Theoretical estimation and experimentation of sound absorption coefficient using CT imaging in wheat straw

In this paper, a theoretical estimation of the sound absorption coefficient of bundled Japanese wheat straw is carried out: the surface area of the skeleton and the volume of the voids are determined on the basis of CT-scan tomographic images. By approximating the gap between two parallel planes, the propagation constants and characteristic impedance in the voids were determined. These values were substituted into a one-dimensional transfer matrix for sound pressure and volume velocity to obtain the normal incidence sound absorption coefficient. To obtain more accurate theoretical values, the tortuosity of the wheat straw was measured and introduced into the theoretical calculations. As the bundled wheat straw is close to a two-dimensional structure, the number of images used in the calculations was reduced to lower the processing cost of the images handled by making it nearly a two-dimensional problem. Differences between theoretical and experimental values were investigated using SEM images, and it was confirmed by SEM images and simulations that the porous part of the thick-walled part of the wheat straw was not involved in sound absorption. The theoretical values obtained by considering this in the CT images were in good agreement with the experimental values.