Finite element analysis of sound absorbing material with periodic decreasing hole profiles

The propagation of acoustic waves in porous materials attenuates with distance. When the thickness of the porous material is equal to the acoustic penetration depth, or critical depth, the sound absorption coefficient reaches an asymptotic value so that any additional thickness of the porous layer provides no significant increase of the sound absorption. To overcome the limitations of the critical depth, a design of sound absorbing material containing periodic holes with decreasing profile diameter is proposed in this paper. The finite element method is used to demonstrate the improved sound absorption over a large frequency band. An extraordinary improvement of the sound absorption coefficient using a periodic conical hole is demonstrated where the critical depth of the porous material is eliminated. The results using the finite element method are compared with theoretical results from a transfer matrix method using a double porosity model, and a good agreement is obtained. A parametric analysis is presented using finite element simulations to illustrate the effects of the different parameters of the decreasing hole profile diameter on the sound absorption coefficient. Different hole shapes with decreasing profile diameters distributed periodically inside the porous layer are compared, and the results show good acoustic performance. The proposed sound absorbing material is applied in a rectangular room as anechoic termination. The result of the reflection coefficient obtained by a mirror source method is close to zero over a large frequency band. This illustrates good sound attenuation of the proposed design.