Sound absorption · Polyester fibre
ISO NOISE ISO TONE
- Thickness
- 25–50 mm
- Sound absorption (NRC)
- 0.69–0.86
What is the sound absorption coefficient (α)?
In short
The sound absorption coefficient (α) is the share of the sound energy striking a surface that is not reflected back into the room: 0 means everything is reflected, 1 means everything is absorbed. One material has different α values at different frequencies, so it is reported as a table by frequency. NRC, SAA and αw are single numbers worked out from that table in different ways. For porous materials α also depends on thickness and on the air space behind, especially at low frequencies.
How is α measured?
Most values in manufacturers' documents are measured in a reverberation room to ISO 354 or ASTM C423. The laboratory measures how fast sound dies away in the room before and after the specimen is placed; the added absorption divided by the specimen area gives α for each frequency band.
The result is therefore a value for that specimen at that thickness, facing and mounting, not a fixed property of the raw material.
Why are some values above 1?
A manufacturer's table may show values such as 1.05 or 1.15. That does not mean the material absorbs more than 100% of the sound. ASTM C423 explains that diffraction at the edges of the specimen makes it appear larger than its real area, so measured values for highly absorptive specimens can exceed 1 — and the standard warns that these values should be used with caution, because installed areas are usually larger than the test specimen and the sound field in a real room differs from the laboratory.
Per-frequency values vs single numbers
| Value | Standard | Worked out from | Note |
|---|---|---|---|
| --- | --- | --- | --- |
| α per frequency | ISO 354 / ASTM C423 | The measured value in each frequency band | The fullest information; used for band-by-band calculation |
| NRC | ASTM C423 | Average of α at 250, 500, 1000 and 2000 Hz, rounded to the nearest 0.05 | Leaves out 125 Hz and 4000 Hz |
| SAA | ASTM C423 | Average of the one-third-octave bands from 200 to 2500 Hz, rounded to the nearest 0.01 | Finer than NRC, but covers a similar range |
| αw | ISO 11654 | Octave-band values (αp) from 250 to 4000 Hz compared with a reference curve | May carry a letter L, M or H, and is grouped into classes A–E |
Good to know:
- Numbers from different systems cannot be compared directly. NRC 0.85 and αw 0.85 do not mean the same thing
- A letter L, M or H after αw shows the material absorbs well above the reference curve at low, mid or high frequencies; when you see one, open the per-frequency table
- Classes A–E are broad bands — fine for a rough look, not precise enough for calculation
- A single number hides the bass. Two products with the same NRC can differ greatly at 125 Hz
For more on NRC see What is NRC.
Why does thickness matter for low frequencies?
Porous materials — glass wool, stone wool, polyester fibre, open-cell foam — absorb sound through friction as air moves through the pores. But at a hard wall the air hardly moves; the movement is greatest about a quarter of a wavelength away from the wall.
A 1000 Hz sound has a wavelength of about 34 cm, while 125 Hz is about 2.7 m. A 25 mm board fixed flat to a wall therefore sits where the air moves enough for treble, but for bass the whole board is still where the air is almost at rest.
An example from the data sheet of ISO TONE polyester-fibre board (ASTM C423; the document does not state the test mounting):
| Thickness | 125 Hz | 250 Hz | 500 Hz | 1000 Hz | 2000 Hz | 4000 Hz | NRC |
|---|---|---|---|---|---|---|---|
| --- | --- | --- | --- | --- | --- | --- | --- |
| 25 mm | 0.04 | 0.28 | 0.68 | 0.89 | 0.90 | 0.82 | 0.69 |
| 50 mm | 0.15 | 0.75 | 0.89 | 0.93 | 0.86 | 0.89 | 0.86 |
From 1000 Hz upwards the two thicknesses are close, but at 250 Hz the 50 mm board absorbs more than two and a half times as much as the 25 mm board — and at 125 Hz both still absorb little.
How does an air gap behind help?
Fixing the board away from the wall or the old ceiling — on a frame, or as a suspended ceiling — puts the material where the air moves more, so low-frequency absorption improves without extra thickness.
That is why α for the same board differs with the test mounting. ASTM E795 defines the arrangements, such as Type A (laid directly on the surface) and E-400 (board face 400 mm from the surface), while documents in the ISO system usually state the overall depth of the construction or the suspension depth. For the effect on ceiling tiles see Choosing a sound-absorbing ceiling.
How to read a manufacturer's table
- Test standard — ISO 354 or ASTM C423, and whether the summary is NRC, SAA or αw
- Thickness and density of the tested specimen — do they match the product you are buying?
- Test mounting — flat on the surface, or with how much air space? If the document does not say, ask the seller, and do not compare it with values whose mounting is known
- Facing — was it tested with the fabric, perforated foil or paint? Facings change the result. For example, the NRC of 0.65 for 25 mm SCG Cylence Zoftone is stated as tested on bare glass wool before the cloth is applied
- Look at the frequencies of your problem — 500–2000 Hz for speech; for music, machinery or a booming room, check 125 and 250 Hz as well
- Values above 1 — a consequence of the method; do not read them as one material being "better than 100%"
A reading example: 50 mm Zoftone grade M has 0.80 / 1.15 / 1.05 / 0.93 at 250 / 500 / 1000 / 2000 Hz, an average of about 0.98, which rounds in 0.05 steps to the NRC of 1.00 given in the document.
General-purpose stone wool also carries absorption values. ROCKWOOL Thermalrock S, for instance, states NRC 0.90–1.05 depending on the density grade (ISO 354). These are values for the bare slab; behind perforated panels or fabric the overall result also depends on the facing.
Using α
The absorption area of a surface equals area × α. Adding up every surface in the room gives the total absorption area, which is used to estimate reverberation time with Sabine's formula — see the example in Echoey meeting rooms and Reverberation and reverberation time.
α is not a sound insulation value
α and NRC say how little sound a surface reflects back into the room; they do not say how much sound passes through to the other side. Insulation with a high NRC, such as ROCKWOOL Safe 'n' Silent Pro (NRC 0.90–1.00 at 50 mm), is fitted in wall cavities as one part of a sound-insulating wall system. The STC or Rw rating belongs to the complete wall system, not to the insulation alone. See STC and Rw.
Sources
- ASTM C423 — Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method (scope and limitations from the ASTM standard page; NRC and SAA definitions summarised by Intertek)
- ISO 354 — Acoustics — Measurement of sound absorption in a reverberation room
- ISO 11654 — Acoustics — Sound absorbers for use in buildings — Rating of sound absorption (αw, shape indicators, absorption classes), cited via the Rockfon Acoustics FAQ and Arup documentation
- ASTM E795 — Standard Practices for Mounting Test Specimens During Sound Absorption Tests
- Sound On Sound — Choosing & Using Porous Absorbers: effect of thickness and air gap on porous absorbers
- Manufacturer documents: ISO TONE data sheet, SCG Acoustic catalogue (Cylence Zoftone), ROCKWOOL Thermalrock S data sheet, ROCKWOOL Safe 'n' Silent Pro data sheet
Related products
Categories and example products for this topic
- ISO TONE — Data sheet gives per-frequency values at 25 and 50 mm — the thickness effect is clear.
- SCG Cylence Zoftone — Manufacturer's table shows values above 1 and states 25 mm was tested on bare wool.
- ROCKWOOL Thermalrock S — Stone wool slab whose data sheet lists NRC to ISO 354 by density grade.
- ROCKWOOL Safe 'n' Silent Pro — Cavity stone wool with NRC at 50 mm — but the insulation rating belongs to the wall.
Sound absorption · Glass wool
SCG Cylence Zoftone
- Thickness
- 25–50 mm
- Density
- 32–64 kg/m³
- Sound absorption (NRC)
- up to 1.00
Building thermal insulation · Stone wool
ROCKWOOL Thermalrock S
- Thermal conductivity (λ)
- 0.034–0.036 W/m·K
- Sound absorption (NRC)
- 0.90–1.05
Sound insulation · Stone wool
ROCKWOOL Safe 'n' Silent Pro
- Thickness
- 25–100 mm
- Density
- 40–100 kg/m³
- Sound absorption (NRC)
- 0.90–1.00