What is a sound absorption coefficient?
A designer choosing a fabric for a wall panel or a curtain usually meets the acoustic data last, as a column of decimals, letters and frequencies on the second page of a datasheet. Each of those numbers belongs to the construction behind it.
A sound absorption coefficient describes one frequency band. In a reverberation room, the laboratory measures the room's sound absorption empty and again with the specimen installed; Intertek's summary of ASTM C423 describes the coefficient as the difference divided by the area of the specimen, band by band. AS ISO 354:2006, which adopts ISO 354:2003, is the Australian edition of the reverberation room method, and Getzner's datasheets give values measured to DIN EN ISO 354.
On that scale, 0 means the specimen adds no absorption and 1.00 means the absorption measured equals the specimen's area. Measured coefficients can be higher than 1.00: ASTM C423 explains that diffraction usually makes a specimen's apparent area larger than its real area, so a highly absorptive specimen can measure above 1. Getzner's datasheet for Volumic Weave, hung as a gathered curtain with 100 per cent extra fabric at a median 150 mm from the wall (Type G-150) and measured to DIN EN ISO 354, gives one-third-octave coefficients of 1.01 at 2000, 2500 and 3150 Hz, 1.02 at 4000 Hz and 1.06 at 5000 Hz (Getzner datasheet, 14 March 2024). Volumic Bubble, stretched tightly across a sealed frame at 150 mm clearance (Type E-150) and measured to the same standard, reaches 1.03 at 500 Hz on its datasheet of the same date. Getzner marks each of these values with the footnote "Absorption area > specimen area".
What is NRC, and how is it calculated?
NRC is a single number: the average of the absorption coefficients at 250, 500, 1000 and 2000 Hz, rounded to the nearest 0.05, as Intertek summarises ASTM C423. Because it averages four bands, two materials with the same NRC can absorb differently at low or high frequencies, so ask for the band values as well.
Canterbury Acoustic Testing Services in Christchurch (CATS) reports NRC calculated in general accordance with ASTM C423 from AS ISO 354:2006 measurements. For example, Cubic Pico stretched on a 50 mm frame with 20 mm fibreglass infill measured NRC 0.85 (AS ISO 354:2006, CATS report 2026-316.1); the values from all three CATS reports are in the table below.
What is αw, and what do absorption classes A to E mean?
The weighted sound absorption coefficient, αw, is the single number on Getzner's datasheets, rated to DIN EN ISO 11654. AS ISO 11654-2002 is identical with ISO 11654:1997. The standard converts the frequency-dependent coefficients into one number against a reference curve whose lowest band is the 250 Hz octave, and Standards Norway's summary of the standard notes that the rating is not appropriate below that frequency.
A letter after the number is a shape indicator. Optic Braid, for example, is αw 0.80 (H), class B, hung as a gathered curtain with 100 per cent extra fabric at a median 150 mm from the wall (Type G-150), measured to DIN EN ISO 354 and rated to DIN EN ISO 11654 (Getzner datasheet, 14 March 2024). The help pages of Arup's Strutt acoustic software, a secondary source rather than the standard, explain that L marks an absorption peak at 250 Hz, M at 500 Hz or 1 kHz, and H at 2 kHz or 4 kHz.
ISO 11654 also gives, for information, a way to group αw values into absorption classes. The table below shows the class boundaries as BauNetz Wissen, a German building reference, sets them out; they are from that secondary source, not quoted from the standard.
| Absorption class | αw range (BauNetz Wissen) |
|---|---|
| A | 0.90 to 1.00 |
| B | 0.80 to 0.85 |
| C | 0.60 to 0.75 |
| D | 0.30 to 0.55 |
| E | 0.15 to 0.25 |
| Not classified | 0.00 to 0.10 |
Why is NRC not a percentage of sound removed?
A coefficient, an NRC and an αw are decimals that compare the absorption measured with the area of the specimen, and Sabins give the absorption of one object. None of them is a percentage of the sound in a room, and sound passing through a wall is a different property, sound insulation, explained in the difference between absorption and insulation. ASTM C423 itself cautions that laboratory coefficients should be used with care, because real surfaces are usually larger than the test specimen and real sound fields are rarely diffuse, so how much a room's reverberation shortens is a calculation for the whole room.
Why does the mounting change the number?
The same fabric gives different numbers in different constructions. The table below shows the CATS laboratory results for Cubic Pico in five constructions, each with its rating, the band where one applies, and the report.
Tested with Cubic Pico
On a 50 mm frame, Cubic Pico measured NRC 0.55 and αw 0.40, class D, with an empty cavity, and NRC 0.85 and αw 0.65, class C, with 20 mm fibreglass infill (AS ISO 354:2006, CATS report 2026-316.1). The fabric stayed the same; the construction behind it changed.
Getzner's datasheets use two standard mountings: Type E-150, fabric stretched tightly across a sealed frame at 150 mm clearance, and Type G-150, a gathered curtain with 100 per cent extra fabric at a median 150 mm from the wall, without a sealed frame. The next table lists each fabric's datasheet αw and class under its own mounting, and each value is read with its mounting.
Getzner datasheet absorption, by mounting
Optic Structure, for example, is αw 0.90, class A, hung as a gathered curtain with 100 per cent extra fabric at a median 150 mm from the wall (Type G-150), measured to DIN EN ISO 354 in a 203.3 m³ reverberation room and rated to DIN EN ISO 11654 (Getzner datasheet, 14 March 2024). That figure describes a gathered curtain, and a stretched wall panel has figures of its own.
What makes a fabric absorb sound?
Flow resistance belongs to the fabric; absorption in use depends on it and on the construction behind it. Every Getzner datasheet gives the specific flow resistance, measured to DIN EN ISO 29053, and across the Acunic range it runs from 315 Pa s/m for Cubic Mega to 1,886 Pa s/m for Basic. The construction behind the fabric, its air gap, frame depth, infill or fullness, is part of every result, as the two Cubic Pico panels above show.
What should you ask a supplier for?
Ask for figures that can be checked against the construction you will build:
- the measure (NRC, αw with its class, or Sabins per object) and its standard;
- the construction tested: mounting, cavity depth and infill, and for a curtain, the fullness and the distance from the wall;
- the exact fabric and colour tested;
- the laboratory, the report number and the test date;
- the one-third-octave values, as well as the single number.
How are Sabins used to rate objects?
A tower, a hanging sail or a baffle is rated as a whole object. AS ISO 354 measures the equivalent sound absorption area of objects such as furniture and space absorbers, and Intertek's summary of ASTM C423 notes that individual objects are reported per unit at each one-third-octave band. Standards Norway's summary of ISO 11654 says that its single-number rating is often not suitable for single items such as chairs or baffles, which is why objects are rated in Sabins per object.
CATS measured four empty towers wrapped in Cubic Pico, each 0.4 x 0.4 x 1.5 m, at 1.13 Sabins per tower at 500 Hz, with the highest value, 1.86 Sabins per tower, at 5000 Hz (AS ISO 354:2006, CATS report 2026-316.2). The table above gives the towers with batts and the suspended panels, each with its band. Compare Sabins per object with Sabins per object of the same size, measured in the same band, when choosing freestanding elements or ceiling sails.
