Search

The thermal properties of a material with a spatio-temporal modulation in both the thermal conductivity and the mass density are studied. The special configuration studied here consists of a modulation in a wave-like fashion. It is found that these materials behaves, in an effective way, as materials with an internal convection-like term that provides them of non-reciprocal properties, in the sense that the flow of heat has different properties when it propagates in the same direction or in the opposite one to the modulation of the parameters. An effective medium description is presented which accurately describes the modulated material, and numerical simulations supports both the non-reciprocal properties and the effective medium description. It is found that these materials are promising candidates for the design of thermal diodes and other advanced devices for the control of the heat flow at all scales.

Raspberry-like magnetic nanoclusters are synthesized and subsequently self-assembled to form a bulk metamaterial exhibiting strong isotropic optical magnetism in visible light. The magnetic response of the nanoclusters (metamolecules) and of the final assembled material are measured by independent optical experiments. The validity of the effective permeability parameter is probed by spectroscopic ellipsometry at variable incidence. Numerical simulations confirm the measurements.

Availability of standardized morphological and molecular characterization data is essential for the efficient development of breeding programmes in emerging crops. Pepino (Solanum muricatum) is an increasingly important vegetatively propagated vegetable crop for which concurrent data on morphological descriptors and molecular markers are not available. We evaluated 58 morphological traits, using a collection of 14 accessions of pepinos (including local Andean varieties and modern cultivars) and 8 of wild relatives, using the IPGRI and COMAV descriptors lists coupled with 20 EST-SSRs from tomato. High morphological diversity was found in both cultivated and wild accessions; all morphological traits except three were variable. Cultivated pepino and wild relatives were significantly different for 26 traits. Also, local varieties and modern cultivars of pepino were different from each other for 13 morphological traits and were clearly separated in a principal components analysis. Fourteen of the 20 tomato EST-SSRs were polymorphic, with an average number of alleles per locus of 4.07 and a polymorphic information content value of 0.4132. This revealed a high degree of transferability from tomato to pepino and wide molecular diversity in the collection. Cultivated materials manifest high levels of observed heterozygosity, suggesting that it is related to heterosis for yield associated with heterozygosis. SSR data clearly differentiated cultivated and wild materials. Furthermore, for pepinos, the modern varieties were genetically much less diverse than the traditional local varieties. However, both groups of cultivated material expressed a low degree of genetic differentiation. A strong correlation (r = 0.673) between morphological and molecular distances was found. Our results provide foundational information for programmes of germplasm conservation, and that can be used to enhance breeding for this emerging crop.

Many efforts have been devoted to studying the robustness of topologically protected edge states in acoustics; however, the robustness of conventional edge states is rarely reported. In this work we theoretically study interface acoustic states appearing in finite arrays of resonators on a thin plate with topologically protected and conventional designs. Topologically protected interface states are first analyzed by employing the concept of breaking inversion symmetry within the unit cell of a honeycomb lattice for cylindrical and spherical resonators; we further demonstrate the robustness of the wave propagation along a zigzag path containing sharp corners and defects. In parallel, a conventional interface state is also designed and compared to the same situations. We found that the conventional interface state suffers backscattering in the zigzag path while it can show a more confined wave transport in some cases. The presence of a defect along the propagation path scatters the conventional interface wave and in particular can prohibit full propagation in the presence of a localized state at the defect. Then, we show that the immunity of the topologically protected design needs the interface to be surrounded by at least two hexagons of the phononic crystals on both sides, especially at the sharp corners in the zigzag path, while the conventional design only needs one hexagon of bulk media with the advantage of compact wave transport. Position and height disorders are further introduced to the interface pillars for both designs. It is revealed that in both designs, the transmission decreases quasilinearly with position disorder while it exhibits an abrupt drop with height disorder showing a transition threshold. With high disorder perturbation, waves can hardly enter the interface for the topologically protected design, while waves are trapped at the interface for the conventional design. A certain robustness against disorder is exhibited for conventional edge states. This work provides insight into the interface states in micro- and nanoscale characterization and figures out the behaviors for both topologically protected and conventional interface states.

International audience; Materials with properties that are modulated in time are known to display wave phenomena showing energyincreasing with time, with the ratemediated by the modulation. Until nowthere has been no accounting for materialdissipation, which clearly counteracts energy growth. This paper provides an exact expression for the amplitudeof elastic or acoustic waves propagating in lossy materials with properties that are periodically modulated intime. It is found that these materials can support a special propagation regime in which waves travel at constantamplitude, with temporal modulation compensating for the normal energy dissipation. We derive a generalcondition underwhich amplification due to time-dependent properties offsets thematerial dissipation. This identityrelates band-gap properties associated with the temporal modulation and the average of the viscosity coefficient,thereby providing a simple recipe for the design of loss-compensated mechanical metamaterials.

International audience; We present a homogenization method for periodic acoustic composites based on the Plane Wave Expansion (PWE) method. We show that the description of periodic acoustic composites needs constitutive parameters which depend on frequency and wavenumber, meaning that the effective material is resonant and nonlocal. Also, an anisotropic mass density and an additional constitutive parameter, called the Willis term in analogy to its counterpart in elasticity, are found. Numerical calculations compare the present method with the traditional multiple scattering method, showing a good agreement between both theories. However, the method presented here overcomes the limitations of the multiple scattering method, where the incorporation of anisotropy and non-locality implies solving the scattering problem of anisotropic objects with additional boundary-conditions. A final example showing the importance of nonlocal effects is provided. This work shows that acoustic metamaterials are nonlocal materials in general, and provides a tool for the proper modeling of the nonlocal constitutive parameters.

International audience; In this work, we propose the realization of electromagnetic metamaterials in two and three dimensions by means of highly anisotropic rods and spheres. The two-dimensional case is deeply analyzed, and the realization of an anisotropic rod by means of angularly layered cylinders is proposed. The band structure of a periodic arrangement of angularly layered rodsis computed, showing a low frequency band gap and some flat modes within it which increases as a function of the number of sectors in the cylinder. The system is studied by means of an effective medium approach, and it is found that the flat bands correspond to a sharp frequency region where the effective metamaterial is doubly negative. A perfect agreement is found between the dispersion curves obtained by means of the effective medium model and the full band structure computed by the finite element method. It is also found that only four sectors in the angularly layered cylinder are enough to obtain the desired double negativity, which is a promising result in view of the fabrication of the presented structures. Finally, the three-dimensional version of these materials is analyzed by means of anisotropic spheres, showing similar results.

This work presents a comprehensive study of the cloak for bending waves theoretically proposed by Farhat et al. [see Phys. Rev. Lett. 103, 024301 (2009)] and later on experimentally realized by Stenger et al. [see Phys. Rev. Lett. 108, 014301 (2012)]. This study uses a semi-analytical approach, the multilayer scattering method, which is based in the Kirchoff-Love wave equation for flexural waves in thin plates. Our approach was unable to reproduce the predicted behavior of the theoretically proposed cloak. This disagreement is here explained in terms of the simplified wave equation employed in the cloak design, which employed unusual boundary conditions for the cloaking shell. However, our approach reproduces fairly well the measured displacement maps for the fabricated cloak, indicating the validity of our approach. Also, the cloak quality has been here analyzed using the so called averaged visibility and the scattering cross section. The results obtained from both analysis let us to conclude that there is room for further improvements of this type of flexural wave cloak by using better design procedures. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)., This work has been supported by the Spanish Ministerio de Economia y Competitividad and the European Union Fondo Europeo para el Desarrollo Regional (FEDER) under Grant with Ref. TEC2014-53088-C3-1-R. We gratefully acknowledge Nicolas Stenger for providing us the parameters listed in Table I.

We present a dynamical homogenization model for the propagation of acoustic waves in periodic arrangements of scatterers. The model is based on the plane wave expansion method and leads to the effective parameters associated with the propagation. Effective mass density, effective bulk modulus and effective Willis term are frequency and wave vector dependant. The case of a rectangular lattice of cylinders is investigated. The homogenization model in the local approximation fits with the plane wave expansion method for small wave numbers. The branches of the dispersion curves are correctly found because of the anisotropy of the mass density.

In this work, we present a method for the design of gradient index devices for elastic waves in plates. The method allows the design of devices to control the three fundamental modes, despite the fact that their dispersion relation is managed by different elastic constants. It is shown that by means of complex graded phononic crystals and thickness variations it is possible to independently design the three refractive indexes of these waves, allowing therefore their simultaneous control. The effective medium theory required for this purpose is presented and the method is applied to the design of the Luneburg and Maxwell lenses as well as to the design of a flat gradient index lens. Finally, numerical simulations are used to demonstrate the performance of the method in a broadband frequency region.

We present an analysis of acoustic cloaks based on the homogenization of two fluidlike materials, with an emphasis on periodically layered imperfect cloaks, by removing the singularities of the acoustic parameters required for ideal cloaks. The conditions that material layers should satisfy are systematically analyzed and critically discussed according to their feasibility.

We discuss the homogenization of acoustic metamaterials with spatio-temporal modulation. It is shown that the effective medium required for the correct description of these structures is a non-recyprocal Willis material. Analytical expressions are given for the effective parameters, and several numerical examples are shown. The theory is applied as well to the specific case of thermal metamaterials, and it is shown that a thermal diode is feasible by spatio-temporal modulation.

We present an acoustic carpet cloak based on acoustic anomalous reflectors. The cloak consists on a engineered diffraction grating, designed in such a way that a wave incident with a given angle is “retrorefflected,” so that a pyramidal surface reflects a normally incident wave as if it were a flat surface. Different geometries and angles of the surface are tested, showing that the surface can have an arbitrary shape, as long as the retrorreflection conditions be satisfied locally. Finally, the experimental characterization of these cloaks is discussed.

Gradient index devices for the simultaneous control of the S0 and A0 Lamb modes are presented and numerically analysed. It is found a special relationship between the refractive indexes for these two modes which allows their simultaneous control by means of graded phononic crystals and thickness variations of the plate. A flat gradient index lens and a circular Luneburg lens are designed. Numerical simulations show that the performance of these devices is good for the two modes in a broadband frequency region, and that this approach can be used to design more advanced refractive devices for the total control of guided vibrational modes.Copyright © 2015 by ASME

Spatial gradients in refractive index have been used extensively in acoustic metamaterial applications to control wave propagation through phase delay. This study reports the design and experimental realization of an acoustic gradient index lens using a sonic crystal lattice that is impedance matched to water over a broad bandwidth. In contrast to previous designs, the underlying lattice features refractive indices that are lower than the water background, which facilitates propagation control based on a phase advance as opposed to a delay. The index gradient is achieved by varying the filling fraction of hollow, air-filled aluminum tubes that individually exhibit a higher sound speed than water and matched impedance. Acoustic focusing is observed over a broad bandwidth of frequencies in the homogenization limit of the lattice, with intensity magnifications in excess of 7 dB. An anisotropic lattice design facilitates a flat-faceted geometry with low backscattering at 18 dB below the incident sound pressure level. Three dimensional Rayleigh-Sommerfeld integration that accounts for the anisotropic refraction is used to accurately predict the experimentally measured focal patterns., Under Review, 9 pages, 5 figures, note that Figs. 5(b,c) would not compile correctly on arxiv.org, likely because arxiv does not support revtex4.1. Please see final published article for the correct version of Fig. 5

International audience; We derive a general theory of effective properties in metasolids based on phononic crystals with low frequency resonances. We demonstrate that in general these structures need to be described by means of a frequencydependent and nonlocal anisotropic mass density, stiffness tensor and a third-rank coupling tensor, which shows that they behave like a nonlocal Willis medium. The effect of nonlocality and coupling tensormanifest themselves for some particular resonances, whereas they become negligible for other resonances. Considering the example of a two-dimensional phononic crystal, consisting of triangular arrangements of cylindrical shells in an elastic matrix, we show that its mass density tensor is strongly resonant and anisotropic presenting both positive and negative divergent values, while becoming scalar in the quasistatic limit. Moreover, it is found that the negative value of transverse component of the mass density is induced by a dipolar resonance, while that of the vertical component is induced by a monopolar one. Finally, the dispersion relation obtained by the effective parameters of the crystal is compared with the band structure, showing good agreement for the low-wave-number region, although the nonlocal effects are important given the existence of some resonant values of the wave number.

International audience; A feasible approach is proposed in this work to realize an acoustic omnidirectional invisible lens for flexural waves in thin elastic plates. These devices require extreme values of the refractive index close to its center, and in this work this drawback is overcome by reducing gradually the thickness of the elastic plate, which has the same effect as increasing proportionally the refractive index. Although ideally the thickness of the plate should be zero at the central point, it is shown that a finite but small minimum thickness is enough to provide a good invisibility effect. Numerical simulations are performed by the finite element method, which supports the theoretical predictions, showing that an invisible lens is feasible for flexural waves.

Different omnidirectional refractive devices for flexural waves in thin plates are proposed and numerically analyzed. Their realization is explained by means phononic crystal plates, where a previously developed homogenization theory is employed for the design of graded index refractive devices. These devices consist of a circular cluster of inclusions with properly designed gradient in their radius. With this approach, the Luneburg and Maxwell lenses and a family of beam splitters for flexural waves are proposed and analyzed. Results show that these devices work properly in a broadband frequency region, being therefore an efficient approach for the design of refractive devices specially interesting for nano-scale applications.

The absorption enhancement by lossy dielectric layers backed by metallic metasurfaces has been studied. A rigorous analysis based on mode matching theory has been used for obtaining analytical expressions for the peak position and for total absorption.

An effective medium theory for resonant and nonresonant metamaterials for flexural waves in thin plates is presented. The theory provides closed-form expressions for the effective mass density, rigidity, and Poisson's ratio of arrangements of isotropic scatterers in thin plates, valid for low frequencies and moderate filling fractions. It is found that the effective Young's modulus and Poisson's ratio are induced by a combination of the monopolar and quadrupolar scattering coefficient, as it happens for bulk elastic waves, while the effective mass density is induced by the monopolar coefficient, contrarily as it happens for bulk elastic waves, where the effective mass density is induced by the dipolar coefficient. It is shown that resonant positive or negative effective elastic parameters are possible, being therefore the monopolar resonance responsible for creating an effective medium with negative mass density. Several examples are given for both nonresonant and resonant effective parameters and the results are numerically verified by multiple scattering theory and finite element analysis.

The absorption of electromagnetic waves by a thin dielectric slab backed by a metasurface has been comprehensively studied and discussed at microwave frequencies. The metasurface consists of a metallic plate decorated with a periodic distribution of coaxial- or annular-type cavities. Analytical expressions for the absorbance have been obtained by using a mode-matching method. For P-polarized waves it is predicted that a low-frequency peak of perfect absorption is possible by properly choosing the lossy component of the permittivity of the dielectric slab, with the position of this peak being easily tunable by the cavity length. It is also shown that non-Bravais lattices, containing several cavities in the unit cell, and the excitation of guided waves by the periodic array of resonators are complementary absorption mechanisms in the studied structures., This work was partially supported by the Spanish Ministerio de Economia y Competitividad (MINECO) under contract TEC2010-19751 and by the US Office of Naval Research.

We report the design of quasi-two-dimensional artificial structures that acoustically behave as positive, single negative, double negative or density-near-zero metamaterials. The scattering units consist of a cavity drilled in one surface of a 2D waveguide and they have an inner structure whose geometrical parameters can be selected in order to obtain the desired dynamical behavior. Finally, we report the practical realization of two samples as well as their experimental characterization showing metamaterial features.

A review of the recent advances on acoustic cloaking for airborne sound is here reported. It is shown that engineering acoustic cloaks based on transformation acoustics has a main drawback: there are no passive materials for increasing the sound speed with respect to that of air at room temperature. Such limitation is here overcome by introducing a cloaking shell with a temperature gradient. We also describe the design, fabrication and characterization of two-dimensional and three-dimensional axisymmetric acoustic cloaks based on scattering cancellation. The broadband and omnidirectional operation of these cloaks remain as a challenge for the future.

We report the design and the characterization of artificial structures made of periodical distributions of structured cylindrical scatterers embedded in a two-dimensional (2D) waveguide. For certain values of their geometrical parameters they show simultaneously negative effective bulk modulus and negative effective mass density. Here our analysis is focused on the frequencies where they behave like materials with negative density or density near zero (DNZ). The scattering units consist of a rigid cylindrical core surrounded by an anisotropic shell divided in angular sectors. The units are embedded in a 2D waveguide whose height is smaller than the length of the cylinders, which makes the structure quasi-2D. We have obtained the dispersion relation of the surface acoustic waves excited at frequencies with negative effective density. Also, we report phenomena associated with their DNZ behavior, such as tunneling through narrow channels, control of the radiation field, perfect transmission through sharp corners, and power splitting. Preliminary experiments performed on samples with millimeter-scale dimensions demonstrated their single-negative behavior, with the main drawback being the strong losses measured at the frequencies where the negative behavior is observed., Work partially supported by the Spanish Ministry of Economy and Competitivity with References No. TEC 2010-19751 and No. CSD2008-00066 (Consolider Program). The authors also acknowledge support from the U.S. Office of Naval Research (Grant No. N000142161). The authors thanks Francisco Cervera for his technical help in the fabrication of samples and to Matthew Guild for the critical reading of the manuscript.

This work presents a gradient index device for insulating from vibrations a circular area of a thin plate. The gradient of the refractive index is achieved exploiting the thickness-dependence of the dispersion relation of flexural waves in thin plates. A well-like thickness profile in an annular region of the plate is used to mimic the combination of an attractive and repulsive potentials, focusing waves at its bottom and dissipating them by means of an absorptive layer placed on top. The central area is therefore isolated from vibrations, while they are dissipated at the bottom of the well. Simulations have been done using the multilayer multiple scattering method and the results prove their broadband efficiency and omnidirectional properties., This work has been supported by the U.S. Office of Naval Research under Grant No. N000140910554.

We present the proposal and the theoretical characterization of the elastic analog of the so called photonic black-hole. The structure has a cylindrical symmetry and consists of two parts, a shell that bends the flexural waves and a core where the energy concentrates. The shell behaves like a gradient index lens and the variation of the refractive index has been obtained by changing the thickness of the plate. The simulations based on a multiple scattering algorithm demonstrate the broadband and omnidirectional properties of the structure.

Metamaterial shells based on radially dependent anisotropic parameters are analyzed in view of achieving different types of applications. The theoretical proposal is reviewed together with some possible applications of these resonant structures. Position sensors, energy harvesters or wireless energy transfer systems are briefly presented.

An elastic analogue of graphene is presented. It consists in a honeycomb arrangement of spring-mass resonators attached to a thin elastic plate. It is shown that the band structure for the propagation of flexural waves of the resulting system presents Dirac points. Also, it is shown that finite “slabs” of this honeycomb arrangement can be used as waveguides due to the presence of “edge states”, a special type of guided states that does not appear in the dispersion relation of the infinite system. Finally, multiple scattering simulations of finite clusters of resonators are shown supporting the theory.

Introduction: The ability to ascertain survival information is important for retrospective and prospective studies. Two databases that can be used are the Social Security Death Index (SSDI) and the National Death Index (NDI). Although the NDI is more complete, there are advantages to the SSDI such as ease of use and cost. The intent of this study was to determine accuracy of the SSDI. Methods: Publically available data on all known deceased individuals in the state of Ohio in 2003 were obtained from the State of Ohio Department of Health. A random sample of 63,557 of these were compared to the SSDI to identify risk factor for inclusion/exclusion. Results: Overall, 94.7% of all death records were confirmed by the SSDI. Age at death, gender, race, ethnicity, and cause of death were all found to significantly affect the likelihood of inclusion. Specifically, people aged 18-24 were included only 79.8% of the time compared to 96.2% for those over the age of 65. Also, malignancy as cause of death resulted in a 95.3% inclusion while trauma as a cause of death led to 86.5% inclusion. While Caucasians had an inclusion of 95.6%, African Americans were included only 87.8% of the time. Hispanics and women also had lower inclusion rates. Discussion: The SSDI is a strong tool for following up on participants lost to follow up in certain populations but is weaker in others. The SSDI would be particularly useful in a population that is largely older, Caucasian, or has malignant disease.

Acoustic cloaking is the mechanism representing the ideal acoustic stealth. We introduce and discuss the acoustic cloak, a material shell that renders an object acoustically ‘invisible’ thanks to its presence surrounding the object. It has been shown that cloaking shells require very complex parameters to be realized. This complexity comes from the fact that their acoustic parameters must be anisotropic, inhomogeneous and divergent near the cloaked object. This chapter explains how to engineer artificial structures, which have been called acoustic metamaterials or metafluids, that respond dynamically as anisotropic and inhomogeneous materials. The metafluids are made from arrays of isotropic and homogeneous elastic cylinders or by metallic plates cylindrically corrugated. We also propose solutions to remove the divergences appearing in the design of cloaking shells. It is therefore predicted that, although difficult to realize, cloaking shells are not impossible by using metafluids based on the homogenization of periodic structures.

An elastic analog of graphene is introduced and analyzed. The system consists of a honeycomb arrangement of spring-mass resonators attached to a thin elastic layer, and the propagation properties of flexural waves along it is studied. The band-structure calculation shows the presence of Dirac points near the K point of the Brillouin zone. Analytical expressions are found for both Dirac frequency and velocity as a function of the resonator s parameters. Finally, the bounded modes of infinitely long ribbons of this honeycomb arrangement are analyzed. The presence of edge states, which are studied using multiple scattering theory, is shown. It is concluded that these structures can be used to control the propagation of flexural waves in thin plates., This work was partially supported by the Spanish Ministerio de Ciencia e Innovacion (MICINN) under Contracts No. TEC2010-19751 and No. CSD2008-66 (the CONSOLIDER program) and by the US Office of Naval Research.

Omnidirectional beam splitters for the simultaneous control of the three fundamental Lamb modes in an elastic plate are designed and numerically studied. Beam splitters consist in radially symmetric and inhomogeneous lenses designed to redirect the incoming energy towards a given angle. In this work, these devices are designed by means of graded phononic crystals combined with thickness variations of the plate. Numerical simulations are presented to show the performance of the designed devices.

A multiple-scattering theory is applied to study the homogenization of clusters of elastic cylinders distributed in a isotropic lattice and embedded in a viscous fluid. Asymptotic relations are derived and employed to obtain analytical formulas for the effective parameters of homogenized clusters in which the underlying lattice has a low filling fraction. It is concluded that such clusters behave, in the low frequency limit, as an effective elastic medium. Particularly, it is found that the effective dynamical mass density follows the static estimate; i.e., the homogenization procedure does not recover the non-linear behavior obtained for the inviscid case. Moreover, the longitudinal and transversal sound speeds do not show any dependence on fluid viscosity. Numerical simulations performed for clusters made of brass cylinders embedded in glycerin support the reliability of the effective parameters resulting from the homogenization procedure reported here. (C) 2012 Acoustical Society of America. [http://dx.doi.org/10.1121/1.4744933], This work was partially supported by the U.S. Office of Naval Research (Award No. N000140910554) and by the Spanish Ministry of Science and Innovation under Contracts Nos. TEC2010-19751 and CSD2008-66 (CONSOLIDER program). D. T. acknowledges a research grant provided by the program "Campus de Excelencia 2010 UPV."

Based on the concepts of artificially microstructured materials, i.e. metamaterials, we present here the first practical realization of a radial wave crystal. This type of device was introduced as a theoretical proposal in the field of acoustics, and can be briefly defined as a structured medium with radial symmetry, where the constitutive parameters are invariant under radial geometrical translations. Our practical demonstration is realized in the electromagnetic microwave spectrum, because of the equivalence between the wave problems in both fields. A device has been designed, fabricated and experimentally characterized. It is able to perform beam shaping of punctual wave sources, and also to sense position and frequency of external radiators. Owing to the flexibility offered by the design concept, other possible applications are discussed., This work was supported in part by the Spanish Ministry of Science and Innovation under Grants TEC 2010-19751 and CSD2008-00066 (Consolider program) and by the U.S. Office of Naval Research under Grant N000140910554.

We present the experimental realization and characterization of an acoustic metamaterial with negative bulk modulus. The metamaterial consists of a two-dimensional array of cylindrical cavities, and the bulk modulus is controlled by their radius size and length. Experiments are performed in a two-dimensional waveguide where a slab of seven layers is used to extract the parameters of the metamaterial. A complete characterization of the constructed structure is reported, including the dispersion relation of the acoustic bands and the skin depth effect, which both have been measured, and the data are well supported by semianalytical models and by finite-element simulations. © 2012 American Physical Society., This work was supported by the Spanish MICINN under Contracts No. TEC2010-19751 and No. CSD2008-0066 (CONSOLIDER program), and by the USA Office of Naval Research. We acknowledge the technical help by A. Diaz-Rubio and A. Climente. J.S.-D. acknowledges useful discussions with A. Broatch and A. Krokhin. D.T. acknowledges the postdoctoral grant provided by the UPV under the program Campus de excelencia internacional.

We demonstrate the presence of Dirac cones in the dispersion relation of acoustic waves propagating on the surface of a plate of methyl methacrylate containing a honeycomb lattice of cylindrical boreholes. This structure represents the acoustic analogue of graphene, the cylindrical cavities playing the role of carbon atoms while acoustic surface waves are the equivalent of electronic waves in graphene. Analytical expressions for the Dirac frequency and Dirac velocity in acoustics are given as a function of the radius and depth of boreholes. These parameters have been experimentally determined for a constructed structure and the data are in fairly good agreement with the predicted values. © 2012 American Physical Society., Work supported by the Spanish Ministry of Science and Innovation (MICINN) under Contracts No. TEC2010-19751 and No. CSD2008-66 (CONSOLIDER program). Daniel Torrent acknowledges support from the program "Campus de Excelencia Internacional 2010 UPV." Both authors thank Didier Mayou for useful discussions.

We present the design, construction, and experimental characterization of the acoustic analogue of the so called photonic black-hole. The fabricated sample has cylindrical symmetry and consists of two parts, a shell that bends the sound towards the center and a core that dissipates its energy. The shell is made of a metamaterial that perfectly matches the acoustic impedance of air and behaves like a gradient index lens. The experimental data obtained in a multi-modal impedance chamber demonstrate that the proposed acoustic black-hole acts like an onmidirectional broadband absorber with strong absorbing efficiency. © 2012 American Institute of Physics., Work supported by the U.S. Office of Naval Research. The authors acknowledge V.M. Garcia-Chocano and F. Cervera for useful discussions and technical help. D.T. acknowledges a fellowship provided by the program Campus de Excelencia Internacional 2010 UPV.

This letter presents the design of a reduced acoustic cloak that uses a temperature gradient in order to obtain sound speeds larger than in air. The cloak consists of a circular acoustic crystal made of ten concentric layers of rigid cylinders whose surfaces are heated or cooled in order to get the temperature gradient needed for cloaking behavior. The total pressure field produced by the scattering of sound waves impinging this complex structure is computed and it is shown how acoustic waves are bent in a way similar to that predicted for perfect cloaking devices. © 2012 American Institute of Physics., Work partially supported by the Spanish MICINN under Grant Nos. TEC2010-19751 and CSD2008-66 (CONSOLIDER program) and by the U.S. Office of Naval Research. J.S.-D. acknowledges useful discussions with L.-W. Cai.

We analyze the properties of acoustic and electromagnetic metamaterials with anisotropic constitutive parameters. Particularly, we analyze the so-called Radial Wave Crystals, which are radially periodic structures verifying the Bloch theorem. This type of crystals can be designed and implemented in acoustics as well as in electromagnetism by using anisotropic metamaterials. In acoustics, we have previously predicted that they can be employed as acoustic cavities with huge quality factors and also like dynamically driven antennas. Similar functionalities are here proven in the electromagnetic domain with, in particular, an analysis of the functionality of practical devices operating in the microwave regime. Starting from our recent works on anisotropic structures and their comparison in both application fields, we present a complete discussion concerning their properties in acoustics and electromagnetics.

A multiple scattering formulation of two-dimensional (2D) acoustic metamaterials is presented. This approach is comprehensive and can lead to frequency-dependent effective parameters (scalar bulk modulus and tensorial mass density), as it is possible to have not only positive or negative ellipsoidal refractive index, but also positive or negative hyperbolic refractive index. The correction due to multiple scattering interactions is included in the theory and it is demonstrated that its contribution is important only for lattices with high filling fractions. Since the surface fields on the scatterers are mainly responsible for the anomalous behavior of the resulting effective medium, complex scatterers can be used to engineer the frequency response. Anisotropic effects are also discussed within this formulation and some numerical examples are reported. A homogenization theory is also extended to electromagnetic wave propagation in 2D lattices of dielectric structures, where Mie resonances are found to be responsible for the metamaterial behavior. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft., This work was partially supported by the US Office of Naval Research under grant number N000140910554 and the Spanish Ministry of Science and Innovation under contract numbers TEC2010-19751 and CSD2008-66 (the CONSOLIDER program). Daniel Torrent also acknowledges support from the contract provided by the program 'Campus de Excelencia Internacional' 2010 UPV.

A systematic study of noise barriers based on sonic crystals made of cylinders that use recycled materials like absorbing component is reported here. The barriers consist of only three rows of perforated metal shells filled with rubber crumb. Measurements of reflectance and transmittance by these barriers are reported. Their attenuation properties result from a combination of sound absorption by the rubber crumb and reflection by the periodic distribution of scatterers. It is concluded that the porous cylinders can be used as building blocks whose physical parameters can be optimized in order to design efficient barriers adapted to different noisy environments. © 2011 Acoustical Society of America., This work has been partially supported by the Spanish MICINN under Grant Nos. TEC2007-67239 and CSD2008-66 (CONSOLIDER Program). D.T. acknowledges a research contract provided by the program Campus de Excelencia 2010 UPV. The authors acknowledge F. Payri and A. Broatch for providing the technical facilities that made possible sonic of the measurements.

We review recent advances on the topic of acoustic metamaterials based on the homogenization of periodic arrangements of sonic scatterers in a fluid or gas background. Particular emphasis is given in the application of these structures for gradient index sonic lenses and acoustic cloaking.

Anisotropic characteristics of cylindrically corrugated microstructures are analyzed in terms of their acoustic and electromagnetic (EM) behavior paying special attention to their differences and similarities. A simple analytical model has been developed using effective medium theory to understand the anisotropic features of both types of waves in terms of radial and angular components of the wave propagation velocity. The anisotropic constituent parameters have been obtained by measuring the resonances of cylindrical cavities, as well as from numerical simulations. This permits one to characterize propagation of acoustic and EM waves and to compare the fundamental anisotropic features generated by the corrugated effective medium. Anisotropic coefficients match closely in both physics fields but other relevant parameters show significant differences in the behavior of both types of waves. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft., The authors acknowledge financial support from the Spanish MICINN (TEC 2010-19751 and Consolider CSD2008-00066) and from the US Office of Naval Research (N000140910554). DT also acknowledges support from the program 'Campus de Excelencia Internacional 2010 UPV'.

This letter demonstrates the mechanical tuning of modes confined in two-dimensional acoustic cavities based on anisotropic metafluids. The employed metafluids have been designed with effective parameters such that the radial component of the sound speed tensor keeps constant and the acoustic impedance remains finite along the tuning. It is shown that mode frequencies can be mechanically down shifted to extremely low values. Experiments confirm the model predictions and let us to conclude that this type of resonators can be employed for the miniaturization of standard resonators based on isotropic fluids such as air. © 2011 American Institute of Physics., We acknowledge support from Office of Naval Research (Grant No. N00014-09-1-0554) and from the Spanish Ministerio de Ciencia e Innovacion under projects with Grant Nos. TEC2010-19751 and CSD2008-00066 (CONSOLIDER Program). D.T. acknowledges a fellowship provided by the program Campus de Excelencia Internacional 2010 UPV.

This Letter reports physical realization of acoustic metamaterials with anisotropic mass density. These metamaterials consist of a superlattice of two fluidlike components radially periodic. Several structures are spectroscopically characterized at large wavelengths (homogenization limit) by studying the acoustic resonances existing in the circular cavity where they are embedded. This characterization method allows us to extract the diagonal components of the sound speed tensor. Analytical expressions describing the anisotropic behavior as a function of the corrugation parameter are also developed and their predictions are in agreement with measurements.

This work introduces the so named radial sonic crystals, which are a new type of acoustic materials characterized by the fact that their acoustical properties are periodic in the radial direction, either in two dimensions or in three dimensions.These periodic conditions allow the application of Bloch theorem to the corresponding wave equation for sound propagation in spherical (3D) and cylindrical (2D) coordinates, respectively. The mentioned conditions leave the radial wave equation invariant under translation along r and can be achieved by using anisotropic fluid-like materials that can be engineered by using structures already proposed. We will report the rich variety of properties and potential applications of radial sonic crystals.