Effects of compression on sound absorption of transversely isotropic fibrous materials at oblique incidence

When a fibrous layer, used as absorbing material, is compressed, its thickness is reduced, and on the other hand, its properties are changed. Thickness reduction usually worsens the performance of the layer, while the changes in properties occasionally improve it. Therefore, compression may have positive or negative effects on the absorption coefficient of the layer through simultaneous production of the aforementioned phenomena. Up to now, the effects of compression on the performance of an isotropic fibrous layer at normal incidence has been analyzed. In this work, the absorption behavior of a transversely isotropic fibrous layer after compression is investigated. Instead of normal incidence, oblique incidence is considered. For this purpose, the acoustical properties of the compressed fibrous layer, such as viscous characteristic length and flow resistivity, are modified in the direction parallel to its surface along with the normal direction. The propagation of sound wave in the layer is described by total displacement representation of Biot’s equations, and the absorption coefficient of the layer is calculated using transfer matrix method (TMM). The obtained results indicate that increases in compression ratio usually decrease the absorption coefficient in the low and medium frequency region, whereas for high frequencies, compression improves the absorption coefficient or has no effect. In addition, for low initial thicknesses of the layer and at low and moderate angle of incidences, an optimum compression ratio can be found.