Characterizing porous materials using the two-cavity method: Numerical considerations and predictive accuracy

The characteristic impedance and propagation constant are intrinsic parameters for understanding the acoustic absorption performance of a porous material. These parameters can be characterized by testing a bulk absorber in a normal incidence impedance tube via the two-thickness or two-cavity method. The two-thickness method requires two samples backed by a hard wall. The two-cavity method requires only one sample to acquire these intrinsic parameters–a useful feature for characterizing materials expensive or difficult to manufacture. This study uses the two-cavity method to predict the surface impedance and absorption coefficient of various bulk materials. The samples tested include open-cell metallic and non-metallic foam, and additively manufactured diamond-type Triply Periodic Minimal Surface (TPMS) samples. The predictions derived from the two-cavity method are compared to the predictions derived from the two-thickness method and the measured results. Certain numerical conditions of impedance in the two-cavity calculations, dependent on selected cavity depths, impacted local predictive accuracy. Cavity depth sizing guidelines to avoid these numerical conditions and improve predictive accuracy are provided.