Estimation of sound absorption coefficient by nano-CT scan images in foam materials with membranes in the skeletal window

This study investigated polyurethane foam, a porous sound-absorbing material. Samples with and without membranes sealing the framework openings were imaged using nano-computed tomography (nano-CT), and the internal voids were modeled as slit-shaped gaps between two parallel planes. The normal incidence sound absorption coefficient was estimated using the transfer matrix method, incorporating the measured tortuosity using the ultrasonic method, and the theoretical results were compared with the experimental results. Experimentally, the normal incidence sound absorption coefficient was determined using a two-microphone impedance tube. Image processing techniques were applied to extract the skeletal cross-section and reduce residual noise from the CT images. Stable sound absorption coefficients were obtained for the original and membrane-removed foam samples through image processing of high-resolution CT scans, even with variations in binarization threshold. By accounting for tortuosity and adjusting the correction factor for the skeletal surface area, the theoretical estimates for the foam samples closely matched the experimental values.