Metamaterial-based absorbers for simultaneous absorption of air-borne sound and structural vibration

In this work, the idea of metamaterial-based absorbers is further extended to develop the concept of sound and vibration absorbers (SVA) capable of simultaneously absorbing sound waves propagating in air and structure. Each SVA unit cell consists of an outer coating and an inner core. The outer coating is a resilient closed-cell foam, superseding the commonly used open-cell porous materials to significantly improve the structural integrity and service endurance. The inner core embedded in the outer coating provides multidirectional inertia to introduce multi-modal interferences with the outer coating. Utilizing a combination of analytic and numerical techniques, we reveal the underlying physics of the SVA for the simultaneous absorption of sound and vibration. A proof-of-concept SVA sample was fabricated using an expanded polyethylene foam as the outer coating and a polylactide hollow cylinder as the inner core. Good agreements are observed between the simulated and experimental results in terms of the normal incident sound absorption coefficients and the frequency response functions. We find that the SVA exhibits two nearly-perfect sound absorption peaks at 850 Hz and 1210 Hz owing to the resonant-induced impedance matching. Meanwhile, the SVA can effectively attenuate structural vibration across an ultra-wide frequency range relying on the effects of the locally-resonant bandgap and the structural damping dissipation. Finally, the noise reduction feasibility of the SVA sample applied in a plate-cavity coupled system was demonstrated. Our reported findings open alternative perspectives for the construction of practical metamaterial-based absorbers to attenuate noise propagating in complex transmission paths.