What is the sound absorption coefficient ?

The absorption coefficient of a material is ideally the fraction of the
randomly incident sound power which is absorbed, or otherwise not
reflected. It can be determined in two main ways, and there are often
variations in the results depending upon the method of measurement
chosen. It is standard practice to measure the coefficient at the
preferred octave frequencies over the range of at least 125Hz – 4kHz.

For the purposes of architectural design, the Sabine coefficient
(calculated from reverberation chamber measurements) is preferred.
Interestingly some absorbent materials are found to have a Sabine
coefficient in excess of unity at higher frequencies. This is due to
edge effects and when this occurs the value can be taken as 1.0

The Odeon computer program includes a file of absorption coefficients.