Javier Redondo

Theoretical prediction of the nondiffractive propagation of sonic waves through periodic acoustic media

We predict theoretically the nondiffractive propagation of sonic waves in periodic acoustic media (sonic crystals), by expansion into a set of plane waves (Bloch mode expansion), and by finite difference time domain calculations of finite beams. We also give analytical evaluations of the parameters for nondiffractive propagation, as well as the minimum size of the nondiffractively propagating acoustic beams.

Prediction of the Random-Incidence Scattering Coefficient Using a FDTD Scheme

This paper is focused on the evaluation of the scattering coefficient of sound diffusers which are based on the incoherency of diffusely reflected sound. A new approach for predicting the scattering coefficient is proposed; the method is based on a finite difference time domain (FDTD) scheme. Two established scattering coefficient measurement methods, proposed by Mommertz and Vorlander [1], are simulated; these correspond to measurements in a reverberant chamber, and free field. The results are also compared to those obtained in a previous paper [2], wherein it was demonstrated that FDTD schemes can be used to predict polar responses. These free field polar responses are used to find the correlation scattering coefficient, which in turn is used to validate the free field case. In modelling the reverberant chamber method, 2D simulations have been used to reduce computation time; hence it is necessary to derive a diffuse field formulation for a 2D reverberation chamber, which is presented. In this case, 1:5 scale experimental data is used for validation.

Application of Structured Light System Technique for Authentication of Wooden Panel Paintings.

This paper presents a new application of photogrammetric techniques for protecting cultural heritage. The accuracy of the method and the fact that it can be used to carry out different tests without contact between the sample and the instruments can make this technique very useful for authenticating and cataloging artworks. The application focuses on the field of pictorial artworks, and wooden panel paintings in particular. In these works, the orography formed by the brushstrokes can be easily digitalized using a photogrammetric technique, called Structured Light System, with submillimeter accuracy. Thus, some of the physical characteristics of the brushstrokes, like minimum and maximum heights or slopes become a fingerprint of the painting. We explain in detail the general principles of the Structured Light System Technique and the specific characteristics of the commercial set-up used in this work. Some experiments are carried out on a sample painted by us to check the accuracy limits of the technique and to propose some tests that can help to stablish a methodology for authentication purposes. Finally, some preliminary results obtained on a real pictorial artwork are presented, providing geometrical information of its metric features as an example of the possibilities of this application.

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.

Application of ultrasound phase-shift analysis to authenticate wooden panel paintings

Artworks are a valuable part of the Worlds cultural and historical heritage. Conservation and authentication of authorship are important aspects to consider in the protection of cultural patrimony. In this paper we present a novel application of a well-known method based on the phase-shift analysis of an ultrasonic signal, providing an integrated encoding system that enables authentication of the authorship of wooden panel paintings. The method has been evaluated in comparison with optical analysis and shows promising results. The proposed method provides an integrated fingerprint of the artwork, and could be used to enrich the cataloging and protection of artworks. Other advantages that make particularly attractive the proposed technique are its robustness and the use of low-cost sensors.

The Parpalló Cave: A singular archaeological acoustic site

The Parpallo Cave is located in the slopes of Montduber, in Valencia (Spain). It is one of the most important Palaeolithic sites, not only in Spain, but in the world. It was a privileged location for its inhabitants from the Upper Palaeolithic onwards. It has one of the most spectacular collections of Palaeolithic art mobilier found to date. The Parpallo cave features a considerably large opening which, undoubtedly, influences its acoustic properties. Indeed, the sound pressure field inside the cave is not excessively reverberant and intelligibility is significantly better than in similar enclosures. In this work a study of the acoustic properties of the Parpallo cave is performed by using numerical simulation.

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.

The Potential for Phononic Sound Diffusers (PSD)

Although periodic structures theory gave rise to sound diffusers almost 40 years ago, it has been only in recent years when several authors regarded the possible application of a particular kind of periodic structures, phononic crystals, in the field on room acoustics. This paper explores the possible application of periodic structures used as sound diffusers in rooms. Preliminary simulations, carried out with FDTD, shows that due to the inherent time spreading provided by a periodic structure, they can be used to reduce, or even to eliminate, pernicious effects associated with the low frequency modes of the room.

Time-domain simulation of ultrasound propagation in a tissue-like medium based on the resolution of the nonlinear acoustic constitutive relations

The authors acknowledge financial support from the FPI program of the Universitat Politecnica de Valencia.

The Uncertainty in Absorption Coefficients Measured in Reverberant Chambers: A Case Study

The aim of this work is to study the uncertainties that are present in measurements of the sound absorption coefficients of acoustical materials by means of the determination of the reverberation time (according to ISO 354 [1]). A detailed study of the uncertainty in the measurements of sound absorption coefficients has been performed. As an additional feature we present a representation of the absorption coefficient and its relative uncertainty.