Absorbers soak up sound energy instead of reflecting it back into the room. Used in the right places, they tighten the sound and sharpen the stereo image. Used everywhere, they make a room sound dead.
Absorption basics
Beginner
How it works
Porous materials such as mineral wool turn sound into a tiny amount of heat as air moves through the fibres. The deeper the material, the lower the frequencies it can absorb.
Thickness matters most
Thin panels only absorb the treble, which leaves the midrange and bass untouched and makes the room sound dull. Floyd Toole recommends porous absorbers of at least 75 mm so they work evenly above about 200 Hz.
| Material (mounted flat on a wall) | 125 Hz | 250 Hz | 500 Hz | 1 kHz | 2 kHz | 4 kHz |
|---|---|---|---|---|---|---|
| ROCKWOOL RWA45, 25 mm | 0.10 | 0.25 | 0.60 | 0.75 | 0.95 | 1.00 |
| ROCKWOOL RWA45, 100 mm | 0.65 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
| Owens Corning 703, 50 mm | 0.10 | 0.71 | 1.14 | 1.14 | 1.03 | 0.95 |
| Owens Corning 703, 100 mm | 0.51 | 1.19 | 1.24 | 1.13 | 1.04 | 0.94 |
1.00 means all the sound hitting the panel at that frequency is absorbed. Figures above 1.00 are a quirk of the lab test, not extra absorption. The 25 mm panel barely touches the bass, while 100 mm works well from 250 Hz up.
Getting it right
- Use mineral wool or glass wool, not thin foam. Mineral wool slabs like RWA45 are Euroclass A1, non-combustible. Acoustic foam is typically a much lower fire class.
- Cover panels in a breathable fabric so sound can get in.
- Leave an air gap behind the panel. GIK Acoustics suggests a gap at least as deep as the panel. It helps the panel reach lower frequencies.
- Start with the first reflection points before anything else. See Treatment Placement.
Figures come from the manufacturers’ own laboratory tests.
Absorption in more depth
Advanced
Why depth matters: the quarter wavelength
A porous absorber works best where air moves fastest. Next to a wall air can’t move, so absorption is weak. Air movement peaks a quarter of a wavelength out from the wall. With sound at 343 m/s, a quarter wavelength is 0.86 m at 100 Hz and 0.43 m at 200 Hz. That’s why spacing a panel off the wall, or making it deeper, extends it downwards.
Air gap data
Owens Corning’s data for a 405 mm air space shows 50 mm 703 improving at 125 Hz from 0.17 to 0.40. That’s a ceiling-void gap, not a typical 50 to 100 mm stand-off, and we couldn’t find laboratory data for realistic domestic gaps. Treat air gaps as a helpful rule of thumb rather than a precise figure.
Reading the numbers
- NRC, used in the US, averages 250 Hz to 2 kHz and ignores 125 Hz, so it tells you little about bass.
- The UK and Europe use αw and absorption classes A to E instead (BS EN ISO 11654).
- Owens Corning’s 2023 data sheet shows lower bass figures than its older design guide. We’ve used the current sheet.
Fire safety in the UK
Building regulations classify linings using Euroclasses (EN 13501-1). Mineral wool slabs such as RWA45 are A1. A UK retailer reports polyurethane foam is typically class C to F, and that the old Class 0 rating alone no longer satisfies Building Control for new work. If in doubt, check with your local Building Control.
Sources
- Hunecke: Porous absorbers
- Toole (2006): Loudspeakers and rooms for sound reproduction, a scientific review, JAES
- ROCKWOOL RWA45 acoustic test report (2025)
- Owens Corning 703 and 705 technical data sheet (2023)
- GIK Acoustics: How thick should acoustic panels be?
- Noise reduction coefficient (overview)
- ROCKWOOL slabs data sheet (fire classification)
- Sonio: Fire-rated acoustic panels and Euroclass (retailer guide)