Applications and Limitations of a Four-Microphone Impedance Tube in the Liquid Media in the Low-Frequency Range

A four-microphone impedance tube is generally used for the absorption measurements of an acoustic material in the gaseous medium, as presented in the standard ASTM E2611. Corresponding dimensional requirements for the experimental setup can be adapted for the application in a liquid medium. However, the standard does not account for the waveguide-related wavefront curvature in the impedance tube with an elastic wall. In this paper a four-microphone impedance tube is v alidated for the low-frequency transmission-loss measurements in liquids, complying with the restriction of plane-wave sound propagation. Furthermore, the transmission-loss calculation requires knowledge of the acoustic properties of the propagation medium. Therefore, methods for on-site measurements of the group velocity and the complex wavenumber, which includes phase velocity and attenuation constant, were investigated. Existing methods, applicable to the liquidfilled impedance tube, were compared to the new approach of problem-specific methods. Tube termination related limitations were observed and analysed.