Treatment
Absorption and NRC
How porous absorbers work, what an NRC rating does and doesn’t tell you, and why thickness beats area.
7 min read7 sourcesChapter 5 of 24
Schematic
The same wave meeting three thicknesses of porous absorber. Thin material stops only the short wavelengths at the top of the range; each doubling of thickness reaches roughly an octave lower. The bars show typical absorption by frequency for each.
Animated illustration of the principle. Not measured data.
Absorption is the conversion of sound energy into heat. Every surface does a little of it; a treatment panel does a lot. The number printed on the panel — its NRC — is the most quoted and most misread figure in acoustics, so it is worth understanding exactly what it measures.
How a porous absorber works
Mineral wool, glass wool, polyester fibre and open-cell foam are all mazes of tiny air passages. When a sound wave enters, the air inside is pushed back and forth through those passages, and friction against the fibres converts the motion to heat. The wave that emerges is weaker.
Air moves most where pressure changes least — a quarter of a wavelength away from the hard wall behind the panel. So a panel approaches its highest absorption at frequencies whose quarter-wavelength falls inside its thickness, tapering off gradually below — a rough guide, since the material itself also shapes the curve — and mounting it off the wall with an air gap moves that useful region lower without adding material.
What NRC measures
The Noise Reduction Coefficient is a single number from 0 to about 1 that averages the material’s absorption coefficients at 250, 500, 1000 and 2000 Hz, rounded to the nearest 0.05. It is measured in a reverberation room to ASTM C423. An NRC near 0 means very little average absorption across those bands; near 1.0 means nearly complete average absorption across them — not perfect absorption at every frequency. Values above 1.0 can appear on test reports because of edge effects and the way the test method counts specimen area; they are not a literal fraction of the sound reaching the face.
Read carefully, NRC is honest. Read as “how good is this panel”, it is misleading in two ways. It says nothing about frequencies below 250 Hz, where many small-room problems live. And two panels with the same NRC can behave very differently across the range — one flat, one strong at 2 kHz and weak at 250 Hz.
A newer single number, the Sound Absorption Average (SAA), averages twelve third-octave bands from 200 to 2500 Hz. It is a little more informative but has the same blind spot below 200 Hz.
Typical values
Figures below are representative ranges from published tests, useful for a sense of scale. Any specific product should be judged from its own test report, with the mounting method noted.
- Painted plaster, brick, concrete, glass: NRC about 0.02–0.05. Essentially reflective.
- Carpet: roughly 0.2–0.35 laid directly on concrete, 0.4–0.7 on an underlay or cushion.
- 25 mm (1″) glass-fibre board on the wall: about 0.70–0.75, falling away quickly below 500 Hz (about 0.25 at 250 Hz).
- 50 mm (2″) board on the wall: about 1.0, still useful at 250 Hz (0.7–0.85) but down to 0.15–0.2 at 125 Hz.
- 100 mm (4″) board on the wall: about 1.1, with meaningful absorption at 125 Hz (around 0.55–0.6).
Thickness beats area
The most common mistake in treated rooms is covering a large area with thin material. It produces a room that is dead at high frequencies and untouched below — the “boomy and dull” result. A smaller area of thick absorber at the right locations does more, sounds more natural, and often costs less.
For the same reason, fabric matters. A closed-weave or backed fabric in front of a panel reflects the high frequencies before they reach the absorber. The test for acoustically transparent fabric is simple: hold it to your mouth and breathe through it. If it resists, it will reflect.
For integrators and architectsThe numbers, the standards, and what to specify
- Test method and mounting
- ASTM C423 / ISO 354 reverberation-room tests. Always read the mounting type (Type A: against a solid backing; Type E mountings: with an air space behind, the number giving the depth in mm). Mounting changes the low-frequency result dramatically and a spec sheet without it is incomplete.
- Flow resistivity
- The property that governs how a fibrous material absorbs: too low and sound passes through, too high and it reflects. Typical panel-grade mineral wool sits in the tens of thousands of Pa·s/m²; density is a rough proxy but not the same thing.
- Quarter-wavelength rule
- High absorption from roughly f ≈ c / (4 × depth) upwards, where depth includes any air gap: 100 mm ≈ 860 Hz, 200 mm total ≈ 430 Hz — a rough guide, since the material and mounting also set the curve. Below that the coefficient falls off gradually, not abruptly — tested 50 mm board is still about 0.7 at 250 Hz.
- Per-band, not NRC
- Design and specify with the full absorption curve (125 Hz–4 kHz octave bands minimum). Products with the same NRC can differ by 0.3–0.45 in a single low band: two 25 mm boards both rated NRC 0.70, one plain and one foil-faced, measure 0.26 and 0.71 at 250 Hz.
- Facing
- Perforated or slotted facings (wood slats, perforated metal) change the panel into a partial resonant absorber: they extend low-frequency absorption and reduce high-frequency absorption. Useful when deliberate; a surprise when not.
What to write into the brief
- Absorption coefficients per octave band with the mounting type, not NRC alone.
- Panel depth as a design variable: 50 mm minimum for reflection control, 100 mm or more where low-mid absorption is the goal.
- Fabric specified as acoustically transparent with the breathability test as acceptance criterion.
Sources
- 01ASTM C423-23e1 — Sound absorption and sound absorption coefficients by the reverberation room method
- 02Intertek — ASTM C423 testing (definitions of NRC and SAA)Accredited test laboratory’s summary of how NRC and SAA are calculated.
- 03ISO 354:2003 — Measurement of sound absorption in a reverberation room
- 04ASTM E795-23 — Mounting test specimens during sound absorption testsDefines the mounting types (A, E…) that change low-frequency results.
- 05ISO 11654:1997 — Sound absorbers for use in buildings: rating of sound absorption
- 06Owens Corning, Fiberglas 700 Series Board — Product Data Sheet (ASTM C423, Type A mounting)Measured absorption coefficients for 25, 50, 75 and 100 mm board.
- 07Carpet and Rug Institute — Acoustical Characteristics of Carpet (technical bulletin)NRC of carpet on concrete and on cushion, ASTM C423.
External links open in a new tab. Figures quoted in this chapter are taken from these sources; a Science of Sound measurement is only ever presented as one, with its file, date and instrument.
Keep reading