly reflective and 1.00 is nominally full absorption. It is reported per octave band (125 Hz–4 kHz) because absorption is strongly frequency-dependent — most porous materials absorb treble far better than bass, so read a whole row, not just the NRC.

These are measured chamber values for materials that have been tested. If instead you are designing a panel that does not exist yet — some thickness of wool or fibreglass, mounted some distance off the wall — the porous absorber calculator predicts its coefficient curve from thickness, airflow resistivity and air gap.

The JSON distribution carries the same 60 rows with each row's full source reference attached, plus a field dictionary, the measurement method and the limitations — use it if you are reading this data with anything other than a spreadsheet.

Click any column header to sort — e.g. sort by 125 Hz to rank materials by genuine bass absorption rather than by NRC.

Absorption coefficients by material category

The 60 materials above fall into 9 categories. The filter on the table is interactive, so here is the same breakdown as a fixed summary — how wide each category's range actually is at 125 Hz (bass) and by NRC (broadband), and which member leads on each. Every figure is computed from the table above.

What the categories do and don't tell you

• The category label is a poor guide to bass absorption. The 13 porous absorbers span α 0.05–0.65 at 125 Hz — about a 13-fold difference on the displayed values — while their NRC values span only 0.45–1.00. Thickness and air gap decide the low end, not the material class.
• Only one category is reliably a bass absorber. Every bass trap here reads at least α 0.45 at 125 Hz — higher than any entry under hard surfaces, floors, glass & wood, soft furnishings or people. The best porous absorbers, acoustic panels and ceiling do reach into that range, so a thick porous absorber can match a trap — but nothing thin does.
• "Acoustic" on the label does not mean it absorbs bass. Ordinary window glass reads α 0.35 at 125 Hz, higher than 4 of the 6 acoustic panels listed — glass absorbs low frequencies by flexing, while a thin panel is simply too shallow to act at those wavelengths.
• The most trustworthy category label is bass traps, whose members vary only about 1.7× at 125 Hz (0.45–0.75). Everywhere else, read the row rather than the category.

About this dataset

Coefficients are random-incidence values measured per ISO 354 / ASTM C423. They are nominal published figures compiled from standard references and manufacturer datasheets — for product-specific design, check the current manufacturer datasheet. NRC is the average of the 250/500/1000/2000 Hz values rounded to the nearest 0.05 — a quick comparison figure that ignores 125 Hz, so read the full row for bass. See the methodology page for how these feed the calculators.

New to these numbers? What is the sound absorption coefficient? explains what α means, why it has no unit, how it is measured, and why published values sometimes exceed 1.00.

Deep dives on individual materials: acoustic foam absorption coefficient · mineral wool absorption coefficient · carpet absorption coefficient · curtains absorption coefficient · gypsum board absorption coefficient.

Sources

• Vorländer, M. — Auralization (Springer), standard material tables.
• Cox, T. J. & D’Antonio, P. — Acoustic Absorbers and Diffusers (3rd ed., CRC Press).
• Owens Corning 700-series rigid fiberglass board published datasheet.
• Knauf / Rockwool mineral-wool acoustic board published datasheets.
• Standard architectural-acoustics coefficient tables (ISO 354 / ASTM C423 measured).
• Bies, D. & Hansen, C. — Engineering Noise Control, appendix absorption tables.
• Beranek, L. & Mellow, T. — Acoustics: Sound Fields and Transducers; standard audience/seating absorption tables.

Worth cross-checking against independent databases such as the PTB Absorption Coefficient Database (Physikalisch-Technische Bundesanstalt, Germany). Coefficients here are nominal published values aggregated for planning, not original measurements — spotted one you'd source differently? It's CC BY 4.0, so corrections are welcome.

FAQ

What is a sound absorption coefficient?

A sound absorption coefficient (α) is the fraction of incident sound energy a surface absorbs instead of reflecting, on a 0–1 scale: 0.00 is perfectly reflective and 1.00 is nominally full absorption. It is quoted per octave band because absorption is frequency-dependent — most porous materials absorb high frequencies far better than bass.

Can an absorption coefficient be greater than 1.0?

Yes. The reverberation-room method (ISO 354 / ASTM C423) derives the coefficient from decay times over a sample's geometric area, but sound diffracts around the panel's edges, so a finite specimen can absorb as if it were larger than its measured area. That pushes reported figures above 1.0, especially for thick or edge-exposed samples at mid/high frequencies. It is a known artefact of the test method, not a data error; for planning, treat such values as essentially total absorption.

What counts as a "good" absorption coefficient?

It depends on the frequency you need to treat. For broadband speech or mixing rooms an NRC around 0.90–1.00 marks a strong absorber. For bass control, read the 125 Hz column instead of the NRC — many high-NRC materials still read well below 0.5 at 125 Hz, so click the 125 Hz header to sort the table and rank materials by genuine low-frequency absorption (why NRC hides bass). The mineral wool page shows how thickness and an air gap change the low-frequency figures.

Why do published coefficients for the same material differ between sources?

Absorption depends on how the sample was mounted (direct to a wall vs. over an air gap — ASTM E795 mountings), its thickness and density, and the test lab. The same porous panel mounted with an air gap can read substantially higher at low frequencies than mounted directly on the wall. That is why every row here is individually sourced and why cross-checking an independent database such as the PTB database is worthwhile.

How do I turn these coefficients into a reverberation time (RT60)?

Reverberation time is driven by the total absorption in a room — each surface's area multiplied by its absorption coefficient, summed (the Sabine relationship). Rather than doing it by hand, enter your room and surfaces into the RT60 calculator, which uses these same coefficients, or size broadband panels with the acoustic panel calculator.

Which category of material absorbs bass best?

Bass traps. Every bass trap in this dataset reads at least α 0.45 at 125 Hz — higher than any entry under hard surfaces, floors, glass & wood, soft furnishings or people. The best porous absorbers, acoustic panels and ceiling do overlap that range, so a thick porous absorber can match a trap at 125 Hz — but no thin surface treatment does.

Do all porous absorbers absorb about the same?

No — "porous absorber" is the least informative label in the table. The 13 porous absorbers here span α 0.05 to 0.65 at 125 Hz, roughly a 13-fold difference computed from those displayed values, even though their NRC values span only 0.45 to 1.00. Thickness and air gap drive the low end, not the material class: Rigid fiberglass 703, 100 mm reaches α 0.65 at 125 Hz while thin foam in the same category reads α 0.05.

Does an acoustic panel always absorb more bass than a plain hard surface?

No. Ordinary window glass reads α 0.35 at 125 Hz — higher than 4 of the 6 acoustic panels in this table, because a light panel is too thin to work at long wavelengths while glass absorbs bass by flexing. Judge a panel by its own 125 Hz figure, not by the fact that it is sold as acoustic treatment.