Rubén Picó

Extraordinary absorption of sound in porous lamella-crystals

We present the design of a structured material supporting complete absorption of sound with a broadband response and functional for any direction of incident radiation. The structure which is fabricated out of porous lamellas is arranged into a low-density crystal and backed by a reflecting support. Experimental measurements show that strong all-angle sound absorption with almost zero reflectance takes place for a frequency range exceeding two octaves. We demonstrate that lowering the crystal filling fraction increases the wave interaction time and is responsible for the enhancement of intrinsic material dissipation, making the system more absorptive with less material.

Enhancement of sound in chirped sonic crystals

We propose and experimentally demonstrate a mechanism of sound wave concentration based on soft reflections in chirped sonic crystals. The reported controlled field enhancement occurs at around particular (bright) planes in the crystal and is related to a progressive slowing down of the sound wave as it propagates along the material. At these bright planes, a substantial concentration of the energy (with a local increase up to 20 times) was obtained for a linear chirp and for frequencies around the first band gap. A simple couple mode theory is proposed that interprets and estimates the observed effects. Wave concentration energy can be applied to increase the efficiency of detectors and absorbers.

Formation of collimated sound beams by three-dimensional sonic crystals

A theoretical and experimental study of the propagation of sound beams in- and behind three-dimensional sonic crystals at frequencies close to the band edges is presented. An efficient collimation of the beam behind the crystal is predicted and experimentally demonstrated. This effect could allow the design of sources of high spatial quality sound beams.

Wave focusing using symmetry matching in axisymmetric acoustic gradient index lenses

The symmetry matching between the source and the lens results in fundamental interest for lensing applications. In this work, we have modeled an axisymmetric gradient index (GRIN) lens made of rigid toroidal scatterers embedded in air considering this symmetry matching with radially symmetric sources. The sound amplification obtained in the focal spot of the reported lens (8.24 dB experimentally) shows the efficiency of the axisymmetric lenses with respect to the previous Cartesian acoustic GRIN lenses. The axisymmetric design opens new possibilities in lensing applications in different branches of science and technology.

Broadband quasi perfect absorption using chirped multi-layer porous materials

This work theoretically analyzes the sound absorption properties of a chirped multi-layer porous material including transmission, in particular showing the broadband unidirectional absorption properties of the system. Using the combination of the impedance matching condition and the balance between the leakage and the intrinsic losses, the system is designed to have broadband unidirectional and quasi perfect absorption. The transfer and scattering matrix formalism, together with numerical simulations based on the finite element method are used to demonstrate the results showing excellent agreement between them. The proposed system allows to construct broadband sound absorbers with improved absorption in the low frequency regime using less amount of material than the complete bulk porous layer.

Acoustic Bessel-like beam formation by an axisymmetric grating

We report Bessel-like beam formation of acoustic waves by means of an axisymmetric grating of rigid tori. The results show that the generated beam pattern is similar to that of Bessel beams, characterized by elongated non-diffracting focal spots. A multiple foci structure is observed, due to the finite size of the lens. The dependence of the focal distance on the frequency is also discussed, on the basis of an extended grating theory. Experimental validation of acoustic Bessel-like beam formation is also reported for sound waves. The results can be generalized to wave beams of different nature, as optical or matter waves.

Acoustically penetrable sonic crystals based on fluid-like scatterers

We propose a periodic structure that behaves as a fluid–fluid composite for sound waves, where the building blocks are clusters of rigid scatterers. Such building-blocks are penetrable for acoustic waves, and their properties can be tuned by selecting the filling fraction. The equivalence with a fluid–fluid system of such a doubly periodic composite is tested analytical and experimentally. Because of the fluid-like character of the scatterers, sound structure interaction is negligible, and the propagation can be described by scalar models, analogous to those used in electromagnetics. As an example, the case of focusing of evanescent waves and the guided propagation of acoustic waves along an array of penetrable elements is discussed in detail. The proposed structure may be a real alternative to design a low contrast and acoustically penetrable medium where new properties as those shown in this work could be experimentally realized.

Enhancement of sound by soft reflections in exponentially chirped crystals

The enhancement of sound inside a two dimensional exponentially chirped crystal during the soft reflections of waves is experimentally and theoretically explored in this work. The control of this enhancement is achieved by a gradual variation of the dispersion in the system by means of a chirp of the lattice constant. The sound enhancement is produced at some planes of the crystal in which the wave is softly reflected due to a progressive slowing down of the sound wave. We find that the character of the sound enhancement depends on the function of the variation of dispersion, i.e., on the function of the chirp. A simple coupled mode theory is proposed to find the analytical solutions of the sound wave enhancement in the exponentially chirped crystal. Harmonic and time domain numerical simulations are performed to interpret the concept of the soft reflections, and to check the analytically calculated field distributions both in good agreement with experiments. Specially we obtain stronger sound enhancement than in linearly chirped crystals. This sound enhancement could motivate applications in energy harvesting, e.g., to increase the efficiency of detectors and absorbers.

Sound diffusers based on sonic crystals

A sound diffuser is proposed based on sonic crystals, structures formed by a periodic distribution of cylindrical scatterers in a host medium, which is usually air. The diffuser is a so-called biperiodic structure, as formed by two arrays of sonic crystals with slightly different periodicities. Large diffusivity at low frequencies is achieved when the typical scale of the blocks is much larger than the periodicity of the crystals, determined by its lattice constant. An interpretation of the low frequency behavior of the diffuser is given in the homogenization limit in terms of multiple reflections and interference between the fields scattered by the different blocks. It is also shown that sonic crystal based diffusers enhance time spreading in comparison with other conventional diffusers.

Phase calibration of sonar systems using standard targets and dual-frequency transmission pulses

The phase angle component of the complex frequency response of a sonar system operating near transducer resonance is usually distorted. Interpretation and classification of the received sonar signal benefits from the preservation of waveform fidelity over the full bandwidth. A calibration process that measures the phase response in addition to the amplitude response is thus required. This paper describes an extension to the standard-target calibration method to include phase angle, without affecting the experimental apparatus, by using dual-frequency transmission pulses and frequency-domain data processing. This approach reduces the impact of unknown range and sound speed parameters upon phase calibration accuracy, as target phase is determined from the relationship of the two frequency components instead of relying on a local phase reference. Tungsten carbide spheres of various sizes were used to simultaneously calibrate the amplitude and phase response of an active sonar system in a laboratory tank. Experimental measurements of target phase spectra are in good agreement with values predicted from a theoretical model based upon full-wave analysis, over an operating frequency of 50-125 kHz.