Numerical calculation on measurement method of acoustic impedance of perforated plate

The flow duct test is often employed to educe acoustic impedance of a perforated plate facing to grazing flow. A conventional method, an application of the so-called direct method, has been proposed to measure the acoustic impedance in a flow duct. In this method, evenly spaced four microphones aligned parallel to the flow duct are used to calculate propagation constants. In this study, to validate this measurement method we conducted numerical analyses simulating the cases that the flow was absent. The sound pressures at the positions of the four microphones in a flow duct whose one wall had finite acoustic impedance were calculated by boundary element method. Then, we confirmed that the prescribed acoustic impedance on the wall was successively reproduced by the proposed measurement method. Moreover, we examined errors introduced by the displacement in the positions of microphones, which could be inevitable in measurement.