Monitoring method of scattering at specimen in high-frequency measurement of normal-incidence sound-absorption-coefficient utilizing impedance tube

This study deals with development of a high-frequency measurement method of normal-incidence sound absorption coefficient in a cylindrical impedance tube. The authors have previously proposed the method using multiple microphones which enables the normal-incidence absorption coefficient measurement in the frequency range beyond the cut-on frequencies of the higher order acoustic modes. It is confirmed that the proposed method is valid for commonly used absorbing materials at which the scattering does not occur. However, it is difficult to judge whether the scattering occurs or not, only from measured absorption coefficient. Hence, in this study, a method that evaluates the effect of the scattering is proposed to confirm the reliability of the measured results. As an indicator of the scattering effects, a normal-incidence (1, 0)-mode-reflection coefficient and a total power reflection coefficient in a tube are defined. They are derived from the difference between the signals of the two microphones placed on opposite sides of the tube. Numerical simulations were conducted to prove the theory. Finally, the experimental results are shown, confirming the validity and the feasibility of the proposed method.