Constanza Rubio

Acoustic barriers based on periodic arrays of scatterers

It is well known that certain periodic structures built by repetition of elements produce sound attenuation effects as a consequence of the destructive interference of the scattered waves by these elements. The sound attenuation results that we got from transmission experiments with these kind of structures, so-called sonic crystals (SCs), led us to think that SCs could be used as an acoustic barrier. Until now, most of the transmission experiments with these periodic arrays of scatterers have been performed under controlled conditions, so how they would behave outdoors is still not well known. In this letter we present outdoor-experimental results for two-dimensional SCs and from these it can be concluded that periodic arrays of scatterers are a suitable device to reduce noise in free-field conditions.

Reflectance properties of two-dimensional sonic band-gap crystals

An analysis of the reflectance of sonic band-gap crystals consisting of square arrays of rigid cylinders in air is presented. The standing wave formed in front of the structures is studied both experimentally and theoretically. Experiments have been performed with a mobile robotized microphone that obtains pressure maps on the plane perpendicular to the axes of the cylinders. Enhancements of the standing wave ratio (SWR) are observed in frequency regions where attenuation bands appear in zero-order transmission experiments. Also, the SWR presents oscillations that can be related to the finite dimension of the structure (Fabry-Perot effect). Both features are well described by calculations based on a double-scattering approach.

The existence of full gaps and deaf bands in two-dimensional sonic crystals

Theoretical and experimental determination of sonic band structures of two-dimensional (2-D) arrays of rigid cylinders in air is reported. We present measurements for square and triangular lattices. A variational method is employed to calculate the acoustic dispersion relation. Experimentally, a transmission technique and the analysis of the phase delay between the incident and scattered waves by the structure are used to construct the acoustic bands. The comparison between theory and experiments allows to fully characterize the band gaps and it has also demonstrated the existence of deaf bands; i.e., bands which cannot be excited due to symmetry reasons. For the case of square lattice we show that the structure with a filling fraction of 0.41 has a full acoustic gap.

Design of lightweight multilayer partitions based on sonic crystals

The sound transmission coefficient of different multilayer partitions commonly encountered in buildings has been measured as a function of frequency. Most of the samples studied showed an increase in the sound transmission coefficient over a specific frequency, called the critical frequency, depending on the layer material. However, for partitions built with the same materials, but built with a periodic arrangement of layers, this behavior has not been observed. This kind of periodic multilayer partition can be considered as a sonic crystal, because the stopband corresponding to a one-dimensional sonic crystal with a constant lattice equal to the modulation of the partition is in the same range as the critical frequency of the panel.

Sound attenuation by a two dimensional array of cylinders

Here, it is shown, for the first time, a systematic analysis (experiment and theory) of the acoustic transmission of a two‐dimensional periodic array of rigid cylinders with two different configurations: square and triangular. The influence of the filling fractions, ranging from 0.06 up to 0.41, on the appearance of the pseudogaps and full band gaps is studied. Above a certain filling fraction, an overlap is observed between the attenuation peaks measured along the two high symmetry directions of the Brillouin zone. This effect is considered as the fingerprint of the existence of a full acoustic gap. Nevertheless, the comparison with our calculation of band structures shows that the lattice has band states in that frequency range. These bands are called deaf bands (i.e., they cannot be excited by experiments of sound transmission) [Sanchez‐Perez et al., Phys. Rev. Lett. (to be published)].

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.

Subwavelength slit acoustic metamaterial barrier

Reduction of noise in the transmission path is a very important environmental problem. The standard method to reduce this noise level is the use of acoustic barriers. In this paper, an acoustic metamaterial based on sound transmission through subwavelength slits, is tailored to be used as an acoustic barrier. This system consists of two rows of periodic repetition of vertical rigid pickets separated by a slit of subwavelength width, embedded in air. Here, both the experimental and the numerical analyses are presented. These analyses have facilitated the identification of the parameters that affect the insertion loss performance. The results demonstrated that the proposed barrier can be tuned to mitigate a band noise in a mechanical plant for buildings where openings for air flow are required as well as industrial noise, without excessive barrier thickness.

Ultrasonic lens based on a subwavelength slit surrounded by grooves.

The lensing capabilities of a single subwavelength slit surrounded by a finite array of grooves milled into a brass plate is presented. The modulation of the beam intensity of this ultrasonic lens can be adjusted by varying the groove depth. Numerical simulations as well as experimental validations at 290 kHz are shown. The experimental results are in good agreement with the numerical simulations. This system is believed to have potential applications for medical ultrasound fields such as tomography and therapy.

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.