BROADBAND SOUND ABSORPTION OF A METAPOROUS LAYER WITH PERIODIC RIGID PARTITIONS

The sound absorption performance of an acoustic porous material works well mainly in a high frequency range, which is due to the dimensional relation between the thickness of an installed porous material and the wavelength of an incident sound wave. To enhance the sound absorption performance of a porous layer over a broad frequency band including a low-frequency range, we propose a metaporous layer, which is supposed to break the dimensional constraint considerably. The proposed metaporous layer is a porous layer with subwavelength-periodic rigid partitions that are embedded in parallel and perpendicularly to sound wave incidence. The porous layer is assumed to be hard-backed. We demonstrate how much the sound absorbing performance can be enhanced with the proposed metaporous layer under the same thickness constraint of a nominal homogenous porous layer. The physical mechanisms behind the enhanced performance including the lowered lower bound working frequency of sound absorption are explained by using an effective medium theory developed for the proposed metaporous layer. Because the unit cell forming the metaporous layer has multiply-tuned thickness resonances at which the surface impedance is matched better with the impedance of surrounding air, the overall sound absorption can be greatly enhanced. The broadening of the working frequency range of effective sound absorption covering a lower-frequency range is due to elongation in the effective layer thickness. Several metaporous layer configurations are suggested, which exhibit considerably improved sound absorption performance over the homogeneous porous layer under the same layer thickness.