Sound Absorption Coefficient

The sound absorption coefficient indicates how much of the sound is absorbed in the actual material. The absorption coefficient can be expressed as:

α = I_a / I_i (1)

where

I_a = sound intensity absorbed (W/m²)

I_i = incident sound intensity (W/m²)

Absorption coefficient - α - for some common materials can be found in the table below:

Material	Sound Absorption Coefficient - α
Plaster walls	0.01 - 0.03
Unpainted brickwork	0.02 - 0.05
Painted brickwork	0.01 - 0.02
3 mm plywood panel	0.01 - 0.02
6 mm cork sheet	0.1 - 0.2
6 mm porous rubber sheet	0.1 - 0.2
12 mm fiberboard on battens	0.3 - 0.4
25 mm wood wool cement on battens	0.6 - 0.07
50 mm slag wool or glass silk	0.8 - 0.9
12 mm acoustic belt	0.5 - 0.5
Hardwood	0.3
25 mm sprayed asbestos	0.6 - 0.7
Persons, each	2.0 - 5.0
Acoustic tiles	0.4 - 0.8

Note! The absorption coefficient varies with the frequency of sound.

Total Room Sound Absorption

The total sound absorption in a room can be expressed as:

A = S₁ α₁ + S₂ α₂ + .. + S_n α_n = ∑ S_i α_i (2)

where

A = the absorption of the room (m² sabine)

S_n = area of the actual surface (m²)

α_n = absorption coefficient of the actual surface

Mean Absorption Coefficient

The mean absorption coefficient for the room can be expressed as:

a_m = A / S (3)

where

a_m = mean absorption coefficient

A = the absorption of the room (m² sabine)

S = total surface in the room (m²)

A rooms acoustic characteristics can be calculated with the formulas above, or estimated for typical rooms.