Absorption Coefficient

A measure of the attenuation caused by absorption of energy that results from its passage through a medium. Absorption coefficients are usually expressed in units of reciprocal distance.

The sum of the absorption coefficient and the scattering coefficient is the attenuation coefficient.
Absorption coefficient is sufficient for describing energy propagation through a homogeneous system only. Propagation through a heterogeneous system is affected by scattering. There is more general term attenuation that takes into account both absorption and scattering. It is widely used in acoustics as attenuation coefficient for characterizing particle size distribution.

The fraction of energy which is absorbed on striking any surface. It therefore takes values between 0 and 1, and is usually frequency dependent. In room ACOUSTICS, this coefficient, given the symbol a, is expressed as a fraction of the perfect ABSORPTION at an open window of equal area.
A table such as the following might be consulted for choosing suitable materials for SOUND INSULATION in a room or building.

Material	128 Hz	256 Hz	512 Hz	1,024 Hz	2,048 Hz	4,096 Hz
Draperies hung straight, in contact with wall, cotton fabric, 10 oz. per square yard	0.04	0.05	0.11	0.18	0.30	0.44
The same, velour, 18 oz. per square yard	0.05	0.12	0.35	0.45	0.40	0.44
Same as above, hung 4 inches from wall	0.09	0.33	0.45	0.52	0.50	0.44
Felt, all hair, contact with wall	0.13	0.41	0.56	0.69	0.65	0.49
Rock wool (1 inch)	0.35	0.49	0.63	0.80	0.83	------
Carpet on concrete (0.4 inch)	0.09	0.08	0.21	0.26	0.27	0.37
Carpet, on 1/8 inch felt, on concrete (0.4 inch)	0.11	0.14	0.37	0.43	0.27	0.27
Concrete, unpainted	0.010	0.012	0.016	0.019	0.023	0.035
Wood sheeting, pine (0.8 inch)	0.10	0.11	0.10	0.08	0.08	0.11
Brick wall, painted	0.012	0.013	0.017	0.020	0.023	0.025
Plaster, lime on wood studs, rough finish (1/2 inch)	0.039	0.056	0.061	0.089	0.054	0.070

Also See:
Sound Absorption Coefficient