Absorption Coefficient

A measure (0-1) of how much sound energy a material absorbs versus reflects.

The absorption coefficient (α, alpha) measures a material's ability to absorb sound energy rather than reflect it. The value ranges from 0 to 1:

• α = 0: Perfect reflector (no absorption) • α = 1: Perfect absorber (all sound absorbed) • Open window: α ≈ 1 (sound escapes, doesn't return)

Absorption coefficients vary significantly with frequency. Most porous absorbers (fiberglass, rockwool, foam) absorb high frequencies easily but require greater thickness for low-frequency absorption. A 2" panel might have α = 0.9 at 1 kHz but only α = 0.2 at 125 Hz. Published numbers are test-condition dependent — thickness, mounting type, air gap, density, and facing all shift the result, so treat catalog values as planning estimates and confirm against the manufacturer's actual test data before specifying.

The coefficient is measured in a reverberation chamber according to ISO 354 or ASTM C423. Values sometimes exceed 1.0 due to diffraction and edge effects inherent to the reverberation-room method — ASTM C423 itself cautions that these are operationally defined, apparent coefficients, not a claim that a material absorbs more sound energy than physically arrives at it.

Absorption coefficients are essential for calculating room acoustics using the Sabine or Eyring equations.

Practical Example

Calculating total absorption for a room

Wall area: 40 m² with α = 0.1 → Absorption = 40 × 0.1 = 4 Sabins

Adding panels with α = 0.8 covering 10 m² adds 8 Sabins, doubling the total absorption.

Standards: ISO 354, ASTM C423

Related Terms

NRC · Sabine Equation · RT60

Glossary · Diffuser Designer

ResonAia is an interactive acoustic design application. Enable JavaScript for the full experience, or follow the links above to explore the platform.