The present paper focuses on effect of flow and hole cross-sectional shape on acoustic absorption performance of perforated plate with background air space. We experimentally measured the flow rate, acoustic pressure, and transfer function using the improved acoustic impedance tube. The normal incidence absorption coefficient was calculated from the measured transfer function using transfer function methods. As the bias flow velocity passing through the orifices increased, the peak level of the acoustic absorption coefficient increased. The acoustic absorption performance has improved using taper orifices of perforated plate. The peak frequency of the acoustic absorption coefficient depended on the taper angle of hole of perforated plate. Velocity measurements were performed around perforated plate by means of particle image velocimetry. The vortices were formed clearly at the downstream of the orifices due to incident sound of pure tone. We discussed the relation between flow field and acoustic absorption performance of perforated plate with bias flow.
