Actively tunable transverse waves in soft membrane-type acoustic metamaterials

Membrane-type metamaterials have shown a fantastic capacity for manipulating acoustic waves in the low frequency range. They have the advantages of simple geometry, light weight, and active tunability. In general, these membrane-type metamaterials contain a rigid frame support, leading to a fixed configuration. However, in some instances, flexible and reconfigurable devices may be desirable. A soft membrane-type acoustic metamaterial that is highly flexible and controllable is designed here. Different from the previously designed membrane-type metamaterials, the stiff supporting frame is removed and the stiff mass at the center of each unit cell is replaced by the soft mass, realized by bonding fine metallic particles in the central region. In contrast to the previous studies, the propagation of elastic transverse waves in such a soft metamaterial is investigated by employing the plane wave expansion method. Both the Bragg scattering bandgaps and locally resonant bandgaps are found to coexist in the soft metamaterial. The influences of structural parameters and finite biaxial pre-stretch on the dynamic behavior of this soft metamaterial are carefully examined. It is shown that whether or not the wave propagation characteristics are sensitive to the finite deformation does not depend on the property and pre-stretch of the membrane. In addition, a broadband complete bandgap and a pseudo-gap formed by the combination of two extremely adjacent directional bandgaps are observed in the low-frequency range, and both can be controlled by the finite pre-stretch.