Sergio Castiñeira-Ibáñez

Evidences of evanescent Bloch waves in phononic crystals

We show both experimentally and theoretically the evanescent behavior of modes in the band gap of finite phononic crystal (PC). Based on experimental and numerical data we obtain the imaginary part of the wave vector in good agreement with the complex band structures obtained by the extended plane wave expansion. The calculated and measured acoustic field of a localized mode out of the point defect inside the PC presents also evanescent behavior. The correct understanding of evanescent modes is fundamental for designing narrow filters and waveguides based on PCs with defects.

Overlapping of acoustic bandgaps using fractal geometries

The transmission of acoustic waves in a fractal distribution of rigid scatterers embedded in air is reported in this work. The Sierpinsky fractal is used to produce a compact small device containing several periodicities, therefore the fractal distribution contains several finite sonic crystals. The attenuation band produced by the fractal distribution results from the sum of the Bragg peaks of each periodicity. On the other hand, bandgaps of sonic crystal depend on the well-known filling fraction, thus the radii of the scatterers in the fractal distribution has been optimized using genetic algorithm in order to overlap the bandgaps of each periodicity obtaining a wide and full attenuation band.

Quantitative characterization of bandgap properties of sets of isolated acoustic scatterers arranged using fractal geometries

The improvement in the bandgap properties of a set of acoustic scatterers arranged according to a fractal geometry is theoretically quantified in this work using the multiple scattering theory. The analysis considers the growth process of two different arrangements of rigid cylinders in air created from a starting cluster: a classical triangular crystalline array and an arrangement of cylinders based on a fractal geometry called a Sierpinski triangle. The obtained results, which are experimentally validated, show a dramatic increase in the size of the bandgap when the fractal geometry is used.

Acoustic wave diffraction at the upper edge of a two-dimensional periodic array of finite rigid cylinders. A comprehensive design model of periodicity-based devices

Diffraction at the upper edge of a two-dimensional periodic array of finite rigid cylinders immersed in air as well as the effect on its wave propagation properties are numerically and experimentally reported in this work. The diffraction and the band gap effects, due to the finite length of the cylinders and the periodicity of the array, are the two phenomena that must be taken into account in the design of real devices based on periodicity to control the propagation of waves. We also present a model which allows the separate analysis of each of these two phenomena and provides a comprehensive procedure for designing more efficient devices based on arrays of scatterers, following the concept of tunability developed by some authors.

Analysis of Fresnel Zone Plates Focusing Dependence on Operating Frequency

The focusing properties of Fresnel Zone Plates (FZPs) against frequency are analyzed in this work. It is shown that the FZP focal length depends almost linearly on the operating frequency. Focal depth and focal distortion are also considered, establishing a limit on the frequency span at which the operating frequency can be shifted. An underwater FZP ultrasound focusing system is demonstrated, and experimental results agree with the theoretical analysis and simulations.

Pinhole Zone Plate Lens for Ultrasound Focusing

The focusing capabilities of a pinhole zone plate lens are presented and compared with those of a conventional Fresnel zone plate lens. The focusing properties are examined both experimentally and numerically. The results confirm that a pinhole zone plate lens can be an alternative to a Fresnel lens. A smooth filtering effect is created in pinhole zone plate lenses, giving rise to a reduction of the side lobes around the principal focus associated with the conventional Fresnel zone plate lens. The manufacturing technique of the pinhole zone plate lens allows the designing and constructing of lenses for different focal lengths quickly and economically and without the need to drill new plates.

Design of Acoustical Bessel-Like Beam Formation by Tunable Angular Spectrum in Soret Zone Plate Lens

The image performance of acoustic and ultrasound sensors depends on several fundamental 1 parameters such as depth of focus or spatial resolution. There are currently two different type 2 of acoustic diffractive lenses: those which form a diffraction-limited spot with a shallow depth 3 of focus (zone plates) and lenses which form an extended focus (quasi-Bessel beams). In this 4 paper, we investigate a pupil-masked Soret zone plate which allows the tunability of a normalized 5 angular spectrum. It is shown that the depth of focus and the spatial resolution can be modified, 6 without changing the lens structure, by choosing the size of the amplitude pupil mask. This effect 7 is based on the transformation of spherically converging waves into quasi-conical waves, due to 8 the apodization of the central part of the zone plate. The theoretical analysis is verified with both 9 numerical simulations and experimental measurements. A Soret zone plate immersed in water 10 with D/2F=2.5 and F=4.5λ, changes its depth of focus from 2.84λ to 5.9λ and the spatial resolution 11 increases from 0.81λ to 0.64λ at a frequency of 250 kHz, by modifying the pupil mask dimensions of 12 the Soret zone plate. 13

Frequency dependence of Fresnel zone plates focus

Fresnel zone plates (FZPs) focus waves through constructive interference of diffracted fields. They are used in multiple fields, such as optics, microwave propagation or acoustics, where refractive focusing by conventional lenses is difficult to achieve. FZPs are designed to work and focus at a design frequency. At this frequency, the behavior of the FZP is optimum and focusing at a certain focal length is achieved. In most medical applications using lenses, it is critical to have a fine and dynamic control on the lens focal length. In this work, the variation of the FZP focus parameters when working at operating frequencies different from the design frequency is analyzed, and a focal length control mechanism is proposed. It is shown that the FZP focal length shifts linearly with the operating frequency, becoming a dynamic control parameter that can be useful in many different applications. However, other focusing parameters, such as focal depth and distortion, are also affected by the operating frequency...

Analysis and design of Fresnel zone plates with multiple foci

Fresnel Zone Plates (FZPs) become an interesting alternative to traditional lenses when planar fabrication is advantageous, and are used in a wide range of physical disciplines such as optics, microwave propagation, or ultrasounds. Conventional FZPs produce a single focus, which is optimal in most applications. However, certain medical applications, such as MRI (magnetic resonance imaging) guided ultrasound surgery, require multiple foci ultrasound exposures. In this work, new multi-focus Fresnel lenses (MFFLs) based on conventional FZPs are presented. The advantages and drawbacks of these new MFFL structures are thoroughly analyzed. There is a tradeoff on the number of foci achieved in a single MFFL and its focusing efficiency. Therefore, the most efficient MFFL is that with two foci. A procedure for designing 2-foci MFFLs, in which the focal length of both foci may be selected independently, is established. For each 2-foci MFFL, there are several physical implementations. The focusing properties of all ...

Open Acoustic Barriers: A New Attenuation Mechanism

One of the main environmental problems of the industrialised countries is the noise, which can be defined as an unwanted or unpleasant outdoor sound generated by transport, industry and human activities in general. When it is not possible to reduce the emission of noise acting on the source, the reduction of noise levels in its transmis‐ sion phase using acoustic screens (AS) seems appropriate; such screens are in common use to reduce noise levels and have been extensively studied since the middle of the 20th century. Over the last decades, various acoustic screen designs have been investigated to increase the screening effect. The research carried out focuses on both the reduction of diffraction at the top edge of the screen by varying the shape at the top or adding absorptive materials to the noise screen, but all these screens are basically formed by a continuum rigid material with a superficial density high enough, to reduce transmission of noise through the screen, in accordance with the mass law. At the end of the nineties, another type of screen based on arrangements of isolated scatterers embedded in air, emerged. Among other interesting properties, these screens provide new mechanisms to control the noise based on the Bragg law. First, a Sonic Crystal Acoustic Screen (SCAS) was presented, where the scatterers are arranged following crystalline patterns. After that, a new prototype of AS based on sonic crystals appears, which increases the attenuation capabilities using arrangements based on fractal geometries. The screens designed in this way have been referred to as Fractal‐based Sonic Crystal Acoustic Screens (FSCAS) in this chapter. In both the cases, the mechanism that prevents the transmission of noise, and therefore increases the noise attenuation, is the destructive Bragg interference due to a multiple scattering process. Finally, a new concept of AS based on a periodic arrangement of scatterers, with a slit dimension between them that is smaller than the wavelength is introduced. This latest screen is called Subwavelength Slit Acoustic Screen (SSAS) which presents a Wood anomaly and Fabry‐Perot resonances, being the destructive interferences among the scattered waves, responsible for the attenuation capabilities of these screens. This new kind of AS (SCAS, FSCAS and SSAS) presents interesting properties and can be considered as a real