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The correction of the aberration of transcranial focused ultrasounds is a relevant issue for enhancing various non-invasive medical treatments. Emission through multi-element phased arrays has been the most widely accepted method to reduce aberrations in recent years; however, a new disruptive technology, based on 3D printed acoustic lenses, has recently been proposed with lower cost and comparable accuracy. The number and size of transducers in phased array configurations was a bottleneck limiting the focusing accuracy, but once the submillimeter precision of the latest generation 3D printers has overcome this limitation, the challenge is now to improve the accuracy of the numerical simulations needed to design the lens. This study introduces and evaluates two improvements to the numerical model applied in previous works that proposed 3D lenses, which consist in the direct calculation of the phase pattern from the propagation of oscillating magnitudes in complex form, and in the introduction of the absorption phenomenon into the set of equations that describe the dynamics of the wave in both fluid and solid media. Numerical experiments are performed analysing the quality of the aberrated-corrected focus in different configurations. The results obtained show that the inclusion of absorption significantly improves focusing, especially where the thickness of the skull is more irregular.

A time-domain numerical code based on the constitutive relations of nonlinear acoustics for simulating ultrasound propagation is presented. To model frequency power law attenuation, such as observed in biological media, multiple relaxation processes are included and relaxation parameters are fitted to both exact frequency power law attenuation and empirically measured attenuation of a variety of tissues that does not fit an exact power law. A computational technique based on artificial relaxation is included to correct the non-negligible numerical dispersion of the numerical method and to improve stability when shock waves are present. This technique avoids the use of high order finite difference schemes, leading to fast calculations. The numerical code is especially suitable to study high intensity and focused axisymmetric acoustic beams in tissue-like medium, as it is based on the full constitutive relations that overcomes the limitations of the parabolic approximations, while some specific effects not contemplated by the Westervelt equation can be also studied. The accuracy of the method is discussed by comparing the proposed simulation solutions to one-dimensional analytical ones, to k-space numerical solutions and also to experimental data from a focused beam propagating in a frequency power law attenuation media.

The broadband reduction of the specular reflections by sonic crystals (SCs) is theoretically and experimentally reported in this work. The analysed system consists of a sound source radiating a SC made of acoustically rigid scatterers embedded in water partially covering an open cavity. By comparison with a reference flat reflector, we observe that reflected waves spread in space as a consequence of the spatially modulated properties of the SC. Moreover, due to the different working frequency ranges of the SC a significant noise reduction is produced in a broadband region. Therefore, due to the spreading of the reflected waves, the system produces a broadband noise reduction in the area of the source. In particular, the noise reduction is close to 2 dB for the two octaves emitted by our source, which represents a decrease of 37% of the acoustic energy. The results shown in this work constitute a proof of concept for the use of SCs as broadband-noise reduction systems at the launch pad. An approach to the geometry of the Vega launch vehicle the European Space Agency is proposed and the limitations of the study are discussed.

The stability and dynamic behavior of a two-level, Jϭ0↔Jϭ1, Zeeman laser model is investigated in the limit of large cavity anisotropy. The stability of the steady-state solutions is governed by two different Hopf bifurcations, one affecting the polarization state of the laser light and the other affecting the intensity dynamics. Above these bifurcations the dynamic behavior exhibited by the model is extremely rich. It has been found that the routes to chaos almost always involve quasiperiodic as well as intermittent dynamics. When this quasiperiodic behavior is locked, type-I and -II intermittencies have been identified. When unlocked, the torus can destabilize through two different scenarios leading to chaos: a ''quasiperiodic intermittency'' or a cascade of period-doubling bifurcations. On-off intermittency has also been found.

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Herrero Durá, JM.; Blasco, X.; Sánchez Pérez, JV.; Redondo, J. (2016). Design of sound phase diffusers by means of multiobjective optimization approach using ev-MOGA evolutionary algorithm. Structural and Multidisciplinary Optimization. 53(4):861-879. doi:10.1007/s00158-015-1367-0.Design of sound phase diffusers by means of multiobjective optimization approach using ev-MOGA evolutionary algorithm J.M. Herrero · X. Blasco · J.V. Sánchez-Pérez · J. RedondoReceived: date / Accepted: date Abstract In this paper a new approach to design sound phase diffusers is presented. The acoustic properties of such diffusers are usually increased by using single objective optimization methods. Here we propose the use of a multiobjective (MO) approach to design them in order to take into account several conflicting characteristic simultaneously. Three different MO problems are posed to consider various scenarios where fundamentally the objective is to maximize the normalized diffusion coefficient (following the corresponding Audio Engineering Society standard) for the so-called medium frequencies. This single objective could be divided into other several objectives to adjust performances to designer preferences. A multi-objective evolutionary algorithm (called ev-MOGA) is used to characterize the Pareto front in a smart way. ev-MOGA is modified, by using integer codification and tuning some of its genetic operators, to adapt it to the new requirements. Special interest is posed in selecting the diffusers codification properly to eliminate duplicities that would produce a multimodal problem. Precision in the manufacturing process is taking into account in the diffuser codification causing, that the number of different diffusers are quantified. Robust considerations related with the precision manufacturing process are considered in the decision making process. Finally, an optimal diffuser is selected considering designer preferences.