Sound Absorption Coefficients

What an absorption coefficient actually tells you

An absorption coefficient is a number between 0 and 1 describing how much sound energy a surface swallows instead of bouncing back. A coefficient of 0.01 — smooth concrete, say — reflects essentially everything that hits it. A coefficient of 0.90 absorbs almost everything.

The reason there are six numbers per material rather than one is that almost nothing absorbs evenly. This is the single most useful thing on this page. Look at thin carpet: 0.02 at 125 Hz and 0.40 at 4 kHz. It does nearly nothing to low frequencies and quite a lot to high ones. That is why a room can get a rug and still sound boomy — the rug never addressed the frequencies causing the boom.

It is also why foam panels disappoint so often. Thin foam behaves like thin carpet: strong at the top, weak underneath. If your complaint is that the room feels heavy or boxy, a material with good numbers only above 1 kHz will not fix it, however much of it you buy.

What is NRC?

NRC — Noise Reduction Coefficient — is the average of the values at 250, 500, 1000 and 2000 Hz, rounded to the nearest 0.05. It is the number printed on product packaging, and it is a reasonable shorthand for speech-range absorption.

It also hides things, deliberately. Notice that NRC ignores 125 Hz entirely. Two materials can share an NRC of 0.70 while one absorbs bass well and the other not at all. If you are chasing boominess rather than echo, read the 125 Hz column and ignore the NRC.

Mounting changes the numbers more than the material does

Look at the curtain entries. Curtains hung flat against a wall perform very differently from the same fabric folded and hung 100 mm away from it — because the air gap behind lets the fabric work at lower frequencies. Some folded curtain entries reach 1.00 in the speech range, which is to say they absorb as well as purpose-built panels.

The same principle applies to panels: a panel spaced off the wall outperforms the identical panel screwed flat to it. If you take one practical thing from this table, make it that — the air gap is free and it works.

How to use these numbers

Multiply a material's coefficient by the area you plan to cover to get its absorption in square metres of equivalent absorption, then compare candidates on that basis. Ten square metres of heavy curtain at 0.65 beats two square metres of excellent panel at 0.95. Coverage usually matters more than product choice, which is the opposite of how most acoustic treatment gets bought.

If you would rather not do the arithmetic, our room acoustics calculator does it for you, and the studio sound check measures what your room is doing right now from a recording of your voice.

Where this data comes from

The figures are drawn from the materials database published with pyroomacoustics, an open-source room acoustics library released under the MIT licence, which compiles published octave-band measurements for common building and finish materials.

Treat them as representative rather than exact. Published coefficients come from laboratory measurements of a specific sample with a specific mounting, and real rooms differ. They are the right tool for comparing options and estimating outcomes, not for certifying a build.