Francisco Belmar

Extraordinary Sound Screening in Perforated Plates

We report extraordinary effects in the transmission of sound through periodically perforated plates, supported by both measurements and theory. In agreement with recent observations in slit arrays, M. H. Lu et al. [Phys. Rev. Lett. 99, 174301 (2007)10.1103/PhysRevLett.99.174301], nearly full transmission is observed at certain resonant frequencies, pointing out similarities of the acoustic phenomena and their optical counterpart. However, acoustic screening well beyond that predicted by the mass law is achieved over a wide range of wavelengths in the vicinity of the period of the array, resulting in fundamentally unique behavior of the sound as compared to light.

Sound transmission through plates perforated with two periodic subwavelength hole arrays

Theoretical results on sound transmission through plates perforated with compound periodic hole arrays are presented. Calculations are obtained by means of a model under the rigid-solid assumption, which is thoroughly derived. A complex interplay between resonant transmission peaks, resonance interference, and Wood anomaly minima is observed. At high frequencies the resonant peaks overcome the Wood anomaly dips, leading to a different behaviour when compared with simple hole arrays.

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.

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.

A tunable acoustic barrier based on periodic arrays of subwavelength slits

The most usual method to reduce undesirable enviromental noise levels during its transmission is the use of acoustic barriers. A novel type of acoustic barrier based on sound transmission through subwavelength slits is presented. This system consists of two rows of periodic repetition of vertical rigid pickets separated by a slit of subwavelength width and with a misalignment between them. Here, both the experimental and the numerical analyses are presented. The acoustic barrier proposed can be easily built and is frequency tunable. The results demonstrated that the proposed barrier can be tuned to mitigate a band noise without excesive barrier thickness. The use of this system as an environmental acoustic barrier has certain advantages with regard to the ones currently used both from the constructive and the acoustical point of view.

Ultrasonic transmission through multiple-sublattice subwavelength holes arrays.

The ultrasonic transmission through plates perforated with 2 × 2 or 3 × 3 square array of subwavelength holes per unit cell are studied by numerical simulations. Calculations are obtained by means of a theoretical model under the rigid-solid assumption. It is demonstrated that when the inter-hole distance within the unit cell is reduced, new transmission dips appear resulting from Wood anomalies that have influence on the second and the third order Fabry-Perot peak. When the inter-hole distance within the unit cell is reduced, the transmission spectrum of the multiple-sublattice holes arrays tends to the transmission spectrum of a plate perforated with only one hole in the unit cell.

SEL for electrically hauled trains

Abstract The aim of this work is the search of an easy method to estimate the Sound Exposure Level (SEL), referring to noise emitted by trains. The theoretical model is based considering the train as a directional linear noise source, providing the SEL value in function of both source-observer distance and train speed. Experimental data obtained from electrically hauled trains and theoretical model results are compared in order to study the correlation between them.

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

Geometrical isotropy in perforated plates with subwavelength holes decorated with Archimedean patterns

The design and use of small apertures perforated in opaque plates to control the transmission of ultrasonic waves has been widely studied in recent years. The ultrasonic transmission response of brass plates perforated with Archimedean patterns of subwavelength hole arrays immersed in water is reported, both numerically and experimentally, in this work. It is shown that an increase in the geometrical isotropy of the elementary cells of the Archimedean patterns gives rise to a suppression of both minimum and maximum transmission corresponding to the destructive and constructive interferences, leading to uniformity within the angle-dependent transmitted sound power coefficient. The experimental results are in close agreement with the calculated ones. This property can be used to design ultrasonic devices such as filters and sensors.