Why an absorption coefficient can be larger than 1

An absorption coefficient is the fraction of the incident sound energy that is not reflected. By that definition it cannot exceed 1: nothing can absorb more than all of the energy. Yet published tables sometimes list values of 1.1 or 1.2. The explanation says a lot about how absorption is measured.

How the coefficient is measured

The usual method uses a reverberation chamber: a hard room of about 200 m³ in which the sound field is as diffuse as possible. First, the reverberation time of the empty room is measured. Then a test sample of absorbing material, typically about 10 m², is laid on the floor, and the reverberation time is measured again. The shortening of the reverberation time tells how much absorption the sample adds. The Sabine equation converts the two reverberation times into an equivalent absorption area, and dividing by the sample area gives the coefficient. See RT60 and absorption.

Why it can exceed 1

The calculation assumes the sample absorbs only the sound that hits its surface. In reality:

1. Edge effects. Sound waves bend around the edges of the sample (diffraction), so it absorbs sound that did not directly hit its area. A smaller sample has relatively more edge, which increases the apparent absorption.
2. Sample thickness. A sample that sticks out of the floor, with its sides exposed, has extra surface area that is not counted.
3. Non-diffuse field. The method assumes a perfectly diffuse field, which real chambers only approximate. The deviations act as an error that can push the result above 1.
4. Measurement uncertainty. Differences of reverberation times are small and sensitive to temperature and humidity changes, so a coefficient of 1.05 may be within the error of a true value of 0.95.

Values between 1 and about 1.2 are therefore artefacts of the method, not physical absorption of more than all energy.

What to do with them

• In calculations, limit the value to 1. A tool that sums areas times coefficients may use the measured value, but a mean absorption above 1 would break the Eyring formula, which has a logarithm of 1 − α.
• Standards such as ISO 11654 define a practical absorption coefficient per octave band, averaged from the third-octave values and capped at 1.0, and a single number rating from it.
• When comparing products, look at the thickness, the air gap behind and the mounting type, which matter as much as the coefficient.

Why Sabine's formula still works

The equivalent absorption area of a room is the sum of the absorption of all its surfaces. A value of 1.1 m² per m² of sample is simply a statement about how the sample influenced the chamber. In room design the effect averages out over many surfaces.

Try it

The RT60 tool caps the absorption of each material below 1, so that the Eyring formula remains defined.

Draft: written by an AI assistant, awaiting technical review.