An absorption coefficient is the fraction of incident energy absorbed at one frequency, so a product needs six numbers, not one; NRC averages four mid bands and therefore says nothing about 125 Hz.
Absorption coefficient: why NRC hides the octave you care about
checked 2026-09-16responsible Room Tone Register (a named register, not a person)
The coefficient table
| Material | 125 Hz | 500 Hz | 2 kHz | 4 kHz | NRC |
|---|---|---|---|---|---|
| Bare plaster | 0.02 | 0.02 | 0.03 | 0.04 | 0.05 |
| 50 mm porous panel | 0.15 | 0.60 | 0.95 | 0.90 | 0.65 |
| 100 mm porous panel | 0.45 | 0.90 | 0.98 | 0.95 | 0.85 |
| Heavy curtain, hung loose | 0.14 | 0.35 | 0.55 | 0.60 | 0.40 |
| Moving blanket, single layer | 0.10 | 0.35 | 0.55 | 0.60 | 0.40 |
Read the first row against the others. Bare plaster is a hard reflector at every frequency, which is why an empty room with hard walls is worse than its floor level suggests: the reflections are nearly as loud as the direct sound.
The rule that follows from the table
A porous absorber's useful bandwidth is set by its thickness. Roughly, a panel works from the frequency whose quarter-wavelength fits inside it. At 125 Hz the quarter wavelength is 686 mm, so a 50 mm panel cannot touch it — not because it is badly made, but because the air particle velocity near a wall is near zero at low frequency and there is nothing for the material to work against. Low-frequency absorption needs either depth or a different mechanism.
Two honest caveats
- A published coefficient is measured in a reverberation room by ASTM C423, which over-reports coefficients above about 0.9 because of edge diffraction. Treat α = 1.05 in a datasheet as a measurement artefact, not a material better than a perfect absorber.
- Coefficients are not additive by area when panels are placed in a corner. A corner-mounted panel behaves differently from the same panel on a flat wall, and the datasheet usually describes the flat-wall case.
This is the register's most contestable position: for a typical untreated domestic room, four well-placed 100 mm panels beat twelve thin ones of the same total area and cost. The measurement literature supports it; the retail market is organised against it.
Near kin: NRC, SAA, diffusion and transmission loss
NRC and SAA are single-number averages of the coefficients at four mid bands, which is why both say nothing about 125 Hz. Diffusion scatters reflected energy in time and direction without removing it, so a diffuser can make a small room sound larger without making it drier. Transmission loss is a completely different property: it describes how much energy passes through a partition, and it is what stops sound, not what absorbs it.
| Term | What it is | How it differs from absorption |
|---|---|---|
| NRC / SAA | A four-band average of coefficients | A summary; conceals the low octaves |
| Diffusion | Scattering without removing energy | Changes the room's sound without drying it |
| Transmission loss | Energy passing through a partition | Stops sound; absorption does not |
| Sabin | The unit of total absorption (m²) | An absolute quantity; the coefficient is a ratio |
The common misread is reading an absorption coefficient as a soundproofing figure. A coefficient of 0.95 means 95% of the energy arriving at the surface is not reflected; it says nothing about how much passes through the wall to the next room. Treating a wall with absorptive material does not reduce what the neighbour hears.
Source log
- 01Absorption of Sound, Penn State Acousticssupports the definition of absorption and the frequency dependence of porous absorbers
- 02ASTM C423: Standard test method for sound absorption and sound absorption coefficientssupports the reverberation-room test method behind published coefficients
- 03ASTM C423 explained (Intertek)supports the practical reading of an NRC rating against octave-band data
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