Alfonso Climente

Sound focusing by gradient index sonic lenses

Gradient index sonic lenses based on two-dimensional sonic crystals are here designed, fabricated, and characterized. The index-gradient is achieved in these type of flat lenses by a gradual modification of the sonic crystal filling fraction along the direction perpendicular to the lens axis. The focusing performance is well described by an analytical model based on ray theory as well as by numerical simulations based on the multiple-scattering theory.

Omnidirectional broadband acoustic absorber based on metamaterials

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.

Gradient index lenses for flexural waves based on thickness variations

This work presents a method for the realization of gradient index devices for flexural waves in thin plates. Unlike recent approaches based on phononic crystals, the present approach is based on the thickness-dependence of the dispersion relation of flexural waves, which is used to create gradient index devices by means of local variations of the plates thickness. Numerical simulations of known circularly symmetrical gradient index lenses have been performed. These simulations have been done using the multilayer multiple scattering method and the results prove their broadband efficiency and omnidirectional properties. Finally, finite element simulations employing the full three-dimensional elasticity equations also support the validity of the designed approach.

Omnidirectional broadband insulating device for flexural waves in thin plates

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.

Scattering of flexural waves from a hole in a thin plate with an internal beam

The scattering of flexural waves by a hole in a thin plate traversed by a beam is modeled here by coupling the Kirchhoff-Love and the Euler-Bernoulli theories. A closed form expression is obtained for the transfer matrix (T-matrix) relating the incident wave to the scattered cylindrical waves. For this purpose, a general method has been developed, based on an analogous impedance method for acoustic waves, for calculating the T-matrix for flexural wave scattering problems. The T-matrix for the problem considered displays a simple structure, composed of distinct sub-matrices which decouple the inside and the outside fields. The conservation of energy principle and numerical comparisons with a commercial finite element simulator have been used to prove the theory.

Noise Reduction by Perfect Absorbers Based on Acoustic Metamaterials

We have designed a cylindrical perfect absorber based on acoustic metamaterials. The absorber consists of a metamaterial shell that surrounds a center that dissipates the acoustic energy. The metamaterial shell is designed so that perfectly matches the acoustic impedance of the air background and guides the sound to the center. Numerical simulations are reported about the efficiency of the absorber as a function of the absorbing material employed at the center.Copyright

Scattering of flexural waves from an N-beam resonator in a thin plate

The impedance matrix method is applied to study the scattering of flexural waves propagating in an infinite thin plate containing an N-beam resonator. The resonator consists of a circular hole containing a smaller plate connected to the background plate by a number N of rectangular beams. After representing the boundary conditions in a modal multipole expansion form, a compact expression is obtained for the T-matrix, which relates the incident and the scattered transverse (out-of-plane) waves. The analysis of the scattering cross-section reveals interesting scattering features, like resonances and anisotropy, associated to this type of resonators. Numerical experiments performed within the framework of the finite element method support the accuracy of the model here developed.

Analysis of cloaks for flexural waves

We report a comprehensive analysis of the cloaks proposed for bending waves propagating in a thin metallic plate. The analysis uses a semi-analytical model which is able to reproduce the experimental data reported by Stenger and coworkers [see Phys. Rev. Lett., vol. 108, 014301 (2012)]. The model is based on the Kirkoff-Love equation of motion and employs the multilayer scattering algorithm to calculate the interaction of the propagating waves with the cloak. The boundary equations apply to the designed device give a complete description of the data without using simplified algorithms solved in a finite element framework. The performance of the cloak is characterized with the visibility factor, a parameter already employed in several acoustic cloaks. A discussion will be given of the performance of the cloak for flexural waves in comparison with acoustic cloaks. [Work supported by ONR.]

Mechanical rainbow trapping and Bloch oscillations in chirped metallic beams

The mechanical rainbow trapping effect and the mechanical Bloch oscillations for torsional waves propagating in chirped mechanical structures are here experimentally demonstrated. After extensive simulations, three quasi-one-dimensional chirped structures were designed, constructed and experimentally characterized by Doppler spectroscopy. When the chirp intensity vanishes, a perfect periodic system, with bands and gaps, is obtained. The mechanical rainbow trapping effect is experimentally characterized for small values of the chirp intensity. The wave packet traveling along the structure is progressively slowing down and is reflected back at a certain depth, which depends on its central frequency. For larger values of the chirping parameter the rainbow trapping yields the penetration length where the mechanical Bloch oscillations emerge. Numerical simulations based on the transfer matrix method show an excellent agreement with experimental data.

Acoustic cloaking for airborne sound based on inclusions of rigid scatterers

Acoustic cloaking is a phenomenon whose physical realization depends on the ability of designing metamaterials with the appropriate parameters. When dealing with airborne sound almost any solid behaves as an acoustically rigid material, so cloaks based on rigid scatterers are here studied. Since these structures are not able to increase the effective sound speed with respect to the background, additional mechanisms should be introduced to allow this effect. Here we will report an acoustic cloak based on temperature gradients. Another possibility of hiding objects from an external sound source consists of using a set of external layers that cancels the scattered field by such objects at selected frequencies. We present the practical realization of this approach for 2D and 3D structures. [Work supported by MINECO from Spain and ONR from United States.]