Absorption Characteristics of Acoustic Material with Perforated Facings

This paper extends and unifies the analysis of sound absorptive structures consisting of (1) a layer of porous material separated from (2) a rigid wall by (3) an air cavity, which may or may not contain (4) separating partitions; and the porous material may or may not be covered with (5) a perforated facing. The normal impedance of the porous layer is expressed in a series form in which the first term suffices for a very thin layer. Equations and design charts are given for the impedance and the absorption coefficient, as functions of angle of incidence, for the complete structure with and without partitions. The statistical coefficient, averaged over angle, is plotted for each case over the commonly encountered ranges of all design variables. Calculated coefficients for eight different designs are compared with those measured by the reverberation method and found to be in good agreement.