Frequency dependent coefficients commonly used to describe the fraction of acoustic energy absorbed at a boundary.
A value of 1.0 indicates all energy is absorbed and there will be no resultant reflection from this surface. A value of 0 indicates no energy is absorbed. These limiting values are theoretical, and in practice some energy will always be absorbed / reflected.
NOTE: Measured a values are often quoted as being greater than 1. This is a result of the measurement technique and not actually achievable. 1.0 is actually the practical limit.