On the units of Sabine absorption and the Sabine absorption coefficient

A century ago, W. C. Sabine demonstrated that sound absorption is fundamental to room acoustics, and introduced the idea for a measure of absorption that has become known as Sabine absorption. Because Sabine absorption and area have the same dimension, e.g., square-meters, many acousticians presume that Sabine absorption has the units of area. For a number of years Robert W. Young has been trying to convince us otherwise, arguing that a dimensionless logarithmic unit distinguishes Sabine absorption from area. [By dimensionless logarithmic unit is meant a scale factor that defines unity on a log scale. The neper (Np) and the decibel are two such quantities.] The metric sabin (Sa) is a unit of Sabine absorption. Defining the metric sabin to be one-half neper square-meter (1 Sa=0.5 Np m2) introduces a logarithmic unit while preserving the numerical scaling of current measurement practice. It follows that the unit of the Sabine absorption coefficient is the half-neper, and a difference between the Sabine absorption coefficient and coefficients from other methods becomes explicit. Among other things, a partial explanation for the experimental observation that Sabine absorption coefficients can exceed unity is provided.