Enhancement of the low frequency sound absorption of porous materials by addition of a thin high flow-resistivity cover layer

The general performance of a porous material against a rigid backing shows very good absorption qualities at high frequencies, but at lower frequencies, where the thickness becomes much less than a wavelength of sound, the absorption is considerably reduced. Based on observations noted during outdoor sound propagation studies and published literature on layered sound absorbing structures, the addition of a thin cover with a high flow-resistivity cover layer can enhance low frequency sound absorption. The objective of this studies was to examine the possibility of increasing low frequency without significantly reducing higher frequency absorption, which usually occurs. The sound absorbing materials were modelled as porous media with complex characteristic impedance and wave number that were obtained using a single flow resistivity parameter based on a widely-used semi-empirical model due to Delany and Bazley (1971). After a thorough parametrical study, experiments were performed to validate the modelling approach, an optimisations was run in order to seek the best combination of flow resistances and layer thicknesses for a given performance requirement.