High-frequency measurement method of normal-incidence sound absorption coefficient with a square cross-section impedance tube using eight microphones

The authors have proposed a high-frequency measurement method of normal-incidence absorption coefficient with a circular cross-section impedance tube using eight microphones. This method enables measurement at frequencies about three times higher than the conventional two-microphone method. This study investigates the case of using an impedance tube with a square cross-section. Four microphones are placed at intersections of the (2, 2) acoustic mode nodal lines in one cross-section. When the signals of these microphones are summed, the effects of higher-order acoustic modes whose cut-on frequencies are lower than that of the (4, 0) acoustic mode are canceled. In this case, normal-incidence sound absorption coefficients can be measured at four times higher frequency than the conventional one. Numerical and experimental results are presented to demonstrate the validity and feasibility of the method.