NWAA's Ron Sauro on Shape and Absorption - a mini treatise on absorption
https://www.stereophile.com/content/nwaa-labs-measurement-beyond-atomic-level-shape-and-absorption"Absorption is the most common treatment used in listening rooms. The measurement of absorption was first described by Wallace Sabine, who compared the reverberation times of a room with and without absorption. This difference was then converted to units of absorption using this formula:
A = 0.9210(V*d/c)
where
A = equivalent absorption area in m2,
V = volume of reverberation room in m,
c = speed of sound at ambient temperature and humidity in m/s,
and d = decay rate in dB/s.
"Most of what we think we know about absorption is wrong! Absorption calculations in use today can err by as much as 85%. The biggest error concerns how important the area of absorption is in the calculation of how much absorption is needed."