Acoustic performance of cavities with gradient changes of cavity numbers under low lattice constant in a soft elastic medium

Air cavities with particular arrangements embedded in elastic layers are generally used as anechoic coatings. In this paper, an idea of cavity numbers with gradient changes is introduced in a soft elastic medium that comprises periodical cylindrical cavities to enhance the low-frequency absorption capacity effectively. The presented model provides a sound absorption coefficient that is higher than 0.5 in the frequency ranges of 75 to 95 Hz and 380 to 720 Hz. Thus, validating the idea can effectively enhance the low-frequency absorption capacity compared with those in previous literature. Analytical results based on effective medium approximation theory are compared with those obtained numerically using the finite element method. These results indicate that the invalidity of the analytical model is due to the coupling between cavities under low lattice constant. Power density dissipation and displacement vectors are plotted to study sound absorption mechanism, thereby showing that the dissipated energy is mainly resulted from the multiple-scattering effect. The scattering between cavities induces the transverse wave caused by the wave mode conversion dissipates in the elastic medium and results in sound absorption. Simulation results show that the absorption peaks can be effectively manipulated by tuning the gradient profile of the cavity numbers. The proposed model is then verified by the corresponding experiment. Results obtained numerically and experimentally are agreed well, and its feasibility is verified. The presented model would offer a new approach for the metamaterial design on low-frequency broadband sound absorption and could have potential applications on the design of anechoic coatings.