An alternative coefficient for sound absorption

The acoustic absorption coefficient is a number that indicates which fraction of the incident acoustic power impinging on a surface is being absorbed. The incident acoustic power is obtained by spatial integration of the incident intensity, which is (classically) defined as the time-averaged intensity associated with the incident sound field. The measurement of the effective, in situ, sound absorption coefficient is problematic as the determination thus requires a decomposition of the sound field in an incident and reflected field which, generally, is virtually impossible to do. This paper introduces an alternative coefficient with which the effective acoustic absorption can be expressed. This coefficient is based on an alternative definition of the incident intensity; the time average of the positive values of the instantaneous intensity. The alternative coefficient is much easier to use in a sense that it follows directly from an in situ, instantaneous intensity measurement. The coefficient does not rely on any assumptions other than the assumption that the linearized wave equation is satisfied (and thus the acoustic energy corollary). As a result, one does not need to decompose the sound field in incident and reflected waves. Hence, one does not need to have prior information about the incident sound field. Accordingly, one does not need to have prior information about the source. The coefficient can be determined in any sound field, either transient or stationary, free field and diffuse/(semi-) reverberant sound fields. The alternative coefficient is illustrated by means of several numerical examples.