Theoretical Analysis and Experiments on the Sound Absorption Properties of Foam Sound Absorbers with Thin Membranes Naturally Present in Foams Using Nano-Computed Tomography Scan Images

Foam sound-absorbing materials develop a fine cellular structure during manufacturing, resulting in variations in porosity, cell size, and the proportion of naturally occurring thin membranes that obstruct skeletal openings. This membrane proportion significantly affects sound absorption. In this study, we utilized cross-sectional images obtained from a submicron resolution computer tomography (CT) scanner (nano-CT) that can capture membrane structures to theoretically assess the sound absorption of foam materials with membranes. We processed these cross-sectional images using techniques, including binarization, to extract the contours of the foam skeletons and the cross-sectional areas of the voids. By modeling the foam’s cross-section as the clearance between two planes, we were able to determine the propagation constant and characteristic impedance within this clearance. The effective density was adjusted based on measured tortuosity. The normal-incidence sound absorption coefficient (SAC), derived from the transfer matrix method, was then compared with experimental values obtained from a two-microphone impedance tube. Image processing techniques helped extract the skeleton cross-section and reduce residual noise, thereby minimizing the effect of variations in the binarization threshold on theoretical values. The accuracy of the theoretical model was enhanced by incorporating a correction factor for the skeleton surface area.