Theoretical Estimation Based on CT Images and Experiments on the Sound Absorption Coefficient of Foam Materials with Removed Membrane.

The structure of foam sound absorbers is not strictly regular, and it is difficult to create a geometric model. In this study, a method for estimating the sound absorption properties of foam sound absorbers with the membrane removed was proposed based on computed tomography (CT) scan images: the circumference of the structure and the cross-sectional area of the voids in the foam cross-section were determined from CT scans of foam materials. The propagation constant and characteristic impedance at the voids were obtained by approximating the foam material cross-section as the clearance between two planes, and the transfer matrix method was used to calculate the normal incident sound absorption coefficient. Further, the sound absorption coefficient was theoretically derived using the effective density with the measured tortuosity applied and compared with the experimental value using a two-microphone impedance measuring tube. By extracting the skeletal part of foam materials by using image processing and removing the residual noise in CT images, and then varying the correction factor for the skeleton surface area, the theoretical value of the sound-absorbing foam without a membrane was closer to the measured value.