Membrane-type smart metamaterials for multi-modal sound insulation.

Metamaterial provides a promising way to control low-frequency noise, but its narrow bandgap limits its applications. To end this, a membrane-type smart metamaterial with multi-modal sound insulation property is studied. The proposed metamaterial consists of an aluminum membrane bonded with multi-modal resonant piezoelectric resonators. Both simulated and experimental results show that the proposed metamaterial can broaden the locally resonant bandgaps because of the effect of the multi-modal resonance (the percent bandwidths are 0.19 and 0.22 for the lowest mode and higher two modes, respectively). Large multi-modal sound insulations (over 37 dB) are obtained around the designed resonant frequencies in low frequency regime (<2000 Hz) with an ultra-thin thickness (over 1000 times thinner than the acoustic wavelength). It is also demonstrated that the excellent sound insulation property can be tuned by simply adjusting the external circuits instead of modifying the structure itself. The underlying mechanism of the unusual sound insulation of the proposed metamaterial is attributed to the negative effective bending stiffness derived by the effective medium method. In addition, the parametric study shows that the circuital parameters (capacitances) are inversely related to the sound transmission loss of the proposed multi-resonant metamaterial, which benefits the optimization of insulation effect.