Acoustic imaging in confined and noisy environments using double layer Time Reversal and Field Separation Methods

International audienceMany imaging methods cannot localize precisely unstationary sources in confined and noisy environments. In this paper, the use of a Time Reversal acoustic sink (TRS) method is proposed, in conjunction with a Field Separation Method (FSM). The proposed time reversal (TR) process is based on the measurement of the sound pressure field and its normal derivative on a double layer hemispherical antenna, which bounds the region of interest (ROI). These data are time-reversed and numerically back-propagated to a surface, 0.5 cm away from the source plane. As most imaging methods, the efficiency of this process relies on the use of the most suitable Green functions, which depend on the propagating environment. A way to improve the TR process is to transform numerically the confined space problem into a free field case, for which the Green functions are well-known. The proposed FSM consists in expanding the measured fields on the spherical harmonics functions, thus allowing to compute the outgoing waves. This process allows a precise localization and characterization of the source placed under the antenna, using free-field Green functions. Thanks to this method, the influence of reverberation and acoustic fields radiated by sources outside the ROI can be suppressed. The measurements presented in this paper are performed in an anechoic room, using two acoustic sources. The first one to image in the ROI emits a filtered pulse and the second one, placed outside the ROI, is driven by a Gaussian white noise. In order to assess the reconstruction quality of the proposed imaging process, a reference field is measured in an anechoic room on the back-propagation surface, corresponding to the pressure values when the source laying in the ROI is radiating alone. Comparisons with back-propagated pressures using TRS in conjunction with FSM show a good accuracy both in space and time domains

Kahler-Ricci flow on blowups along submanifolds

In this short note, we study the behavior of Kaher-Ricci flow on Kahler manifolds which contract divisors to smooth submanifolds. We show that the Kahler potentials are Holder continuous and the flow converges sequentially in Gromov-Hausdorff topology to a compact metric space which is homeomorphic to the base manifold

Synthesis of a Mach cone using a speaker array

International audienceThe interest of the authors concerns sniper detection using time-reversal techniques on the Mach cone in a reverberant urban environment. In order to setup a safe and reproducible experimental framework at a reduced scale, it is possible to synthesize a N-wave with a conical geometry by means of loudspeakers disposed along a hypothetical ring axis. The supersonic nature of the simulated displacement leads to a set of constraints, both on spatial and temporal samplings, correlated to the structure of the medium and to the digital sampling of the N-shaped signal. Those constraints are theoretically studied to ensure reconstruction of the conical wavefront. A rst experiment has been realized, that allowed the synthesis of a Mach wave using 15 speakers spaced by 4.36 cm. Taking into account the directivity of each speaker and the diraction eects due to the line array, the symmetry of revolution of the cone is studied. Since the loudspeakers are in their linear regime, nonlinear behaviors of the wave are no longer present. However, inverse ltering methods are possible for improving the quality of the signal. We show that it is possible to visualize the spatio-temporal evolution of the pressure eld in planes containing the ring axis using a linear microphone array mounted on a translation robot. Comparisons between experiments and simulations show encouraging results for the following. PACS no. 43.28.We, 43.28.M

Evaluation of the viscous characteristic length of air-saturated porous materials from the ultrasonic dispersion curve

International audienceThis Note describes a method to determine the viscous characteristic length $\gamma$ from an analysis and a linear regression of the dispersion curve. This approach is valid in the high frequency domain, above 20 kHz, where some asymptotic expansions simplify the theoretical equations. It is shown that there is a linear relationship between the squared propagation index and the inverse of the square root of the frequency which has a slope proportional to $\gamma$, the thermal characteristic length $\gamma'$ and some physical constants. The value of $\gamma'$ is determined using the Brunauer, Emmet and Teller (BET) method. Low frequency ultrasound, in the 40-200 kHz range, is used to perform the experiments. The computation of the dispersion curve from the experimental waveforms is done by using the phase spectrum method, as was outlined in a previous Note

Méthodes d'identification et de caractérisation de source de bruit en environnement réverbérant

Ce travail de thèse à été financé par le projet LICORVE (Développement de garnitures légères, innovantes, recyclables et poly-sensorielles pour les applications de coffres de véhicule). Il consiste à développer une méthodologie de mesure pour localiser et caractériser les sources de bruit dans un coffre de voiture. L'environnement de mesure se caractérise par un petit volume où les réflexions de la source d'intérêt et des sources perturbatrices sur les parois ne peuvent être négligées. La méthode doit donc permettre de séparer les différents contributions pour estimer le plus précisément possible les sources étudiées (déconfinement). Dans un premier temps, deux méthodes de séparation : Double Layer SONAH (Statistically Optimal Near el Acoustical Holography) et Field Separation Method (FSM) sont étudiées numériquement. Les limites et avantages de chacune ont été déterminés dans un environnement de mesure confiné. Cela a permis de choisir la méthode la plus adaptée à notre problématique. Dans un deuxième temps les principales conclusions de l'étude numérique sont validées expérimentalement. Dans ce cadre, un ensemble de mesures sont réalisées dans une maquette avec la méthode FSM pour localiser et caractériser des sources maitrisées. Après avoir validée la fiabilité de la méthode de séparation, FSM a été mise en œuvre dans le coffre d'une Peugeot 508 sw en condition de roulement. Les résultats obtenus ont permis d'orienter le choix des garnitures pour un traitement acoustique optimal.This thesis consists in developing, through the LICORVE project (light garnitures, innovative, recyclable and multi-sensorial for vehicle boots applications), a measurement method for localizing and characterizing noise sources in a vehicle trunk. The measuring environment is distinguished by a small volume where the reflections on the partitions generated by the source of interest and the interfering sources cannot be neglected. Therefore, the method must allow the separation of the different contributions in order to assess accurately the studied sources. As a first step, two separation methods : Double Layer SONAH (Statistically Optimized Near-Field Acoustical Holography) and Field Separation Method (FSM) are numerically studied. The limitations and advantages of each of them are determined in a confined measuring environment; this allowed to select the most appropriate method to tackle our problem. As a second step, the main conclusions of the numerical study are confirmed experimentally. In this context, measurements are performed, using the FSM method, in a trunk mock-up to localize and characterize the controlled sources. So confirmed the reliability of the separation method, it has been tested in the boot of a Peugeot 508 SW on a roller bench. The obtained results allowed guiding the selection of garniture for the acoustic treatment.PARIS-CNAM (751032301) / SudocSudocFranceF

Determination of the viscous and thermal characteristic lengths of plastic foams by ultrasonic measurements in helium and air

International audienceThe acoustic wave most commonly transmitted and detected in the high-porosity absorbent materials used in noise control is generally the airborne slow compressional wave. In a new experiment, the air saturating the sample is replaced by helium and the transmission is studied at ultrasonic frequencies ͑70–600 kHz. The experiment is quite easily performed using standard ultrasonics and vacuum equipment. The main purpose of this work is to propose a method to determine simultaneously both the viscous and thermal characteristic lengths with the same precision. These two parameters characterize the viscous and the thermal interactions between the frame and the fluid at high frequencies. The characteristic lengths are deduced from the high-frequency asymptotic behavior of either the velocity or the attenuation curves obtained in the sample saturated by air and by helium. It also appears that due to the properties of helium, the discrepancy previously observed between predictions and measurements is shifted toward higher frequencies

Le retournement temporel en milieu réverbérant pour localiser une source supersonique

International audienceLocaliser l'axe du cône de Mach causé par le déplacement supersonique d'un objet balistique dans un environnement urbain réverbérant représente un défi scientifique et technique considérable, notamment en raison des réflexions multiples sur les murs. Le retournement temporel rend possible la localisation de sources en milieu réverbérant mais n'a pas encoré eté testé lorsque les sources sont en mouvement supersonique. Le présent article expose lapremì ere approche d'un telprobì eme dans le cadre d'un modèle géométrique simple, reproduisant un espace réverbérant constitué de deux murs et d'un sol d'impédance infinie. Le principe de Huyghens permet de synthétiser un cône de Mach par superposition de fronts d'ondes monopolaires. Dans le cadre de cetté etude, le principe est utilisé pour des simulations numériques et pour une validation expérimentale en laboratoire, o` u une ligne de haut-parleursémetparleurs´parleursémet le cône de Mach artificiel. Une méthode faisant appel aux fonctions de Green des sources images modélise la réverbération pour la propagation directe et la rétro-propagation par renversement du temps, grâcè a un réseau de microphones déployé sur le sol. Un calcul numérique rétro-propage ensuite les données mesurées jusque dans des tranches verticales intersectant l'axe du cône de Mach. La maximisation d'un critère statistique d'ordre 4, qui supprime les forts niveaux dus aux microphones, détermine le point d'intersection en question. Une méthode de tri permet de garder les meilleures estimations servantàservant`servantà l'interpolation géométrique de l'axe. La méthode proposée présente une précision angulaire de 1 • et une distance entre les axes de 1 cm, ` a la fois sur les simulations et les mesures expérimentales

Characterization of acoustic materials at arbitrary incidence angle using sound field synthesis

Standardized methods for measuring sound absorption such as the impedance tube and reverberation chamber methods are limited to normal or diffuse incidence, respectively. Two research axes have been generally followed in the literature to develop alternative techniques, the first one focusing on the measurement part, that is from the two-microphone technique to the use of microphone arrays or pressure-velocity sensors. The second axis focuses on the excitation part with for instance the use of sound field synthesis techniques. Since acoustic impedance and sound absorption coefficient of materials are classically defined under normal and oblique plane wave excitation, synthesizing an “ideal” plane wave using a loudspeaker array would allow measuring these acoustics quantities using a simple microphone pair. In this article, the effect of the different parameters of a loudspeaker array on acoustic plane waves reproduction on a material’s surface is first numerically studied. Then, numerical and experimental results for the estimation of both impedance and absorption coefficients are reported. These results show that sound field synthesis allows to characterize a material for arbitrary incidence angles over a wide frequency range, thus offering an alternative method to standard techniques and an improvement over existing works

Overview of recent TJ-II stellarator results

The main results obtained in the TJ-II stellarator in the last two years are reported. The most important topics investigated have been modelling and validation of impurity transport, validation of gyrokinetic simulations, turbulence characterisation, effect of magnetic configuration on transport, fuelling with pellet injection, fast particles and liquid metal plasma facing components. As regards impurity transport research, a number of working lines exploring several recently discovered effects have been developed: the effect of tangential drifts on stellarator neoclassical transport, the impurity flux driven by electric fields tangent to magnetic surfaces and attempts of experimental validation with Doppler reflectometry of the variation of the radial electric field on the flux surface. Concerning gyrokinetic simulations, two validation activities have been performed, the comparison with measurements of zonal flow relaxation in pellet-induced fast transients and the comparison with experimental poloidal variation of fluctuations amplitude. The impact of radial electric fields on turbulence spreading in the edge and scrape-off layer has been also experimentally characterized using a 2D Langmuir probe array. Another remarkable piece of work has been the investigation of the radial propagation of small temperature perturbations using transfer entropy. Research on the physics and modelling of plasma core fuelling with pellet and tracer-encapsulated solid-pellet injection has produced also relevant results. Neutral beam injection driven Alfvénic activity and its possible control by electron cyclotron current drive has been examined as well in TJ-II. Finally, recent results on alternative plasma facing components based on liquid metals are also presentedThis work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 under Grant Agreement No. 633053. It has been partially funded by the Ministerio de Ciencia, Inovación y Universidades of Spain under projects ENE2013-48109-P, ENE2015-70142-P and FIS2017-88892-P. It has also received funds from the Spanish Government via mobility grant PRX17/00425. The authors thankfully acknowledge the computer resources at MareNostrum and the technical support provided by the Barcelona S.C. It has been supported as well by The Science and Technology Center in Ukraine (STCU), Project P-507F

Spread of a SARS-CoV-2 variant through Europe in the summer of 2020

[EN] Following its emergence in late 2019, the spread of SARS-CoV-21,2 has been tracked by phylogenetic analysis of viral genome sequences in unprecedented detail3,4,5. Although the virus spread globally in early 2020 before borders closed, intercontinental travel has since been greatly reduced. However, travel within Europe resumed in the summer of 2020. Here we report on a SARS-CoV-2 variant, 20E (EU1), that was identified in Spain in early summer 2020 and subsequently spread across Europe. We find no evidence that this variant has increased transmissibility, but instead demonstrate how rising incidence in Spain, resumption of travel, and lack of effective screening and containment may explain the variant’s success. Despite travel restrictions, we estimate that 20E (EU1) was introduced hundreds of times to European countries by summertime travellers, which is likely to have undermined local efforts to minimize infection with SARS-CoV-2. Our results illustrate how a variant can rapidly become dominant even in the absence of a substantial transmission advantage in favourable epidemiological settings. Genomic surveillance is critical for understanding how travel can affect transmission of SARS-CoV-2, and thus for informing future containment strategies as travel resumes.S