Attenuation of Sound Wave by Porous Metal inside a Flow Duct

Sound attenuation by porous metal was investigated inside a flow duct with and without air flow. The porosity and the structure of porous metal samples were varied. The energy dissipation coefficient and the axial wave number of the sound wave passing on the porous sample were reported and discussed. The convective effect was significant within the observed frequency, 250 to 2000 Hz. However, the refractive effect becomes prominent in the high-frequency range for the sample with high porosity in the case of upstream propagation (reverse mode). Furthermore, the impedance of the porous sample was educed using a uniform flow profile and shear flow profile. The educed impedance from both flow profiles is similar in the case forward mode for the samples with high porosity. However, the use of shear flow profile resulted in different educed impedance in reverse mode for high porosity porous material. For the porous sample with low porosity, shear flow profile affected the educed impedance in both forward and reverse mode.