Tunable phononic metamaterials for controlling the sound propagation
We describe some details on different heterogeneous devices that display new and unexpected responses such as negative index of refraction, subwavelength imaging and negative bulk modulus which, in general are referred as metamaterials, particularly the mechanical and acoustic properties of pentamode metamaterials, also known as phononic metamaterials (PnM). We present a short review on the principal advances in tunable PnM including own results on this topic. Some own experimental and simulation evidences of the tunability of metamaterials-like phononic crystals are presented. We describe the tunability of the sound propagation in two-dimensional phononic crystals (PnCs) based on steel cylinders periodically embedded in deionized water, comparing the steel cylinder diameter and lattice parameters and the effect of have a lens-like structure. Analysis is doing by observing the sound transmission spectrum. We observe band gaps dependent on the geometrical features of the PnCs.
Metamaterials (MMs) are 1D, 2D or 3D novel artificial material devices, displaying interesting, unexpected and unusual properties not existing in nature, which will be completely determined by the disposition, structure or composition of its components [1, 2]. Thanks to advances in theoretical physics, computational modeling techniques, fabrication and characterization tools, in the past decade many different MMs devices has been possible to engineering i.e., photonic metamaterials for the light propagation control , plasmonic MMs controlling plasmon resonances , an class of planar MMs with abilities to mold light flow, called plasmonic metasurfaces , tunable MMs based on liquid crystals where the main metamaterial ingredient is a liquid crystal included in the metamaterial structure , MMs based on graphene layers composites , superconducting  and quantum MMs presenting nonlinear behaviors or described by quantum structure (dots, wires) , phononic MMs for cloaking, tuning or collimating mechanic waves [10-12], and others.
Non-natural properties of metamaterial-like devices includes for instance, negative refractive index , ultra high  or zero refraction , subwavelength imaging , negative bulk modulus or mass density , artificial magnetism , tunable electric permittivity , and others. Even when the negative refractive index was reported by Veselago  in 60´s, metamaterial term was used firstly by Walser until ending 90´s to describe the material performance of structures with properties beyond the limitations of conventional composites , particularly the electromagnetic metamaterials design. The above mentioned unprecedented properties of MMs have open some current and possible applications like for example, perfect absorber incident waves for antennas or radar devices [21, 22], optical MMS for Raman scattering applications , MMs for nanophotonics and nanocircuits fabrication , MMs used in sensor technologies , MMs has been also used in different biomedical and applications , the construction of superlens, hyper and metalenses for super-resolution imaging [27, 28], the use of metamaterials for cloaking  and invisibility  or electromagnetically induced transparency components for slow light [31, 32], engineered devices for collimate, tune, filter or cloak thermal or sonic waves [11, 12, 33, 34], and among.
In this work authors are interested in describe the principal advances and some applications on phononic metamaterials, putting special attention on the tunability properties MMs-like structures, including own results. The paper is organized as follows: in the next section authors present some details about the general properties of phononic metamaterials. Section 3 is focus to describe the main advances on tunable phononic crystals, describing some works about the tunability process. Sections 4 and 5 contain the experimental and simulations details and the concerning result and discussion, which evidence the tunability effect. We end our paper with some conclusions.
Phononic metamaterials (PnM) are carefully man-engineered structures capable to control and tune both, sound (acoustic vibrations) [11, 35] and heat (thermal vibrations) [33, 36] propagation in ways not observed in nature. In that sense, one can referred to thermal or acoustic PnM depending on its internal structure by virtue of acoustic vibration are in the Hz-KHz frequency range while thermal vibration correspond to THz range [11, 33], such that, the PnM internal structure needs be sensible to these frequencies. Phononic Crystals (PnC) are referred as a PnM kind, an artificial periodic composite metamaterials consisting of sound or heat scatters, periodically disposed in a matrix with high impedance contrast of mass densities and/or elastic moduli with respect to that for scatters. PnC are the analogue photonic crystals but engineering for the phonons controls, photons in the second case. The contrast between the elastic features of the matrix and scatter can give rise to new and unusual phononic features and the existence of phononic band gaps (PnBG), coming from the periodic Bragg scattering as well as localized Mie scatterings from the individual scatters [36, 37]. Recent progress in nanofabrication technologies, however, has made it possible to engineering micro/nano-scale periodic structures allowing to the modulation of wavelength in the GHz frequency range, which are in the domain of thermal excitations . Thus, the manipulation of thermal properties, such as thermal conduction and heat capacity, are possible [39, 40].
One of the main and first observed properties of phononic crystals is concerning with the fact that these composite media typically exhibit stop bands or phononic band gaps (PnBG) in its transmission spectra, i.e., frequency ranges where the propagation of sound and vibrations is strictly forbidden, like in the photonic band gaps in photonic crystals. The first reports on phonons propagating in 1D-periodic elastic composites were done by Dobrzynski and Djafari-Rouhani group in 80s, describing in a first paper an Al-W superlattice made from alternate layers of Al and W , founding the importance of the layer thickness on the vibrational properties of the composite. The same group reported the existence of phononic gaps and surface localized phonons in superlattice consisting of alternating slabs of two different crystals . They develop the theory to easily study all the bulk and surface vibrational properties of a superlattice like the described. This last report is probably the first study on the existence of vibrational gaps.
Inspired in the existence of photonic crystals-like structures, starting 90s, Kushwaha and coworkers developed the first theoretical studies on the existence of phononic band gaps in 2D-periodic structures [43, 44].