Extension of the frequency range of normal-incidence sound absorption coefficient measurement in impedance tube using four or eight microphones

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This study proposes a normal-incidence sound absorption coefficient measurement technique in the high-frequency range in which obliquely propagating waves can exist in an impedance tube. To extract the normal-propagating wave factor through a cross section of a cylindrical tube, four microphones are located with one in each quarter of the circumference and their signals are summed. The normal-incidence absorption coefficient is calculated from the frequency response function between the normal-propagating factors of two cross sections, which are placed at a prescribed distance. Using the proposed method, measurement can be performed at about twice the frequency of the conventional method. To verify the validity of the proposed method, numerical simulations by the finite element method (FEM) in cases when there is a slight inclination of the specimen surface, which causes scattering, were conducted. The simulation results prove that the proposed method can cancel the effect of the (1, 0) and (2, 0) modes and enable the normal-incidence absorption coefficient to be measured in the frequency region in which these modes can propagate. Finally, experimental results show the validity and the feasibility of the proposed method.

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