Estimation of transfer matrix by applying the theoretical acoustic incident impedance of a lightweight powder layer to the two-thickness method

In order to estimate the sound absorption coefficient when the powder layer is connected to other acoustic elements such as the back air layer, it would be more versatile as an acoustic element if the powder layer could be represented by a transfer matrix. Therefore, in this study, the theoretical values of the acoustic incident impedance of the powder layer obtained were applied to the two-thickness method to derive the characteristic impedance, propagation constant and transfer matrix. In order to verify the obtained transfer matrix, the case where other acoustic elements (nonwoven fabric, back air layer) are stacked was investigated. To verify the validity of the proposed method, the sound absorption coefficient of the powder layer with a back air layer was theoretically derived using the obtained transfer matrix. In the powder layer with a back air space, the theoretical values tended to be close to the experimental values near the first-order peak of the sound absorption curve. In addition, the characteristic impedance and propagation constants of pairs of different thicknesses were combined to address the disturbances in theoretical values.