Francisco Cervera

Noise control by sonic crystal barriers made of recycled materials

A systematic study of noise barriers based on sonic crystals made of cylinders that use recycled materials like absorbing component is reported here. The barriers consist of only three rows of perforated metal shells filled with rubber crumb. Measurements of reflectance and transmittance by these barriers are reported. Their attenuation properties result from a combination of sound absorption by the rubber crumb and reflection by the periodic distribution of scatterers. It is concluded that the porous cylinders can be used as building blocks whose physical parameters can be optimized in order to design efficient barriers adapted to different noisy environments.

Sound focusing by flat acoustic lenses without negative refraction

We present the experimental realization of two flat acoustic lenses generated by inverse design. The lenses consist of aperiodic lattices of aluminum cylinders confined in a rectangular area. They were obtained by a design tool that combines multiple scattering theory and a genetic algorithm. The cylinders’ positions are optimized by the genetic algorithm in order to produce maximum sound amplification at the focal point. Both the focus and the dimension of the lenses are arbitrarily chosen. Our approach is illustrated by measurements in two lenses fabricated with aluminum cylinders 2m long and having five and nine layers, respectively. Sound amplification up to 6.4dB is obtained at the focus. The excellent agreement found between experimental pressure patterns and theoretical simulations supports our tool of design. It is concluded that flat acoustic lenses made of aperiodic distribution of scatterers can produce sound focusing with no need of negative refraction, a property that has been demonstrated in...

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.

Acoustic cloak for airborne sound by inverse design

This Letter presents practical realization of a two-dimensional low loss acoustic cloak for airborne sound obtained by inverse design. The cloak consists of 120 aluminum cylinders of 15 mm diameter surrounding the cloaked object—a cylinder of diameter 22.5 cm. The position of each cylinder in the cloak is optimized using the data from two different techniques: genetic algorithm and simulated annealing. The operation frequency of this cloak is 3061 Hz with the bandwidth of about 100 Hz. Being a multi-step approach to the desired cloaking, the inverse design is also valid, in principle, for non-symmetric cylinders and even for three-dimensional objects.

Quenching of acoustic bandgaps by flow noise

We report an experimental study of acoustic effects produced by wind impinging on noise barriers based on two-dimensional sonic crystals with square symmetry. We found that the attenuation strength of sonic-crystal bandgaps decreases for increasing values of flow speed. A quenching of the acoustic bandgap appears at a certain speed value that depends of the barrier filling ratio. For increasing values of flow speed, the data indicate that the barrier becomes a sound source because of its interaction with the wind. We conclude that flow noise should be taken into account in designing acoustic barriers based on sonic crystals.

Directional acoustic source by scattering acoustical elements

Highly directional sources are desirables in a variety of fields for many applications. The authors report an inverse designed scattering acoustical element device that transforms an omnidirectional ultrasonic source into one highly directional. This two-dimensional design shows an overall better modeled performance than other previously proposed, including a half-power angular width less than 5°. The experimental demonstration is performed in the ultrasonic range, using a hydrophone as omnidirectional source and an array of alumina rods as building blocks for the scattering acoustical elements. The measured half-power angular width is 6°, a value that supports the high reliability of the designing tool.

Homogenization of two-dimensional anisotropic dissipative photonic crystal

We present the theoretical and experimental study of the effective permittivity of photonic crystal (PC) of parallel rods in the long-wavelength limit. Considering low-absorbing materials, we obtain analytical formulas for the imaginary part of the permittivity for the dielectric tensor. For the H-polarized mode propagating in a low-loss PC with rectangular symmetry we predict stronger azimuthal anisotropy for the imaginary part of permittivity than that for the real part. Measurements of microwave transmission through a PC of FR4 rods support the proposed theory. The obtained results may be used for optimization and tailoring of electromagnetic losses in artificial periodic structures.

Experimental realization of sonic demultiplexing devices based on inverse designed scattering acoustic elements

We present experiments demonstrating sonic demultiplex devices able to separate spatially several wavelengths. The devices are based on clusters of circular scatterers whose position and size are determined by using an inverse design approach. Two prototypes are built and characterized, a first that spatially separates two wavelengths (i.e., 20.0 and 22.7cm) and a second that separates three wavelengths: 20.0, 21.3, and 22.7cm. Both prototypes are typical examples of acoustic scattering elements, a name here introduced to define inverse designed devices that are characterized by their great adaptability in controlling the scattering image, including both the reflected and transmitted waves.

Sound control by temperature gradients

This work reports experiments showing that airborne sound propagation can be controlled by temperature gradients. A system of two heated tubes is here used to demonstrate the collimation and focusing of an ultrasonic beam by the refractive index profile created by the temperature gradients existing around the tubes. Numerical simulations supporting the experimental findings are also reported.