The prediction of the diffuse field sound absorption of perforated panel systems

This paper studies the diffuse field sound absorption coefficient of a system consisting of a rigid perforated panel with a thin porous woven/matted material glued to its back, which is placed in front of an air cavity with a rigid backing. To cut the cost of trial and error diffuse field sound absorption coefficient measurements, a prediction method was developed. Measurements were made in a two-microphone impedance tube of the complex specific acoustic impedances of the un-perforated rigid panel materials, and of the thin porous materials in front of a rigidly terminated air cavity. These values were used in the transfer matrix method to predict the complex specific acoustic impedances of the perorated panels systems as a function of the angle of incidence of the sound. These calculations assumed the systems to have infinite or finite lateral extent. The measured diffuse field absorption values usually lay between the infinite and finite predictions. The most important variables are the perforation factor of the panel, the acoustic resistance of the thin porous material and the cavity depth.