Victor M. García-Chocano

Noise control by sonic crystal barriers made of recycled materials

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.

Broadband sound absorption by lattices of microperforated cylindrical shells

Absorption of broadband noise by sonic crystals consisting of microperforated cylindrical shells is proposed and experimentally demonstrated. The theoretical study has been performed in the framework of multiple scattering method, where a model for the T matrix of the microperforated shells has been developed. It has been predicted an extraordinary broadband sound absorption that is explained in terms of the multiple scattering phenomena occurring at the surfaces of the absorptive units—the microperforated panels. Our proposal has been supported by experiments performed on a structure consisting of 3 rows of cylindrical shells 3 meters height.

Quenching of acoustic bandgaps by flow noise

We report an experimental study of acoustic effects produced by wind impinging on noise barriers based on two-dimensional sonic crystals with square symmetry. We found that the attenuation strength of sonic-crystal bandgaps decreases for increasing values of flow speed. A quenching of the acoustic bandgap appears at a certain speed value that depends of the barrier filling ratio. For increasing values of flow speed, the data indicate that the barrier becomes a sound source because of its interaction with the wind. We conclude that flow noise should be taken into account in designing acoustic barriers based on sonic crystals.

Acoustic metamaterial absorbers based on multilayered sonic crystals

Through the use of a layered arrangement, it is shown that lossy sonic crystals can be arranged to create a structure with extreme acoustic properties, namely, an acoustic metamaterial. This artificial structure shows different effective fluids and absorptive properties in different orientations. Theoretical, numerical, and experimental results examining thermoviscous losses in sonic crystals are presented, enabling the fabrication and characterization of an acoustic metamaterial absorber with complex-valued anisotropic inertia. To accurately describe and fabricate such an acoustic metamaterial in a realizable experimental configuration, confining structures are needed which modify the effective properties, due to the thermal and viscous boundary layer effects within the sonic crystal lattice. Theoretical formulations are presented which describe the effects of these confined sonic crystals, both individually and as part of an acoustic metamaterial structure. Experimental demonstrations are also reported ...

Reduced acoustic cloaks based on temperature gradients

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.

Anomalous sound absorption in lattices of cylindrical perforated shells

This work reports the enhancement of sound absorption by sonic crystals slabs made of cylindrical perforated shells. These building units, with perforations of millimeter size, show small losses and cannot explain the strong absorption observed at some specific frequencies when the slabs consist of just a few number of rows. It is found that this phenomenon is due to a resonant Wood anomaly which occurs when the incident wave couples with a leaky guided mode supported by the slab. This effect results in an enhancement of the absorption, since the energy transferred to the guided mode travels within the slab, along a direction perpendicular to the incident one. Multiple scattering and finite element simulations give support to the proposed behavior, the transmittance results being in good agreement with experimental data previously reported.

Enhanced inertia from lossy effective fluids using multi-scale sonic crystals

In this work, a recent theoretically predicted phenomenon of enhanced permittivity with electromagnetic waves using lossy materials is investigated for the analogous case of mass density and acoustic waves, which represents inertial enhancement. Starting from fundamental relationships for the homogenized quasi-static effective density of a fluid host with fluid inclusions, theoretical expressions are developed for the conditions on the real and imaginary parts of the constitutive fluids to have inertial enhancement, which are verified with numerical simulations. Realizable structures are designed to demonstrate this phenomenon using multi-scale sonic crystals, which are fabricated using a 3D printer and tested in an acoustic impedance tube, yielding good agreement with the theoretical predictions and demonstrating enhanced inertia.

Resonant excitation of coupled Rayleigh waves in a short and narrow fluid channel clad between two identical metal plates

Transmission of ultrasonic waves through a slit between two water immersed brass plates is studied for sub-wavelength plate thicknesses and slit apertures. Extraordinary high absorption is observed at discrete frequencies corresponding to resonant excitation of Rayleigh waves on the both sides of the channel. The coupling of the Rayleigh waves occurs through the fluid and the corresponding contribution to the dispersion has been theoretically derived and also experimentally confirmed. Symmetric and anti-symmetric modes are predicted but only the symmetric mode resonances have been observed. It follows from the dispersion equation that the coupled Rayleigh waves cannot be excited in a channel with apertures less than the critical one. The calculated critical aperture is in a good agreement with the measured acoustic spectra. These findings could be applied to design a broadband absorptive metamaterial.

Quasi-two-dimensional acoustic metamaterials for sound control in ducts

Acoustic metamaterials are artificial structures with acoustic properties not found in natural materials.1 They consist of periodic arrangements of subwavelength units, each unit being a designed composite made of different materials and shapes. Since the building units and their separation are small in comparison with the sound wavelength, a metamaterial behaves as a homogeneous medium with exotic properties. A fluid or a gas can be acoustically characterized with two parameters, its mass density ( ) and its bulk modulus (B). Although these parameters always take positive values in natural materials, mass anisotropy and negative values of both parameters are possible under dynamical conditions using acoustic metamaterials.1–3 These unusual properties lead to novel phenomena such as negative refraction, and interesting applications such as superlensing or acoustic cloaks. This work focuses on metamaterials intended to work with waves traveling inside two-dimensional ducts. One of the walls defining such a waveguide is drilled in order to insert a periodic arrangement of cylindrical cavities (see Figure 1). Since the cavities’ length L extends along the third dimension, the resulting structures are named quasi-two-dimensional sonic crystals (Q2DSC). We found that they exhibit unusual properties due to the resonances embedded in the cavities. These properties appear at low frequencies, corresponding to wavelengths much larger than the cavity radius, R, and lattice separation, a. We fabricated Q2DSC sample consisting of a cluster made of 15 rows of 9 cylinders with R D 1cm, and covering an area of 47 25cm2 (see Figure 2). We characterized the sample in a Figure 1. Diagram of a simple quasi-2D sonic crystal. It consists of a periodic distribution of cylindrical cavities drilled in the upper surface of a waveguide with height h. The color pattern represents the sound propagation inside the waveguide. R: Cavity radius. L: Length. a: Lattice separation.

Acoustic cloaks for airborne sound: Experimental realizations in two- and three-dimensions

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.