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.

The coefficient is measured in a reverberation chamber according to ISO 354. Published values sometimes exceed 1.0 due to edge effects and mounting conditions. This doesn't violate physics, just reflects measurement methodology.

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

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