Concrete absorption coefficient
A cast concrete floor absorbs α 0.01 at 500 Hz — NRC 0.00, effectively nothing. Coarse, unpainted concrete masonry block absorbs α 0.31 in the same band, at NRC 0.35. Both are "concrete", and most tables publish a single number for the word. That gap is the whole reason this page exists: if you are planning a basement, garage or concrete-framed room, the figure you need depends entirely on which concrete you actually have — and on whether anyone has painted it. Here are the sourced octave-band coefficients pulled straight from our absorption database, plus a reverberation model of the same room under three different shells. We don't sell panels or building materials.
Concrete and masonry 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 first and third rows are the same base material in different surface conditions.
| Surface | 125 Hz | 250 Hz | 500 Hz | 1k Hz | 2k Hz | 4k Hz | NRC |
|---|---|---|---|---|---|---|---|
| Concrete or terrazzo floor | 0.01 | 0.01 | 0.01 | 0.02 | 0.02 | 0.02 | 0.00 |
| Concrete masonry block, painted/sealed | 0.10 | 0.05 | 0.06 | 0.07 | 0.09 | 0.08 | 0.05 |
| Concrete masonry block, coarse/unpainted | 0.36 | 0.44 | 0.31 | 0.29 | 0.39 | 0.25 | 0.35 |
| Plaster on masonry | 0.01 | 0.01 | 0.02 | 0.03 | 0.04 | 0.05 | 0.05 |
| Brick, unglazed | 0.03 | 0.03 | 0.03 | 0.04 | 0.05 | 0.07 | 0.05 |
Source: Standard architectural-acoustics coefficient tables (ISO 354 / ASTM C423 measured). See every value and its source in the full database or download the CSV (CC BY 4.0).
"Concrete" is four different acoustic materials
The single most common mistake in room planning is taking a published "concrete" coefficient and applying it to a wall it does not describe. The rows above separate into four cases, and they are not interchangeable:
- Solid cast concrete (floors, slabs, poured walls, tilt-up panels) — α 0.01–0.02, NRC 0.00. Dense, closed, reflective at every frequency. This is the row that applies to a poured wall or a slab floor.
- Coarse concrete masonry block (lightweight cinder or breeze block, left bare) — NRC 0.35. The open pore structure of lightweight aggregate makes the surface genuinely porous, and it behaves like a shallow porous absorber.
- Painted or sealed concrete block — NRC 0.05. Same wall, pores closed.
- Plastered or rendered masonry — α 0.02 at 500 Hz, NRC 0.05, essentially identical to unglazed brick (NRC 0.05). A skim coat converts a porous block wall into a hard one.
Note what this means for NRC as a shorthand: painted block, plaster on masonry and unglazed brick all score NRC 0.05, and so does gypsum board on studs — which absorbs α 0.29 at 125 Hz against 0.01 for the plastered wall it is rated equal to. Single-number ratings collapse exactly the distinctions that matter here; see what NRC actually measures.
The same room, three shells
Coefficients on their own are hard to weigh. This is an empty 4.2 × 3.6 × 2.4 m room (36.3 m³, 68 m² of surface) — a typical single garage or basement room — computed with the same engine behind our RT60 calculator. The floor is exposed concrete in all three cases; only the walls and ceiling change. Figures are Sabine RT60 in seconds.
| Shell | 125 Hz | 250 Hz | 500 Hz | 1k Hz | 2k Hz | 4k Hz |
|---|---|---|---|---|---|---|
| Block walls, painted or sealed | 1.44 | 2.69 | 2.16 | 1.81 | 1.47 | 1.62 |
| Block walls, coarse and unpainted | 0.42 | 0.35 | 0.48 | 0.51 | 0.38 | 0.59 |
| Stud-framed and plasterboarded | 0.44 | 1.08 | 1.94 | 2.43 | 1.66 | 1.32 |
Planning estimate. An empty room with no furniture, no air absorption and no treatment is a deliberately bare case — real rooms are better than this. Sabine's equation also overstates reverberation as mean absorption rises, so the bare-block row is the least reliable of the three in absolute terms (its Eyring value is 0.44 s at 500 Hz against 0.48 s Sabine). The ratio between shells is the robust part, and it is what the surface finish is worth. See our methodology & sources.
Bare block against the materials sold as treatment
Put the coarse-block row next to the porous absorbers and the shape of the comparison is unusual: block is competitive in the low-mid range and falls behind everywhere above it.
| Material | 125 Hz | 250 Hz | 500 Hz | 1k Hz | 2k Hz | 4k Hz | NRC |
|---|---|---|---|---|---|---|---|
| Concrete masonry block, coarse/unpainted | 0.36 | 0.44 | 0.31 | 0.29 | 0.39 | 0.25 | 0.35 |
| Mineral wool, 50 mm, on wall | 0.15 | 0.55 | 0.90 | 1.00 | 1.00 | 1.00 | 0.85 |
| Open-cell acoustic foam, 50 mm | 0.08 | 0.25 | 0.60 | 0.90 | 0.95 | 0.98 | 0.70 |
| Carpet, heavy, on foam pad | 0.08 | 0.24 | 0.57 | 0.69 | 0.71 | 0.73 | 0.55 |
| Concrete masonry block, painted/sealed | 0.10 | 0.05 | 0.06 | 0.07 | 0.09 | 0.08 | 0.05 |
| Concrete or terrazzo floor | 0.01 | 0.01 | 0.01 | 0.02 | 0.02 | 0.02 | 0.00 |
At 125 Hz coarse block (α 0.36) is ahead of 50 mm mineral wool (0.15), 50 mm acoustic foam (0.08) and heavy carpet on a pad (0.08). By 1 kHz mineral wool is at α 1.00 and the block has fallen to 0.29. Bare block is therefore useful bulk absorption in the range small rooms struggle with, but it is not a treatment plan: the top three octaves, where speech intelligibility and flutter echo live, are still entirely the job of porous panels.
Absorption is not soundproofing
Concrete is the material where this distinction causes the most confusion, because it is genuinely excellent at one of the two. Transmission loss between rooms is governed largely by mass, and a masonry wall has a great deal of it — which is why concrete construction really does keep neighbours' noise out. Absorption is a surface property and is governed by porosity, which a sealed masonry wall has none of. The result is the combination people find so counterintuitive: a concrete room can be genuinely quiet and still sound cavernous, because the two properties are independent. Adding porous panels will not improve isolation, and adding mass will not shorten reverberation.
What this means for treating your room
- Find out which concrete you have before using any coefficient. Poured wall or slab: use the concrete or terrazzo floor row. Lightweight block: check whether it is sealed, because that decides between NRC 0.35 and 0.05.
- If the block is bare, think twice before painting it. In the modelled room that decision alone is worth about 4.5× in mid-band RT60, and it is far cheaper to leave a wall unpainted than to buy back the absorption in panels afterwards.
- Budget more absorption than a generic panel count suggests for a sealed masonry room. The structure is contributing essentially nothing, which is not the assumption a typical "how many panels" rule of thumb is built on.
- Expect a bass problem specifically. A masonry shell returns 90% of the energy at 125 Hz, so modes ring long. A stud-framed room of identical dimensions does not behave the same way.
- Hard floors compound it. Vinyl or linoleum over concrete is NRC 0.05 — it changes the feel underfoot and almost nothing acoustically.
Plan the rest of your room
A concrete or masonry shell changes how much treatment the room needs, so it is worth putting real numbers on it rather than guessing. Size the absorption with the acoustic panel calculator, then check the result against a reverberation target with the RT60 calculator — it lets you set concrete, block or any other material in the database as a surface and see the per-band effect, so you can compare your actual shell against the three modelled above. Because masonry rooms are bass-heavy, find the problem frequencies first with the room mode calculator.
Frequently asked questions
What is the absorption coefficient of concrete?
For a solid concrete or terrazzo floor the sourced octave-band coefficients are α 0.01 / 0.01 / 0.01 / 0.02 / 0.02 / 0.02 at 125 / 250 / 500 / 1000 / 2000 / 4000 Hz, giving an NRC of 0.00. That is as close to a perfect reflector as a real building material gets: it returns about 99% of the sound energy that reaches it at 500 Hz. But "concrete" is not one material acoustically — coarse unpainted concrete masonry block is α 0.31 in the same band, at NRC 0.35 against 0.00 for the slab.
Does concrete absorb sound?
Solid cast concrete does not — α 0.01 to 0.02 across the spectrum, NRC 0.00. Coarse, unpainted concrete masonry block does, and substantially: NRC 0.35, with α 0.44 at 250 Hz. The difference is porosity, not the cement. Open pores in lightweight block let air move and dissipate energy in exactly the way a porous absorber does; a dense cast slab has no such path.
Why does painting a concrete block wall change its absorption so much?
Because the absorption lives in the surface pores, and paint fills them. The same block measures NRC 0.35 coarse and NRC 0.05 once painted or sealed — a factor of 7. At 250 Hz it falls from α 0.44 to 0.05. Nothing structural changes; the pore openings simply close.
Is a concrete basement or garage a bad room for a studio?
If the walls are sealed, yes — and by a large margin. Modelled as an empty 4.2 × 3.6 × 2.4 m room, a painted-block shell gives a mid-band (500 Hz) Sabine RT60 of about 2.16 s, against roughly 0.48 s for the identical room with the block left bare. Critical-listening targets sit at 0.2–0.3 s, so the sealed room is an order of magnitude away and the bare-block room is within reach of it.
Does coarse concrete block absorb more bass than acoustic foam?
At 125 Hz, yes. Coarse block is α 0.36 against 0.08 for 50 mm acoustic foam and 0.15 for 50 mm mineral wool. Above 500 Hz the porous absorbers pull far ahead — mineral wool reaches α 1.00 at 1 kHz where the block is 0.29. Bare block is a broad mid-band absorber, not a substitute for treatment.
Does concrete block soundproof a room?
Isolation and absorption are different properties, and concrete is strong at one and weak at the other. A dense masonry wall is an excellent sound barrier because it is heavy, which is what blocks transmission between rooms. That same wall absorbs almost nothing inside the room once sealed (NRC 0.05). A concrete room can therefore be very quiet from the outside and sound terrible on the inside — those are two separate problems with two separate solutions.
What should I do about a concrete room?
Treat the surface finish as a decision, not a given: if the block is already bare, leaving it unpainted is free absorption worth roughly 4.5× in mid-band reverberation time in the modelled room. Where the shell is already sealed, the whole absorption budget has to be added — plan for more porous absorption than a generic panel count suggests, since the structure contributes essentially none. A sprayed acoustic plaster (α 0.66 at 500 Hz, NRC 0.60) is one way to keep a masonry look while restoring the absorption paint removed.
Related material guides
- Acoustic foam absorption coefficient — NRC by thickness and foam type — and why it barely touches bass.
- 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.
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.