Different studies have been theoretically and experimentally performed to insight the tunability of phononic crystal. Vasseur and coworkers in 2001 [A] demonstrated the existence of absolute stop bands in triangular array of parallel circular steel cylinders in an epoxy matrix, band gaps are independent of the direction of propagation in the plane perpendicular to the cylinders. Khelif [B] showed that the pass band could be tune by controlling the inner radius of the tubular steel inclusions in two-dimensional composite media composed of a square array of hollow steel cylinders embedded in water. The effect of the tube inner radius on the transmission spectrum was founded semiquantitatively separable from the effect of the composite periodicity. Tsong [C] work showed that the elastic band gaps can be enlarged or reduced by adjusting the temperature of the phononic structure. The temperature effect can potentially be utilized for fine-tuning of the phononic band gap frequency and the precise design of filters. Dehesa group [D, E] showed that specially designed two-dimensional arrangements of full elastic cylinders embedded in a nonviscous fluid or gas, define a new class of acoustic metamaterials characterized by a dynamical effective mass density that is anisotropic. Dehesa and colleagues showed an explicit dependence on the lattice filling fraction, the elastic properties of cylinders relative to the background, their positions in the unit cell, and their multiple scattering interactions.
More recently, Robillard [F] founded the feasibility of tuning the band structure of phononic crystals composed of an epoxy matrix and Terfenol-D inclusions, by employing magnetostrictive materials and applying an external magnetic field, which show the contactless tunability of the absolute band gaps of a two-dimensional phononic crystal. The tunable phononic crystal behaves like a transmission switch for elastic waves when the magnitude of an applied magnetic field crosses a threshold. Chin [G] and coworkers presented in 2011 a theoretical study on the tunability of phononic band gaps in two-dimensional phononic crystals consisting of various anisotropic cylinders in an isotropic host. The anisotropic materials used in their work include cubic, hexagonal, trigonal, and tetragonal crystal systems showing that by reorienting the anisotropic cylinders, we show that phononic band gaps for bulk acoustic waves propagating in the phononic crystal can be opened, modulated, and closed.
In the past two years many works have been reported on the tunability of PnC properties using different mechanisms. Bertoldi [H], for instance demonstrated the ability to tune the phononic band gaps of 3D periodic elastomeric structures using deformation. The elastomeric nature of the material makes the transformation of the band gaps a reversible and repeatable process, providing avenues for the design of tunable 3D phononic crystals like sonic switches. The influences of geometrical perturbations from rows to rows in PnC were investigated by Leonoir [I] and colleagues, showing that when either the inner radius of the shells between two adjacent rows is varied, or the spacing between the rows is changed, proposing the change of the steel rows can also by steel-polyethylene bilayers. Feng [J], investigated the compression effect on the tunability of phononic crystals consisting of arrays of cylindrical elements when these systems are excited by a continuous dynamic signal under large static precompression, they support a characteristic band structure whose cutoff frequency can be controlled by changing the alignment angles or the static precompression. The past year, Wang [K] investigated the effects of geometric and material nonlinearities introduced by deformation on the linear dynamic response of two-dimensional phononic crystals. He displayed out that not only the deformation can be effectively used to tune the band gaps and the directionality of the propagating waves, but also revealed how geometric and material nonlinearities contribute to the tunable response of phononic crystals. Pashchenko [P] reports a novel type of tunable phononic crystal based on an electric field-induced piezoelectric effect in a thin ferroelectric film. Using finite element simulation their group revealed the possibility of applying the ferroelectric phononic crystal as an electrically tunable surface acoustic wave filter.
Authors [Q] reported theoretical and experimental results on a novel tunable phononic crystal based on thermosensitive hydrogel and steel cylinders. The reported PnC is an optically responsive sonic structure that can modulate the filtering or transmission of ultrasonic waves through it. The modulation of the ultrasonic waves occurs due the electromagnetically induced volume phase transition of the PNIPAm hydrogel infiltrated within the periodic phononic structure.