The primary goal of this study is to investigate and refine the two-cavity impedance tube method for acoustic characterization of bulk porous materials, specifically addressing previously unexplained inaccuracies in the prediction of surface impedance and absorption coefficients. Unlike the conventional two-thickness approach, the two-cavity method requires only one sample thickness and involves conducting measurements at various air cavity depths behind the sample. The initial analyses revealed previously unidentified numerical instabilities, resulting in anomalous predictions of sound absorption at specific frequencies. Through systematic investigation and use of calculated data, the numerical origins of these anomalies are uncovered and a practical solution, involving the careful selection of cavity depths, is presented. This approach significantly improves predictive accuracy, validating the two-cavity impedance tube method as a robust and effective tool for the acoustic characterization of a wide variety of porous materials, including metallic and nonmetallic open-cell foams and additively manufactured lattice structures.
Ciletti et al. (Sun,) studied this question.