The absorption coefficient is a measure of the efficiency of a surface or material in absorbing sound. If 55 percent of the incident sound energy is absorbed, the absorption coefficient is said to be 0.55. One square foot of this material gives 0.55 absorption units (sabins). An open window is considered a perfect absorber because sound passing through it never returns to the room. It would have an absorption coefficient of 1.0. Ten square feet of open window would give 10 sabins of absorbance.

The absorption coefficient of a material varies with frequency and with the angle at which the sound wave or ray impinges upon the material. In an established sound field in a room, sound is traveling in every imaginable direction. What we need in our calculations are sound absorption coefficients averaged over all possible angles of incidence.

Young has pointed out a long-standing and widespread confusion in the field of acoustics concerning the sound absorption coefficient.1 There really are two kinds, one based on the arithmetic mean reflection coefficient of the several sound absorbing surfaces, a, and the other the geometric mean reflection coefficient, ?, which are related by:

in which

a = Sabine absorption coefficient

? = energy absorption coefficient

We can skirt this problem by concentrating our attention on the Sabine coefficient, a, which is actually what is measured and published in various tables.