Sound absorption is tested to ISO 354 using the reverberation-room method: a sample of the material is placed in a large, hard-walled chamber, and the reduction in the room's sound decay time when the sample is present gives the absorption coefficient (α) for each one-third-octave band, typically from 125 Hz to 4000 Hz. Because the result depends heavily on how the sample is mounted and any air gap behind it, a valid report always states the exact construction and mounting it applies to. A quoted αw or NRC only means something when it traces back to such a report.
How is sound absorption actually tested?
Absorption is measured to ISO 354, the reverberation-room method. A standardised area of the material is placed on the floor of a large chamber with hard, reflective surfaces designed to hold an evenly spread, diffuse sound field. The room is filled with sound, the source is switched off, and the time the sound takes to fade away is recorded across a range of frequencies.
The chamber is measured twice — empty, then with the sample present. The absorptive sample makes the sound die away faster, and that change in decay time is converted, using the same Sabine relationship that governs any room, into the extra absorption the sample added. Divided by the sample's area, this gives the absorption coefficient (α) for each one-third-octave band, usually reported from 125 Hz to 4000 Hz.
Why the mounting and air gap change the result
The single biggest reason two figures for the 'same' panel disagree is the mounting. A panel fixed hard against a rigid wall behaves very differently from the same panel set forward on battens with an air gap — or insulation — behind it. That cavity lets the panel absorb far more at low and mid frequencies, so the identical product can move a whole absorption class depending on the depth behind it.
For this reason ISO 354 defines standardised test mountings, and a report is only meaningful alongside the mounting it used. A figure measured with a deep void behind the panel simply does not apply if you then fix that panel flat to plasterboard. Always match the mounting in the report to the way you actually intend to install.
What a credible ISO 354 report contains
A report you can rely on names four things: the exact construction tested (material, thickness, slat and groove dimensions, backing and any acoustic fleece), the mounting and air gap, the per-band α for every one-third-octave band, and the single-number ratings derived from that data. It should also identify the accredited laboratory and the test date, not just quote a headline value.
The per-band table is the evidence; the single numbers are a summary of it. Under BS EN ISO 11654, the one-third-octave results are weighted into a single rating, αw, and an absorption class from A to E. See how αw and NRC are calculated for what those summaries do and don't tell you about a real room.
Is a headline figure enough on its own?
No. A number with no report behind it, or a number lifted from a different build-up, is not evidence — it is marketing. Absorption depends on the specific construction and mounting, so any figure that cannot be traced to a named ISO 354 report for that exact panel and installation should be treated as unverified.
This is the difference between proven and promised. It is also why credible project examples tie each performance claim to the report it comes from, at the mounting used on site — rather than quoting a single impressive-sounding coefficient with nothing behind it.
How to read the numbers sensibly
Don't fixate on the peak. A panel that absorbs strongly at 2000 Hz but weakly at 250 Hz will tame hiss and sibilance yet do little for a boomy, low-pitched room, so read the whole curve against the problem you actually have. The low bands are where deep air gaps and cavities earn their keep.
You may also see an absorption coefficient printed as slightly above 1.0 in a report. This is a recognised artefact of the finite sample size and edge effects in the reverberation-room test, not a mistake or an exaggeration — but it is one more reason to read the whole method, not just the largest number on the page.
Frequently asked questions
Does an ISO 354 test measure soundproofing?
No. ISO 354 measures how much sound a material absorbs within a room — controlling reverberation, echo and speech clarity. It says nothing about how much sound passes between rooms; that is sound insulation, a completely different measurement governed by different standards, such as those behind Approved Document E. Absorptive panels improve a room's acoustics, they do not soundproof it.
Why is the same panel quoted with different absorption figures?
Because the mounting differs. The same panel tested flat against a wall and tested with an air gap or insulation behind it will absorb differently, especially at low frequencies. Neither figure is wrong — they describe different installations. A test report always states the mounting it applies to, and you should use the one that matches your build-up.
Can an absorption coefficient really be greater than 1?
Occasionally, yes. Because the test uses a finite sample in a diffuse field, edge and diffraction effects can push a measured coefficient slightly above 1.0 in some bands. It does not mean the material absorbs more than 100% of the energy reaching it; it is a known feature of the reverberation-room method rather than an error in the report.
How do I know a supplier's absorption figure is trustworthy?
Ask for the ISO 354 test report, not just the headline number. A trustworthy figure names the exact construction tested, the mounting and any air gap, the per-band α, and the accredited laboratory. If a supplier cannot produce a report for the specific panel and installation, treat the figure as unverified.