Transmission extraordinaire des ondes acoustiques : du concept pour les ondes de volume en régime GHz à l’application pour l’imagerie, par Thibaut Devaux, Laboratory of Applied Solid State Physics Hokkaido University, Sapporo, Japan, invité par Jean-Michel

2018
04
20

Mots Clés :  transmission extraordinaire, imagerie sub-longueur d’onde, microscopie acoustique, ondes de volume, GHz, nano-tube

Concentrer l’énergie des ondes acoustiques dans une région de taille inférieure à la longueur d’onde est un récent sujet d’intérêt dans le domaine des métamatériaux. Une méthode pour y parvenir est le principe de la transmission extraordinaire des ondes, dans lequel une structure résonante est combinée à une ouverture de taille sub-longueur d’onde pour améliorer la transmission de l’onde. Après un bref rappel des travaux existants et des perspectives envisagées dans ce domaine, ce séminaire est articulé autour de deux axes présentants les recherches effectuées sur ce sujet lors de mon post-doctorat à l’Université de Hokkaido au Japon.
La transmission optique extraordinaire (EOT) et la transmission acoustique extraordinaire (EAT) ont déjà montré théoriquement et expérimentalement la possibilité de transmettre une quantité d’énergie supérieure à celle attendue par les seules considérations géométriques. Cependant, cette approche n’a jamais été exploré pour des ondes acoustiques confinées dans une architecture purement solide. La première partie de ce séminaire présente une architecture permettant d’obtenir des caractéristiques d’EAT à des régimes de l’ordre du GHz pour des ondes de volumes. Ce système propose de coupler les résonances acoustiques de Fabry-Pérot à l’intérieur d’un tube de taille nanométrique avec la conversion des ondes longitudinales en ondes de surface à l’interface d’une structure rainurée. A l’aide de simulations numériques, il est possible de montrer une transmission d’énergie jusqu’à ∼340 fois supérieure aux attentes géométriques. En disposant une structure rainurée en aval du tube, il est également possible de modifier la directivité du rayonnement acoustique pour de futures applications en imagerie.
La seconde partie de cet exposé propose d’utiliser le phénomène de transmission extraordinaire acoustique à des fins d’imagerie. Un nouveau prototype de microscope acoustique fonctionnant en champ proche et en régime audible est présenté. Celui-ci exploite les capacités de transmission extraordinaire d’un métamatériau à masse nulle. En utilisant une simple mesure du coefficient de réflexion à l’intérieur d’un tube rempli d’air qui dispose d’une extrémité sur laquelle est disposée une membrane sub-longueur d’onde, nous montrons expérimentalement et numériquement qu’il est possible d’atteindre une résolution en profondeur d’environ 2 fois le diamètre de la membrane et une résolution latérale ∼ 25 fois inférieure à la longueur d’onde émise (λ = 25 cm) à 1400 Hz dans l’air. Les capacités de ce dispositif sont ensuite illustrées avec une application à l’imagerie topographique bi-dimensionnelle. Par ailleurs, les résultats montrent la possibilité d’obtenir une image quantitative d’une surface composée de différents matériaux et ainsi en déduire les informations sur le coefficient de réflexion acoustique de l’échantillon avec une résolution sub-longueur d’onde.

Simulation numérique d’un piano de concert , Patrick Joly, Unité de Mathématiques Appliquées, UMA, Ensta Paris Tech, invité par François Gautier

2018
02
20

Dans cet exposé, je raconterai, un peu comme une histoire, ma collaboration avec Juliette Chabassier et Antoine Chaigne pour la modélisation mathématique d’un piano de concert. J’expliquerai la mise en équations du modèle effectuée à partir d’une description aussi complète que possible de l’instrument, l’étude de la littérature et l’exploitation de résultats expérimentaux. Les principales propriétés mathématiques de ce modèle, en particulier les identités d’énergie, seront présentées. Puis je détaillerai les choix qui sont été faits pour la discrétisation des équations, avec pour but essentiel de garantir la stabilité numérique du schéma numérique et un contrôle optimal des propriétés de dispersion numérique. Plusieurs résultats de simulation numérique seront exposés. Des perspectives de continuation de ce travail seront évoquées.

Désordre et comportement dynamique dans les matériaux. Phénomènes de relaxation., par Nouredine ZEKRI, USTO (Oran), invité par Sohbi Sahraoui

2018
04
02

Même ordonnés, les matériaux présentent toujours des imperfections, qu’ils soient naturels (bois, cuivre, eau etc.) ou bien réalisés par l’homme (super-réseau, nanostructures etc.). Ces imperfections sont soit substitutionnelles (impuretés, alliages, …) ou structurelles (dislocations, amorphisme, …). La réponse aux excitations externes est souvent très influencée par ces imperfections. Dans le cas du transport d’ondes (électroniques, optiques, acoustiques, …), le désordre mène à une transition métal/isolant de type localisation Anderson. Dans le cas du transport de particules ou d’énergie, on parle de diffusion (normale ou anormale). Les phénomènes de croissance obéissent à des lois dynamiques similaires régulés par la rugosité de l’interface. Finalement les phénomènes de relaxations, observés dans tous les domaines combinant le stockage et la diffusion (comme la charge et la décharge d’un condensateur), présentent un comportement non exponentiel à cause du désordre.  
Dans ce séminaire nous nous intéressons à l’influence du désordre sur les phénomènes de relaxation. Ces phénomènes seront abordés dans deux cas différents: les diélectriques (type Debye ou non) et la viscoélasticité (type Zener ou non). Gérés d’habitude par des équations de premier ordre, la présence du désordre conduit à des équations d’ordre fractionnaires. Ces équations semblent correspondre à des phénomènes de relaxations dans des structures de dimension fractale.  Une application à la relaxation des fronts de feu dans le but de déduire la topologie du support sera également discutée comme perspective.

Tunable topological phononic crystals, par Ying WU, Associate Prof. (KAUST), invité par Vincent Tournat

2018
01
23



Evaluation of the biodiesel oxidation stage and characterization of biodiesel blends by spectroscopic methods, par Elton De Lima SAVI, Brésil, invité par Samuel R

2018
05
15

This study showed how the attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) combined with principal component analysis (PCA) can identify a biodiesel sample at different stages of the oxidative process. The biodiesel samples were prepared using the Rancimat technique, from a commercial biodiesel composed of soybean oil and animal fat. The biodiesel degradation was accelerated maintaining the temperature of the sample at 110°C under constant air flux for different times. Properties such as oxidative stability, density, viscosity, calorific value, gas chromatography and UV/Vis were measured for general characterization. These measures served as support to interpreting the final results, some of them are commonly used in standard analyzes. Infrared absorption spectra were measured using the ATR-FTIR technique. Principal component analysis scores were generated from the spectra by a routine in Mathematica software. Component analysis made it possible to distinguish samples at the initial oxidative stage due to changes in the chemical compounds. This method was fast, accurate, low cost and it did not use chemical inputs. The second part of the work had as objective to determine small amounts of biodiesel in blends diesel / biodiesel, using the thermal lens spectroscopy (LT) with photochemical effect in a dual-beam configuration. The blends were prepared in different concentrations, of standard fuels and commercial fuels. Samples properties such as density, viscosity, calorific value, dn/dT, specific heat, infrared absorption spectrum and UV / Vis were performed as general characterization. Thermal lens spectroscopy was operated with the excitation beam at 476.5 nm and 950 nm. Using the TL theoretical model, considering photochemical effect, it was possible to fit the TL data. Therefore, TL spectroscopy can be an useful method for certifying the quality of biodiesel blends.

Keywords: Biodiesel, PCA, ATR-FTIR, diesel/biodiesel blends, thermal lens spectroscopy.

Propagation acoustique dans les résonateurs cylindriques d’instrument à vent en bois., par Henri Boutin, IRCAM

2018
04
10

Le son des instruments à vent est le résultat du contrôle d’un grand nombre de paramètres. Certains sont contrôlés par l’interprète au cours du jeu, et d’autres sont fixés par le facteur, notamment dans la fabrication du résonateur.

Pour les instruments en bois, dans la tradition occidentale, les facteurs choisissent typiquement des essences de bois dures et denses, de façon à garantir une stabilité géométrique de la perce lorsqu’elle est soumise à de fortes variations d’humidité et de température. Puis ils polissent la surface interne et l’imprègnent d’un mélange d’huile (de lin ou d’amande douce) et d’essence de térébenthine. Cette dernière étape est répétée régulièrement au cours de la vie de l’instrument.

Afin d’étudier l’impact de ces étapes de facture sur l’acoustique du résonateur, cette présentation propose un modèle de propagation dans un tuyau cylindrique à paroi poreuse. Il permet d’estimer le facteur d’atténuation dans la perce ainsi que l’impédance de paroi, à partir de mesures d’impédances d’entrée.

Cette méthode non-intrusive est appliquée à plusieurs tuyaux de différentes essences et directions avant polissage, après polissage, puis après huilage de la paroi interne. Elle permet de rendre compte de l’état de la perce au cours de la fabrication du résonateur et présente pour cela un intérêt significatif pour les facteurs.

les différentes équipes de Recherche du Laboratoire de Mécanique, Modélisation et Production et quelques applications traitées pour les problèmes Acoustiques et Vibro-Acoustique, par Mohamed HADDAR, invité par Jean-Luc R

2018
04
17

Cette conférence consiste à présenter les différentes thématiques du LAboratoire de
Mécanique, Modélisation et Productique (LA2MP) de l’Ecole Nationale d’Ingénieurs de
Sfax, en Particulier la thématique « Comportement dynamique et vibro-acoustique des
systèmes mécaniques et des structures ». Ensuite, une présentation générale des
différentes coopérations internationales avec différents Laboratoires de recherche.
A la fin de cette conférence, deux présentations succinctes seront proposées :
« Caractérisation acoustique des matériaux absorbants à base des déchets agricoles » et
« Modélisation numérique du comportement vibro-acoustique d’une boite de vitesse ».
Ces deux présentations correspondent aux résultats de deux thèses réalisées et soutenues
au sein de notre Laboratoire.

Acoustique des mousses : zoom sur la vibration des films, par Caroline Derec, Univ. Paris-Diderot, invitée par Jean-Michel G

2018
06
12

Des travaux récents ont montré que la propagation d'une onde acoustique dans une mousse liquide présente un comportement complexe qui s'explique en évoquant la dynamique à l'échelle des constituants élémentaires de la mousse, en particulier celle des films de savon qui entourent les bulles. Par ailleurs, nous avons observé que des mousses solides présentent sous certaines conditions une absorption fortement amplifiée lorsqu'elles sont à pores fermés, plutôt qu'à pores ouverts. Les films (liquides ou solides) semblent ainsi jouer un rôle important pour la compréhension de la propagation acoustique dans les mousses, et nous avons développé des expériences se focalisant sur l'étude de la vibration de films isolés. 

- Dans une première expérience, un film de savon liquide est mis en vibration sur un pot vibrant et une onde transverse stationnaire s'établit. Nous avons mesuré expérimentalement et calculé la relation de dispersion complexe de cette onde, afin de décrire les effets inertiel, élastique et dissipatif dominants dans la réponse du film.

- Par ailleurs nous effectuons des mesures en tube d'impédance pour mesurer l'absorption acoustique due à la présence d'un film unique (film liquide ou solide élastique), et par diverses expériences et modélisations, nous cherchons à comprendre l'origine de cette absorption.

Je terminerai par la présentation de la "Danse d'un film de savon", expérience développée dans un objectif de médiation scientifique : un film de savon vertical est installé à l'extrémité d'un tube, dont l'autre extrémité présente un haut-parleur. Lorsque le haut-parleur diffuse un morceau de musique, le film de savon s'anime en rythme selon une dynamique riche et complexe.

 

Propagation acoustique dans les mousses : de l’audible à l’ultrason, du liquide au solide, par Julliette Pierre et Mathis , invitée par Jean-Michel G

2018
09
11

Les mousses liquides et solides présentent des propriétés acoustiques surprenantes.
Dans un premier temps, je vous monterai comment le son se propage et se dissipe dans une mousse liquide. Une mousse liquide résulte d’un ensemble compact de bulles de gaz, non sphériques et délimitées par des films de savon très minces. Ces dernières années nous avons montré que cette structure ossature liquide / film liquide est à l’origine de propriétés acoustiques remarquables : forte dissipation du son et densité effective négative [Pierre et al. PRL (2014), Pierre et al. EPJE (2017)].
Plus récemment, nous nous sommes intéressés aux cas de mousses solides fabriquées par solidification de mousses liquides et dont l'intégralité de la structure liquide est conservée (i.e. les pores des mousses solides sont fermés par de fines membranes). Je vous montrerais comment la présence des membranes affecte de manière importante la propagation du son [Gaulon et al. APL (2018)].

14h : Topological Elastic Metastructures: exploiting topological transitions to tailor the mechanical energy flow in thin-walled structures, par Fabio Semperlotti, invité par François G

2018
06
05

Topological acoustics is an emerging branch of physical acoustics that developed in the wake of the research on topological insulators in condensed matter physics. Owing to the similarities between the electromagnetic and the acoustic wave fields, acoustic metamaterials capable of topologically non-trivial transport properties were recently theorized and experimentally validated. This talk will present some recent work from my group that focuses on the design and development of engineered elastic thin-walled structures exhibiting non-trivial topological dynamics. The dispersion and propagation properties of such elastic waveguides are engineered by exploiting a mechanism that is the acoustic analogue of the Quantum Valley Hall Effect (QVHE) in electronic materials. QVHE enables the creation of domain walls inside the lattice that are capable of supporting highly robust and quasi-one-directional edge states. By using a combination of theoretical, numerical, and experimental results this talk will illustrate how QVHE can be effectively achieved in load-bearing structural waveguides therefore enabling accurate control of the mechanical energy flow.

Bio: Dr. Fabio Semperlotti is an Assistant Professor in the School of Mechanical Engineering at Purdue University. Before joining Purdue, Dr. Semperlotti was a faculty in the Aerospace and Mechanical Engineering department at the University of Notre Dame. He received a M.S. in Aerospace Engineering (2000), and a M.S. in Astronautic Engineering (2002) both from the University of Rome “La Sapienza” (IT), and a PhD. in aerospace engineering (2009) from the Pennsylvania State University, (USA). Prior to joining Penn State, Dr. Semperlotti served as structural engineer for a few European aerospace industries, including the French Space Agency (CNES), working on the structural design of space launch systems (such as Ariane 5 and Vega) and satellite platforms.

Dr. Semperlotti is a member of the Ray W. Herrick laboratory and directs the Structural Health Monitoring and Dynamics laboratory (SHMD) where he conducts, together with his research group, research on several aspects of structures and materials including structural dynamics and wave propagation, elastic metamaterials, structural health monitoring, and energy harvesting. His research has received funding from US ARMY, DARPA, National Science Foundation, Air Force, and industrial sponsors. Dr. Semperlotti was the recipient of the National Science Foundation CAREER award (2015) and of the Air Force Office of Scientific Research Young Investigator Program (YIP) (2015).

Research work at the Vibrations and Acoustics Laboratory of the Universidad del Valle, Cali, Colombia., par Joao Ealo et Nicolas Hermant, invités par Laurent S

2018
06
26

In this presentation a description of the most relevant research results is included. From ultrasonic air-coupled applications such as the property extraction and water content estimation in Coffee plant leaves to the design and experimental characterization of broadband ferroelectret-based ultrasonic transducers for airborne ultrasound applications. This include the design of developable surface devices such as omnidirectional radiators and a dual-focalization transducer array, among others. Also, recent results on new designs for vortex beam generation are presented. Special attention is given to the appearance of grating vortices when an array transducer is employed to generate and steer the helical beam and to what we have called active diffracting gratings, in which the geometry of the vibrating surface of the ultrasonic transducer resembles that of a grating.

3 research topics in the MEMS laboratory at City College, par Ioana Voiculescu, city college NY, invitée par Stéphane D.

2018
07
03

This presentation will have three parts and will cover research conducted by two doctoral students and research conducted by the French intern students in the MEMS laboratory at City College where Dr. Voiculescu is director.

MEMS Biosensor with Integrated Impednace Spectroscopy and Gravimetric Measurements for Water Toxicity Testing

Wearable and Stretchable Piezoelectric Nanogenerator for Skin Applications

Stretchable Lab-on-chip Device with Impedance Spectroscopy Capability for Mammalian Cell Studies

_____________________________

* MEMS Biosensor with Integrated Impednace Spectroscopy and Gravimetric Measurements for Water Toxicity Testing

This presentation describes the design, fabrication and characterization of a multiparametric biosensor based on live mammalian cells. This biosensor combines two biosensing techniques; resonant frequency measurements and electric cell-substrate impedance sensing (ECIS) on a single chip. The biosensor is able to simultaneously perform in real-time two different types of electric measurements on the same cell monolayer: (1) monitoring the resonant frequency values that will give information about the progression of cells adhesion and cell viscoelasticity and, (2) recording the impedance spectra of the cells, that will report on cell adhesion progress, shape, growth, motility and viability. The sensor is based on the innovative placement of the working microelectrode for ECIS technique as the upper electrode of a quartz crystal microbalance (QCM) resonator. Bovine aortic endothelial cells (BAECs) with different densities were used as sensitive cells.

In the seminar, the fabrication process of the biosensor will be discussed. Impedimetric and gravimetric measurements conducted with this cell-based hybrid biosensor will be explained. Cell damage induced by toxic water will result in the decrease in impedance, as well as increase in the resonant frequency. The effects of the toxicants: ammonia, nicotine and aldicarb on cells were monitored with both sensors; the QCM and the ECIS technique. The lab on chip was demonstrated to indicate low concentration of toxicants. The responses of BAECs to toxic samples occurred during initial 5 to 20 minutes depending on the type of chemicals and concentrations. A highly linear correlation between signal shifts and chemical concentrations was demonstrated for each toxicant.

 * Wearable and Stretchable Piezoelectric Nanogenerator for Skin Applications

The presentation describes a stretchable piezoelectric nanogenerator (NG) that is intended to harvest energy. The nanogenerator was fabricated from zinc oxide (ZnO) piezoelectric thin film embedded in polymer materials. The microfabricated nanogenerator will be attached on the skin and stretched by the natural movements of arms, legs or neck. We expect that energy harvested by this device will be able to power wearable skin sensors. This is the first stretchable NG fabricated from ZnO uniform film published in the literature. This nanogenerator consists of ZnO films embedded in stretchable poly(dimethylsiloxane) (PDMS). One large gold (Au) electrode for energy collection was fabricated immediately underneath the ZnO film. Circular Au electrodes also for energy collection were fabricated on top of the thin ZnO film. In the prezentation this nanogenerator fabrication will be described and the testing method will be also discussed along with the output power from this device. This research was conducted entirely by French students during their internship at CCNY.

* Stretchable Lab-on-chip Device with Impedance Spectroscopy Capability for Mammalian Cell Studies

This presentation is about the fabrication and testing of electric cell substrate impedance spectroscopy (ECIS) electrodes on a stretchable membrane. The ECIS sensor on stretchable membrane can evaluate the influence of mechanical stimuli on endothelial cell function during cycling stretch. This can be used to simulate and replicate the dynamic environment of organism and enable the cell activity involved in cells attachment and proliferation in vitro.

This is the first time when ECIS electrodes were fabricated on a stretchable substrate and ECIS measurements on mammalian cells exposed to cyclic strain of 4% and 8% were successfully demonstrated.  Bovine aortic endothelial cells (BAEC) were used to evaluate the endothelial function influenced by mechanical stimuli in this research because they undergo in vivo cyclic physiologic elongation produced by the blood circulation in the arteries. Using the impedance measurements, we demonstrated that the cyclic stretch improved cell attachment and proliferation.

 

Ioana Voiculescu graduated with a Ph.D. in Mechanical Engineering from Politehnica University Timisoara, Romania and she obtained the second doctorate degree, Sc.D., in Mechanical Engineering from The George Washington University. She conducts research for chemical and biomedical sensors, actuators, and microsystems based on microelectromechanical system (MEMS), micro total analysis system (or “Lab-on-a-chip”) and semiconductor technologies. She received several research grants funded by the National Science Foundation (NSF) and U.S. Army Research Office. She also worked two years at U.S. Naval Research Laboratory (NRL) in Washington DC. At NRL she conducted research for several Complementary Metal Oxide Semiconductor (CMOS)-Microelectromechanical Systems (MEMS) devices used for detection of explosives and chemical agents such as: a resonant microcantilever beam sensor for the detection of chemical warfare agents, and a micromachined preconcentrator for enhancing trace detection of explosives. She has three US patents on her name and more than 30 peer reviewed journal papers. She also recently edited a book; “Nanocantilever beam, fabrication and applications”.  

Monitoring changes in granite rock under biaxial test: A 4-D imaging application with diffuse waves, par Xie FAN Institute of Geophysics China Earthquake Administration, invité-e par Aroune D.

2018
07
10

Diffuse acoustic or seismic waves are highly sensitive to detect changes of mechanical properties in heterogeneous geological materials. In particular, thanks to acoustoelasticity, we can quantify stress changes by tracking acoustic or seismic relative velocity changes in the material at test. In this presentation, we report our recent work on a small-scale laboratory application  to image spatiotemporal mechanical changes on a granite sample under biaxial loading, using diffuse waves at ultrasonic frequencies (300 kHz to 900 kHz). We demonstrate the ability of the method to image reversible stress evolution and deformation process, together with the development of reversible and irreversible localized microdamage in the specimen at an early stage. Using full-field infrared thermography, we visualize stress-induced temperature changes and validate stress images obtained from diffuse ultrasound. We demonstrate that the inversion with a good resolution can be achieved with only
a limited number of receivers distributed around a single source, all located at the free surface of the specimen. This small-scale experiment is a proof of concept for frictional earthquake-like failure
(e.g., stick-slip) research at laboratory scale as well as large-scale seismic applications, potentially including active fault monitoring.

reference:
Xie, F., Ren, Y., Zhou, Y., Larose, E. & Baillet, L. (2018). Monitoring local changes in granite rock under biaxial test: A spatiotemporal imaging application with diffuse waves. Journal of Geophysical Research:Solid Earth, 123, 2214–2227. https://doi.org/10.1002/2017JB014940

titre à venirséminariste à venir

2018
09
18

en blaum

(titre à venir), par Hadrien Beriot, Siemens

2018
12
31

(résumé à venir)

Picosecond ultrasonics and phonon spectroscopy, par Prof. Andrey Akimov from the School of Physics and Astronomy, The University of Nottingham, UK, invité par Vitaly G et Samuel R.

2018
10
16

Picosecond ultrasonics is an advanced technique which uses short laser pulses to generate and detect terahertz acoustic waves (i.e. coherent phonons). During last decade picosecond ultrasonics have been used for: nanoscopy with nanometer resolution; understanding phonon transport properties in complex nanostructures; and control of optical and electronic devices. Picosecond ultrasonics enables to obtain phonon spectra for compressive and shear acoustic waves in sub-THz and THz frequency ranges in various nanoobjects. In the first part of the talk the basic principles and main achievements of the picosecond ultrasonics and coherent phonon spectroscopy will be given. Recent experiments performed in the University of Nottingham on the studies of van der Waals nanolayers and development of new phonon spectroscopy methods will be described in the second part of the talk.

Modelling approaches for MEMS transducer with moving electrode in form of square clamped plate, par Petr Honzik, invité par Antonin N

2018
11
06

The growing use of the miniaturized electroacoustic transducers can be observed in recent years not only in the domain of consumers audio devices, but also in metrological applications. Since high quality (and low price in case of high number of sensor nodes) is needed, the precise modelling of such devices is of interest.

The geometrical simlicity of the square transducers is avantageous from the point of view of microfabrication (thus decreasing the price). The square moving electrodes of significantly reduced dimensions, unlike the circular membranes in classical microphones, present the behaviour similar to the one of clamped plates.

Recently proposed analytical model of transducers with membrane-type moving electrodes making use of integral formulation for the acoustic behaviour of the thin fluid film loading the moving electrode enables an easy change from the membrane-type to plate-type moving electrode by using appropriate eigenfunctions. The main difficulty is that the exact analytical expression for the eigenfunctions of the clamped plate does not exist.

Two approaches for the solution of the above mentioned problem will be presented:

- The analytical solution using “modal“ functions which do not represent the eigenfunctions of the clamped plate problem, but enables, along with another coupling term and integral formulation for the fluid, to form an exact solution for the displacement of the clamped plate loaded by thin fluid film in the form of series.

- The approximate solution using a simple numerical model of clamped plate (without the fluid load) along with the integral formulation for the fluid, which can, on the other hand, provide models of transducers with more complicated forms of the moving electrodes (nonuniform plate thickness, perforated plates).

Iterative Methods for Efficient Statistics in High Dimensions, par Manolis Zampetakis en phD au MIT, invité par Olivier R

2018
10
10

We provide and analyze iterative algorithms for the classical statistical problems, going back to Galton, Pearson, and Fisher, of estimating, with arbitrary accuracy the parameters of a multivariate normal distribution in the following cases:

(i) the samples are truncated (ii) the population is inhomogeneous.  (i) Truncated samples from a d-variate normal a samples is only revealed if it falls in some subset S of the d-dimensional Euclidean space otherwise the samples are hidden and their count in proportion to the revealed samples is also hidden. We show that the mean and covariance matrix can be estimated with arbitrary accuracy in polynomial-time, as
long as we have oracle access to S, and S has non-trivial measure under the unknown d-variate normal distribution.

 (ii) Inhomogeneous populations lead to the case of Gaussian Mixture Models. The Expectation-Maximization (EM) algorithm is a widely used method for estimating mixtures of Gaussians. Despite its wide use and applications, there are essentially no known convergence guarantees for this method. We provide global convergence guarantees for mixtures of two Gaussians with known covariance matrices.

 Finally we provide the first universal analysis tool for proving global convergence guarantees in iterative methods for non-convex optimization.

 joint works with: Constantinos Daskalakis, Themis Gouleakis and Christos Tzamos.
 

References
  C. Daskalakis, T. Gouleakis, C. Tzamos, and M. Zampetakis. Efficient
statistics, in high dimensions, from truncated samples. FOCS '18
  C. Daskalakis, C. Tzamos, and M. Zampetakis. Ten Steps of EM Suffice
for Mixtures of Two Gaussians. COLT' 17
  C. Daskalakis, C. Tzamos, and M. Zampetakis. A converse to Banach’s
fixed point theorem and its CLS-completeness. STOC '18

Speech synthesis with deep neural networks, par Marie Tahon invitée par Pierrick L

2018
11
13

This talk will introduce techniques of speech synthesis from input text or Text-To-Speech (TTS). After a brief introduction of different approaches for TTS, be they founded on physical models or on data processing, I will mainly focus on deep neural networks. Indeed neural networks bring a nice compromise between speech intelligibility and audio quality. Their inconvenient being the amount of data required for training the models. The various stages necessary in order to set up such systems are described, including data collection, pre-processing, model design and training. The links between data-based approaches and the source-filter acoustic model will also be discussed.

Sound field control - Some engineering applications, par Alain Berry, invité par Manuel M

2018
11
20

Concepts and technologies for controling sound fields over extended regions of space using loudspeaker arrays have been around for some time, especially for audio and music reproduction. In comparison, little has been done on this subject for engineering applications. The presentation will review the basics of the most employed spatial sound field reproduction techniques - Wave Field Synthesis, Ambisonics and inverse approaches. Applications of these techniques will be shown for spatial reproduction of sound fields in vehicles or in workplaces, acoustic absorption measurement, vibroacoustic testing of panels under synthetic sound fields and auralization of vibroacoustics models.

date disponible

2018
11
27

(séminaire en blaum)

Threshold and dynamics of acoustic vaporization of encapsulated microdroplets., par Côme OLIVIER, invité par lui-même ;)

2018
12
11

The vaporization of nanodroplets by an ultrasonic field for medical purposes, either to turn them into contrast agents or to liberate a drug load, is a developing field. The optimization of acoustic droplet vaporization (ADV) will be enhanced by understanding its underlying physical mechanisms, which are currently not totally elucidated. Lacour et al. [J. Acoust. Soc. Am. (2018)] modeled this phenomenon, focusing on the finite size of the droplet and its encapsulation by a thin hyperelastic layer. Numerical simulations reveal that the ADV threshold is mainly determined by droplet surface tension and shell rigidity. To check these theoretical results, experimental vaporization of single droplets is carried out in free space. The threshold pressures to induce ADV and Inertial Cavitation (IC) are simultaneously determined for micron-sized PFC droplets and the fast evolution of the bubbles is evaluated. Depending on the parameters of the fluid, the bubble, and the acoustic excitation, various behaviors are observed after nucleation. Distinguishing between them is crucial to maximize the load delivery by controlling and balancing incidental effects (enhanced extravasation, sonochemistry, etc.) Only by understanding the phenomena at stakes will one be able to accurately model and predict droplets and bubbles behavior.

Sparse directional interpolation of room impulse responses (en blaum), par Elias Zea Marcano de KTH, invité par Mathieu G

2018
12
13

The unprecedented success of compressive sensing (CS) has led to a reduction of samples and sensors in many research disciplines. A problem which requires astronomically many sensors is the measurement of room impulse responses (RIRs). This presentation overviews a novel CS-based method to interpolate RIRs recorded at sparse microphone positions, including a validation against measurements taken in a lecture hall at KTH. It is shown that 67% missing data can be accurately interpolated up to 4 kHz, given an alias-free bandwidth of 5.7 kHz.

Origami: A versatile design method for building mechanical metamaterials JK Yang ssociate Professor in Aeronautics and Astronautics University of Washington, Seattle WA, 98195, USA, , invité par Vincent T

2018
12
18

Mechanical metamaterials are emerging as an enabling technology for manipulating mechanical energy flow at will. The unique mechanical properties of these metamaterials are derived from structural architectures rather than their base materials. A natural question is how we design these architectures in a clever way. In this presentation, I will propose origami as a powerful tool for designing mechanical metamaterials. First, I will show how origami design principle can be exploited to tailor stiffness and stability of metamaterial cells. Then, I will use these cells to construct metamaterials that mitigate structural impact in an efficient way. Specifically, I will demonstrate a counter-intuitive mechanism of converting external compressive impact to tensile waves by using origami-based metamaterials. Next, I will show how origami can be combined with the concept of topology to form dial-in topological metamaterials. I will finish my talk with other potential engineering applications of origami, such as mechanical logic and hyperlens design.

 Biosketch

Jinkyu Yang is an Associate Professor in Aeronautics & Astronautics at the University of Washington (UW). Formerly, he was an Assistant Professor in Mechanical Engineering as well as an interim Program Director of Aerospace Program at the University of South Carolina (2011-2013). He held a postdoctoral researcher position at Caltech (2009-2011) and a senior engineer position at Samsung Electronics (2006-2009). He received his BS degree in Aerospace Engineering from KAIST in 2000 and Ph.D. degree in Aeronautics and Astronautics from Stanford University in 2005. His research has been directed towards developing novel engineered materials and structures, e.g., metamaterials, phononic crystals, and nonconventional composites, for aerospace and mechanical applications under the sponsorship of U.S. National Science Foundation (NSF), Department of Defense, Federal Aviation Agency, and Boeing. He has received the NSF CAREER, University of South Carolina’s Rising Star, and Samsung Think Tank Team Awards.