Sound absorption testing in the reverberation room
Sound absorption is measured by putting the material in a reverberation room and recording how much faster sound decays with it there than without it. That is the whole method, and it is why the number depends as much on how the specimen is mounted as on what it is made of.
What you get back
- Sound absorption coefficient αs and αp, one-third octave and octave bands — the raw result everything else is derived from.
- Weighted sound absorption coefficient αw with shape indicators L, M and H, to ISO 11654 — the single-figure rating European specifications ask for.
- Noise Reduction Coefficient (NRC) and Sound Absorption Average (SAA), to ASTM C423 — the single-figure ratings North American and most Asian specifications ask for.
- Equivalent sound absorption area A, in square metres, for objects that cannot be laid flat — seating, baffles, hanging absorbers.
- Absorption class A to E under ISO 11654, where the specification calls for a class rather than a number.
Materials and products we test
- Acoustic panels and wall linings — fabric-wrapped, perforated timber, perforated metal, PET and felt.
- Suspended ceiling tiles and rafts — mineral fibre, glass wool, gypsum and metal pan.
- Mineral wool, glass wool, rock wool, polyester and melamine foam, loose or faced.
- Perforated and micro-perforated sheet, and slotted timber systems where the cavity behind does the work.
- Carpets, underlays, curtains and drapes, and upholstered seating.
- Sprayed and rendered acoustic finishes, and moss, timber-wool and other decorative absorbers.
- Baffles, banners and free-hanging units, reported as absorption area per unit.
Mounting changes the answer
Mounting is not a detail. The same absorber tested flat against a rigid backing and again with an air cavity behind it can differ by several tenths of an absorption coefficient at low frequency — enough to move an NRC by a full step. ASTM C423 mountings A, E-400 and J and the ISO 354 equivalents describe genuinely different products as far as the result is concerned. Tell us the mounting the material will actually be installed in, and if your specification names one, quote it to us. A figure published without its mounting condition is not comparable to anything.
Turnaround and logistics
- Typical turnaround
- About two weeks from receipt of a complete specimen and an agreed scope
- Urgent work
- An urgent route is available — tell us the date you are working to at enquiry, not after booking
- Before you ship
- Confirm specimen size, mounting and packing first — see sample requirements
- Report language
- Reports issued in English; correspondence in Thai or English
Random-incidence absorption on a full-size specimen in the reverberation room. Reported per one-third-octave band, with the single-number rating your specifier asked for: αw and absorption class to ISO 11654, or NRC and SAA to ASTM C423.
What we test
- Acoustic panels and wall absorbers
- Suspended ceiling tiles and rafts
- Mineral wool, glass wool and foam insulation
- Carpets, underlays and textile floor coverings
- Curtains and drapes
- Automotive and transport interior materials
- Perforated and micro-perforated panels
Mounting changes the number more than the material does
The same 50 mm absorber mounted directly against a rigid backing (Type A) and mounted with a 200 mm air gap (Type E-200) can differ by several tenths of an absorption coefficient at low frequency. Neither result is wrong. If you publish an αw obtained with an air gap and your customer installs the product flat against concrete, you have a warranty problem, not a laboratory problem. Tell us how the product is actually installed and we will test it that way.
Reverberation room
- Specimen area
- 10–12 m² per ISO 354
- Mounting options
- Type A (direct), Type E-200 (200 mm air gap), Type J (suspended grid)
- Frequency range
- 100–5,000 Hz, one-third-octave bands
- Room qualification
- ISO 354 Annex A
- Reported quantities
- αs per band, αp, αw + class (ISO 11654), NRC + SAA (ASTM C423)