Vicente Romero-García

Tunable wideband bandstop acoustic filter based on two-dimensional multiphysical phenomena periodic systems

The physical properties of a periodic distribution of absorbent resonators is used in this work to design a tunable wideband bandstop acoustic filter. Analytical and numerical simulations as well as experimental validations show that the control of the resonances and the absorption of the scatterers along with their periodic arrangement in air introduce high technological possibilities to control noise. Sound manipulation is perhaps the most obvious application of the structures presented in this work. We apply this methodology to develop a device as an alternative to the conventional acoustic barriers with several properties from the acoustical point of view but also with additional esthetic and constructive characteristics.

Complex dispersion relation of surface acoustic waves at a lossy metasurface

The complex dispersion relation of surface acoustic waves (SAWs) at a lossy resonant metasurface is theoretically and experimentally reported. The metasurface consists of the periodic arrangement of borehole resonators in a rigid substrate. The theoretical model relies on a boundary layer approach that provides the effective metasurface admittance governing the complex dispersion relation in the presence of viscous and thermal losses. The model is experimentally validated by measurements in the semi-anechoic chamber. The complex SAW dispersion relation is experimentally retrieved from the analysis of the spatial Laplace transform of the pressure scanned along a line at the metasurface. The geometrical spreading of the energy from the speaker is accounted for, and both the real and imaginary parts of the SAW wavenumber are obtained. The results show that the strong reduction of the SAW group velocity occurs jointly with a drastic attenuation of the wave, leading to the confinement of the field close to the...

Strongly focused vortex beams by using flat Fresnel-spiral lenses

We report geometrically-optimal diffraction gratings for sharp vortex beam focusing using Fresnel-spiral curves. The lenses are built based on the Fresnel-spiral, a spiral curve that combine the focusing properties of Fresnel zone plates and the phase dislocations produced by spiral gratings. On the one hand, the constructive and destructive interferences between open and opaque zones in the grating, in analogy to the Fresnel zone plate, allow sharp beam focusing. On the other hand, the spiral shape of the grating retains the helicity, rotating the phase of the diffracted waves and creating a phase dislocation along the axis. This allows the generation of geometrically optimal focused vortex beams, enhancing the field intensity at the focus up to 170 times. In particular, this system offers a tunable topological charge of the vortex beam by using different arms in the Fresnel-spiral diffraction grating, being the topological charge equal to the number of arms. Two different Fresnel-spiral diffraction gratings with topological charge of 1 and 5 are experimentally tested showing excellent agreement with theory and simulations. These diffraction gratings will allow the design of effective wave-matter interaction systems, with direct applications in industry and biomedical engineering.We report geometrically-optimal diffraction gratings for sharp vortex beam focusing using Fresnel-spiral curves. The lenses are built based on the Fresnel-spiral, a spiral curve that combine the focusing properties of Fresnel zone plates and the phase dislocations produced by spiral gratings. On the one hand, the constructive and destructive interferences between open and opaque zones in the grating, in analogy to the Fresnel zone plate, allow sharp beam focusing. On the other hand, the spiral shape of the grating retains the helicity, rotating the phase of the diffracted waves and creating a phase dislocation along the axis. This allows the generation of geometrically optimal focused vortex beams, enhancing the field intensity at the focus up to 170 times. In particular, this system offers a tunable topological charge of the vortex beam by using different arms in the Fresnel-spiral diffraction grating, being the topological charge equal to the number of arms. Two different Fresnel-spiral diffraction grat...

Complex frequency analysis of the scattering matrix for the design of perfect absorbers in reflection and transmission problems

The design of deep-subwavelength structures for low frequency sound perfect absorption is challenging. Subwavelength perfect absorption implies increasing of the density of states at low frequency while maintaining impedance matching to the surrounding medium. Therefore, the study of the eigenvalues and eigenvectors of the scattering matrix in the complex frequency plane appears extremely powerful to analyze such systems and derive the critical coupling condition. The latter consists in exactly compensating the leakage of the structure by using the intrinsic losses. Two simple structures, the thicknesses of which are 88 times and 40 times smaller than the perfectly absorbed wavelengths, are presented for reflexion and transmission problems, respectively.The design of deep-subwavelength structures for low frequency sound perfect absorption is challenging. Subwavelength perfect absorption implies increasing of the density of states at low frequency while maintaining impedance matching to the surrounding medium. Therefore, the study of the eigenvalues and eigenvectors of the scattering matrix in the complex frequency plane appears extremely powerful to analyze such systems and derive the critical coupling condition. The latter consists in exactly compensating the leakage of the structure by using the intrinsic losses. Two simple structures, the thicknesses of which are 88 times and 40 times smaller than the perfectly absorbed wavelengths, are presented for reflexion and transmission problems, respectively.

Perfect and broadband acoustic absorption in deep sub-wavelength structures for the reflection and transmission problems

The mechanisms to achieve perfect acoustic absorption by sub-wavelength structures in both reflection and transmission problems are reported. While the mechanism consists in critically coupling a single resonance independently of its nature in the reflection problem, the mechanism becomes more complicated in the transmission problem. To tackle these issues, we use asymmetric interacting resonators, whose interaction leads to the perfect absorption condition. The analyzed system consists in a panel with a periodic distribution of thin slits, the upper wall of which being loaded by Helmholtz Resonators. The propagation in the slit is highly dispersive due to the presence of the resonators, producing slow sound conditions and down-shifting the slit resonance to low frequencies. By controlling the geometry of the resonators, the visco-thermal losses are tuned to compensate the leakage of the system and fulfill the perfect absorption condition. In the case of the reflection problem, a single resonator is enoug...

Broadband transmission loss in the audible regime with a 3D sonic crystals by use of resonance overlap

The acoustic properties of a three-dimensional sonic crystal made of square-rod rigid scatterers incorporating a periodic arrangement of quarter wavelength resonators are theoretically and experimentally reported in this work. The periodicity of the system produces Bragg band gaps that can be tuned in frequency by modifying the orientation of the square-rod scatterers with respect to the incident wave. In addition, the quarter wavelength resonators introduce resonant band gaps that can be tuned by coupling the neighbor resonators. Bragg and resonant band gaps can overlap allowing the wave propagation control inside the periodic resonant medium. In particular, we theoretically and experimentally show that this system can produce a broad frequency band gap exceeding two and a half octaves (from 590 Hz to 3220 Hz) with transmission lower than 3%. Finite element methods were used to calculate the dispersion relation of the locally resonant system. The visco-thermal losses were accounted for in the quarter wav...

Acoustic characterization of silica aerogel clamped plates for perfect absorption purpose

Silica aerogel has been widely studied as bulk material for its extremely low density and thermal conductivity. Plates or membranes made of this extremely soft materials exhibits interesting properties for sound absorption. A novel signal processing method for the characterization of an acoustic metamaterial made of silica aerogel clamped plates is presented. The acoustic impedance of a silica aerogel clamped plate is derived from the elastic theory for the flexural waves, while the transfer matrix method is used to model reflection and transmission coefficients of a single plate. Experimental results are obtained by using an acoustic impedance tube. The difference between the measured and modeled reflexion and transmission coefficients is minimized under constraints to recover the acoustic parameters of the silica aerogel plate. Once the properties of the silica aerogel plate are obtained, the perfect absorption condition is derived by studying the reflection coefficient of a aerogel plate rigidly backed...

Evidence of multimodal vibration damping using a single piezoelectric patch with a negative capacitance shunt

In this work, a piezoelectric patch shunted with a negative capacitance circuit has been used to simultaneously damp several modes of a square aluminum plate at low frequencies. The active nature of such electromechanical system leads to regions of instabilities in which the highest vibration attenuation performance appears in the softening region. Once the geometry is fixed, the system has two degrees of freedom, dominated by the electrical parameters of the circuit: the resistance and the negative capacitance. We tune both the value of the negative capacitance, in order to place the structure close to the instability in the softening region, and the resistance of the circuit in such that control the losses of the system. This work shows an optimal design to simultaneously damp several modes with non-zero electromechanical coupling factors using a single shunted patch at low frequencies. The problem is solved numerically and tested experimentally with good agreement. The results show the possibility of c...

Analysis of the reflection properties of sonic crystal devices.

The phenomenon of band‐gaps typical of wave propagation in periodic materials has been a subject of extensive investigations. In acoustics, the interest is mainly related with the ability of such structures, the so‐called sonic crystals, of blocking the propagation of sound at certain frequencies, acting as filters. Recently, there is an increasing interest in the use of sonic crystals to control the beam propagation, and a number of interesting features as focusing and collimation have been predicted and observed. At the root of these phenomena is the angle‐dependent dispersion introduced by the crystal. In propagating regimes, for frequencies outside the gap, the particular form of the dispersion relations in the wavenumber (reciprocal) space allows us to determine the spatial characteristics of the transmitted field. In this work, we merge the two ideas above, and investigate the spatial characteristics of the reflected acoustic field when its frequency belongs to a band‐gap. In a band‐gap, the propaga...

Sonic crystal barriers with absorbent and resonant scatterers.

The ability of periodic distributions of absorbent and resonant scatterers embedded in air for attenuating a wide range of frequencies is studied. The scatterers consist of a rigid core with a resonant cavity, covered by a layer of absorbent material. A preliminary numerical analysis shows the resulting combination of the three main physical phenomena appearing in such a periodic system: scattering, absorptions, and resonance. The scattering reproduces the so‐called band gaps, ranges of frequencies related with the periodicity of the crystal. In addition, resonant scatterers have been designed to present resonances in the range of frequencies below the band gap of the crystal, such that there appear attenuation peaks for the low frequencies. Finally, the resonant scatterers are covered by a layer of absorbent material, a fibrous woolen felt that produces an increase of the whole attenuation spectra without removing the multiple scattering and the resonance phenomena.