Improved low-frequency performance of a composite sound absorption coating

In order to improve the low-frequency acoustic performance, a composite sound absorption coating made by embedding another viscoelastic material into the viscoelastic substrate of the traditional sound absorption coating, has been developed. Compared to the substrate, the transverse wave speed of the embedded-layer is lower. Accounting for the symmetry, the unit cell of the traditional sound absorption coating or the composite sound absorption coating could be approximately regarded as a viscoelastic single-layer cylindrical tube or a viscoelastic double-layer cylindrical tube, respectively. The plane wave normally impinging on the sound absorption coating only excites the axisymmetric wave propagating in the axial direction of the viscoelastic cylindrical tube. If the complex axisymmetric wave number and the effective impedance of the viscoelastic cylindrical tube are obtained, the acoustic performance of the sound absorption coating could be evaluated by using the transfer matrix method, which is suitable for both the single-layer cylindrical tube and the double-layer cylindrical tube. Numerical results show that the peak frequency of the sound absorption coefficient is shifted to the lower location as the proportion of the embedded-layer increases, or the transverse wave speed of the embedded-layer decreases.