Curtains absorption coefficient
"Soundproof" curtains are one of the most oversold ideas in room acoustics — so it's worth knowing exactly what fabric does and doesn't do. Here are the sourced octave-band coefficients (α) and NRC for lightweight and heavy curtains — pulled straight from our absorption database — plus a straight answer to the two questions curtains always raise: do they kill echo, and do they actually block noise? We don't sell curtains, panels or windows.
Curtain 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. Note how both rows are near-zero at 125 Hz and climb through the mids into the treble — and how much the heavy, deep-folded curtain gains over the light one.
| Curtain | 125 Hz | 250 Hz | 500 Hz | 1k Hz | 2k Hz | 4k Hz | NRC |
|---|---|---|---|---|---|---|---|
| Curtains, lightweight, draped | 0.05 | 0.06 | 0.39 | 0.63 | 0.70 | 0.73 | 0.45 |
| Curtains, heavy, deep folds | 0.14 | 0.35 | 0.55 | 0.72 | 0.70 | 0.65 | 0.60 |
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). "Heavy, deep folds" is fabric gathered to roughly double the track width and hung proud of the wall — the fullness and the air gap behind it are doing much of the work (see below).
Do "soundproof" curtains block noise?
This is the misconception the whole product category is built on. Absorption is not soundproofing. A curtain absorbs sound bouncing around inside the room; it does almost nothing to stop sound travelling through the glass or wall into or out of the room. The reason is mass: blocking airborne noise needs a heavy, sealed barrier, and even a thick curtain weighs a tiny fraction of what a pane of glass or a stud wall does. At 125 Hz — where traffic rumble and bass live — even heavy curtains sit at α 0.14.
Curtains cut reverb, not bass
Read across either row and the pattern is the familiar porous-absorber shape: a small number at 125 Hz, a large number by 2–4 kHz. Fabric is a thin porous absorber, and a porous layer only works once its depth (including any air gap behind it) approaches a quarter of the sound's wavelength. Treble wavelengths are a few centimetres, so a hung curtain handles them well; bass wavelengths are metres long, so a fabric layer does essentially nothing down low. Lightweight curtains climb from α 0.05 at 125 Hz to 0.73 at 4 kHz; heavy ones from 0.14 to 0.65.
So curtains are excellent for a bright, echoey room with a lot of glass, and useless for a boomy, bass-heavy one — that low-end problem needs thick porous panels and corner bass traps, which a curtain cannot substitute for.
Fullness and air gap are the real levers
Two identical fabrics can perform very differently depending on how they're hung. The single biggest variable after weight is fullness — how deeply the fabric is gathered. A curtain pleated to roughly double the track width presents far more effective surface and traps air in its folds, which is why the heavy, deep-folded row reaches α 0.35 at 250 Hz against just 0.06 for the light draped one, and 0.55 vs 0.39 at 500 Hz. The second lever is the air gap: hanging the rail 10–15 cm proud of the window lets the fabric work like a porous panel spaced off a wall, extending absorption down into the low mids. If you're choosing curtains partly for sound, buy heavier fabric, order it full, and hang it off the wall.
Curtains vs. a real acoustic panel
At their best — heavy fabric in deep folds — curtains are a useful mid-and-treble absorber and an easy way to treat a window wall you can't hang panels on. But panel for panel, purpose-built absorbers do more, especially through the mids where speech and instruments sit.
| Material | 125 Hz | 250 Hz | 500 Hz | 1k Hz | 2k Hz | 4k Hz | NRC |
|---|---|---|---|---|---|---|---|
| Curtains, heavy, deep folds | 0.14 | 0.35 | 0.55 | 0.72 | 0.70 | 0.65 | 0.60 |
| 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: Standard architectural-acoustics coefficient tables (ISO 354 / ASTM C423 measured). Knauf / Rockwool mineral-wool acoustic board published datasheets. Knauf / Rockwool mineral-wool acoustic board published datasheets. Compare against mineral wool and acoustic foam in detail. An upholstered fabric seat behaves similarly to a heavy curtain (α 0.56 at 500 Hz), which is why a soft-furnished room already sounds calmer than an empty one.
What curtains are good for
- Taming reverb off a glass wall — a hard window wall is one of the most reflective surfaces in a room, and heavy drapes handle its mid-and-treble reflections well.
- An easy, reversible treble absorber where wall panels aren't wanted — rented rooms, living rooms, anywhere aesthetics rule out foam or panels.
- A partner to panels: let the curtains cover the window wall's highs while panels and traps do the mids and corners.
What to keep in mind
- They won't block outside noise. Near-zero at 125 Hz and almost no mass — airborne noise reduction needs mass and sealing at the window, not a curtain.
- They won't fix bass or a boomy room. The low end passes straight through; that's what corner bass traps are for.
- Light fabric barely counts. A thin, flat-hung curtain does little — weight, fullness and an air gap are what make fabric absorb.
Plan the rest of your room
Curtains handle the window wall's highs; size the panels that do the real work with the acoustic panel calculator, then check the result against a reverberation target with the RT60 calculator — it lets you pick curtains (or any material in the database) as a surface and see the per-band effect. If the room sounds boomy, confirm the problem frequencies first with the room mode calculator.
Frequently asked questions
Do soundproof curtains actually block noise?
No, not in the way the name suggests. A curtain is an absorber, not a barrier: it soaks up sound bouncing around inside the room but has almost no mass, so it does little to stop voices, traffic or music passing through the window or wall. Even heavy, deep-folded curtains only reach α 0.14 at 125 Hz — they quieten a room's echo and can slightly cut high-frequency leakage around a rattly window, but blocking airborne noise needs mass and sealing (secondary glazing, a solid-core door), not fabric.
Do curtains absorb sound and reduce echo?
Yes — heavy curtains are a genuine mid-and-treble absorber. Deep-folded heavy curtains reach α 0.55 at 500 Hz and 0.72 at 1 kHz (NRC 0.60), enough to noticeably calm reverb and flutter echo off a hard window wall. Lightweight curtains do much less — only α 0.39 at 500 Hz — so weight, weave and fullness decide how much they help.
Do curtains absorb bass or low frequencies?
Almost none. At 125 Hz lightweight curtains are only α 0.05 and even heavy deep-folded ones reach just 0.14 — a fabric layer hung a few centimetres off the glass is far too shallow for wavelengths that are metres long. Curtains cut the treble reverb and leave the bass untouched, so a room that already sounds boomy will not be fixed by adding drapes.
Are heavy curtains better than lightweight ones for sound?
Considerably. Going from light draped fabric to heavy curtains in deep folds lifts the 250 Hz coefficient from α 0.06 to 0.35 and the 500 Hz value from 0.39 to 0.55. Two things drive it: mass (a denser, tighter weave) and fullness — the same fabric gathered into deep pleats absorbs far more than the same fabric hung flat.
How should I hang curtains to absorb the most sound?
Gather them fuller and space them off the wall. Ordering roughly double the track width so the fabric hangs in deep folds, and mounting the rail 10–15 cm proud of the window, both raise absorption — the fullness increases effective surface area and the air gap behind the fabric behaves like the gap behind a wall panel, pushing absorption down into the low mids. It is the same lever as an air gap behind a porous panel, applied to fabric.
Curtains vs acoustic panels — which treats a room better?
Panels do more per square metre, curtains are the easy win you may already have. A wall of heavy curtains (NRC 0.60) is a real treble-and-mid absorber, but a 50 mm mineral-wool panel (NRC 0.85) absorbs more across the mids — α 0.55 vs 0.35 at 250 Hz — and can be placed at the reflection points that matter. Use heavy curtains to tame the window wall and reverb; use panels and corner bass traps for the mids and low end.
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.
- 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.