During absorption, energy is absorbed. The amount of absorption is the absorption coefficient, absorption factor or absorption coefficient. It is a dimensionless quantity, given in the Greek letter alpha, that indicates how much energy a given material absorbs in sound waves, light waves, or electromagnetic waves.
The absorption coefficient is the quotient of the absorbed energy, that is, the energy that the medium absorbs, and the energy that the medium reflects. The absorption coefficient can assume values between 1.0 and 0.0. It characterizes material-dependent properties and is divided into five absorption classes in acoustics. An absorption coefficient of 0 means that no energy is absorbed, but all energy is reflected. It is different with an absorption coefficient of 1, where the complete energy is absorbed. If the absorption coefficient has a value of 0.60, then 60% of the energy is absorbed and 40% is reflected. The absorption index can be calculated from the absorption coefficient.
In acoustics, the sound absorption coefficient is an important characteristic value for the absorbers, which is determined at several frequencies between 100 Hz and 5 kHz. The right choice of absorber material is crucial for reverberation and room sound.
The pioneer of this absorption concept for sound waves was Wallace C. Sabine (1868 to 1919), after whom the absorption coefficient is named: Sabine.