In this work, Fano resonance is studied in a pillared acoustic metasurface, which originates from the avoided crossing of strongly coupled individual monopolar modes of two dissimilar pillars in one unit cell. The Fano resonance manifests itself as an outofphase compressional (monopolar) vibrations of the two pillars coupled through the plate. The Fano asymmetric transmission profile can be analyzed through the interference between the incident wave and the scattered wave emitted by resonant pillars. The peak in the asymmetric transmission is always maintained either the coupling of the two pillars is strong or weak. It is further found that the Fano resonance can be tuned by varying different geometrical parameters, such as the height/diameter of the pillars, the distance between the two pillars, or introducing an inner hole into the pillars which can possibly be filled with a liquid.

In this chapter, we discuss the vibrational properties of one-dimensional (1D) phononic crystals of both discrete and continuous media. These properties include the dispersion curves of infinite crystals as well as the confined modes and localized (surface, cavity) modes of finite and semi-infinite crystals. A general rule about the existence of localized surface modes in finite and semi-infinite superlattices with free surfaces is presented. We also present the calculations of reflection and transmission coefficients, particularly in view of selective filtering through localized modes. Most of the results presented in this chapter deal with waves propagating along the axis of the superlattice. However, in the last part of the chapter, we also discuss wave propagation out of the normal incidence and, more particularly, we demonstrate the possibility of omnidirectional transmission gap and selective filtering for any incidence angle. A comparison of the theoretical results with experimental data available in the literature is also presented and the reliability of the theoretical predictions is indicated. IntroductionThe one-dimensional (1D) phononic crystals called superlattices (SLs) are of great importance in material science. These structures are, in general, composed of two or several layers repeated periodically along the direction of growth. The layers constituting each cell of the SL can be made of a combination of solid-solid or solid-fluid-layered media. These materials enter now in the category of so-called phononic crystals (see the first chapter of this book and references therein) [1][2][3] constituted by inclusions (spheres, cylinders, etc.) arranged in a host matrix along two-dimensional (2D) and three-dimensional (3D) of the space. After the proposal of SLs by Esaki [4], the study of elementary excitations in multilayered systems has been very active. Among these excitations, acoustic phonons have received increased attention after the first observation by Colvard et al.[5] of a doublet associated to folded longitudinal acoustic phonons by means of Raman scattering. The essential property of these structures is the existence of forbidden frequency bands induced by the difference in acoustic properties of the constituents and the periodicity of these systems leading to unusual physical phenomena in these heterostructures in comparison with bulk materials [6][7][8].With regard to acoustic waves in solid-solid SLs, a number of theoretical and experimental works have been devoted to the study of the band gap structures of periodic SLs [6-10] composed of crystalline, amorphous semi-conductors, or metallic multilayers at the nanometric scale. The theoretical models used are essentially the transfer matrix [7,[11][12][13] and the Green's function methods [6,[14][15][16], whereas the experimental techniques include Raman scattering [5,17,18], ultrasonics [19][20][21][22][23][24][25][26][27][28][29], and time-resolved X-ray diffraction [30]. Besides the existence of the band-gap structures in perfe...

Wave demultiplexers transporting desired wavelengths towards proper directions or ports are attracting numerous interests and applications in both physical and engineering areas. In acoustics, there is still a lack of compact and simple designs to achieve demultiplexers in three-port systems. In this work, we propose such a design using Helmholtz resonators where the frequency selection is based on the phenomenon of acoustically induced transparency (AIT). First, a modified transfer matrix method is derived to analytically describe and analyze the AIT effect with Helmholtz resonators. Then, the good performances of wave routing in these designs are further demonstrated by both simulation and experiment. These AIT based demultiplexers are subwavelength and simple in their designs. Therefore, they are promising for various potential applications such as signal processing, information communication and sensing.