Active control in an anechoic room : Theory and first simulations

International audienceNoise control and source design require the measurement of sound radiation at low frequencies. Anechoic rooms, which are designed for this purpose, allow echo-free measurements at medium or high frequency but passive wall treatment is less effective at low frequency and in practice no facility provides anechoicity below 50Hz. This paper discusses the applicability of an active control algorithm which has been previously introduced to minimize the echoes from a scattering object to the cancellation of the low frequency wall echoes in an anechoic room including wall-embedded secondary sources. At first the paper discusses, in the general case then for a free half-space as a model case, the algorithm key which consists in estimating the scattered acoustic pressure from total pressure measurements. Boundary Element Method computations are secondly used to simulate estimation and active control of error signals accounting for the low-frequency scattered pressure in an anechoic room. The simulations show that control with a few dozen microphones and noise sources allows a large reduction of the noise scattered from the walls at low-frequency

A pH‐Triggered Polymer Degradation or Drug Delivery System by Light‐Mediated Cis / Trans Isomerization of o ‐Hydroxy Cinnamates

A new methodology for the pH-triggered degradation of polymers or for the release of drugs under visible light irradiation based on the cyclization of ortho-hydroxy-cinnamates (oHC) to coumarins is described. The key oHC structural motif can be readily incorporated into the rational design of novel photocleavable polymers via click chemistry. This main-chain moiety undergoes a fast photocleavage when irradiated with 455 nm light provided that a suitable base is added. A series of polyethylene glycol-alt-ortho-hydroxy cinnamate (polyethylene glycol (PEG)n-alt-oHC)-based polymers are synthesized and the time-dependent visible-light initiated cleavage of the photoactive monomer and polymer is investigated in solution by a variety of spectroscopic and chromatographic techniques. The photo-degradation behavior of the water-soluble poly(PEG2000-alt-oHC) is investigated within a broad pH range (pH = 2.1–11.8), demonstrating fast degradation at pH 11.8, while the stability of the polymer is greatly enhanced at pH 2.1. Moreover, the neat polymer shows long-term stability under daylight conditions, thus allowing its storage without special precautions. In addition, two water-soluble PEG-based drug-carrier molecules (mPEG2000-oHC-benzhydrol/phenol) are synthesized and used for drug delivery studies, monitoring the process by UV–vis spectroscopy in an ON/OFF intermittent manner

PROPAGATION D'ONDES VIBRATOIRES DANS UN SOL

L'objet de cet article est l'étude de la propagation d'ondes vibratoires dans un sol stratifié. Le milieu de propagation est tout d'abord modélisé par une couche de matériau viscoélastique, homogène, d'épaisseur constante. Le déplacement est calculé à l'aide de la théorie des distributions et d'une transformée de Fourier spatiale. On suppose ensuite que la couche contient un obstacle de dimensions non négligeables par rapport aux longueurs d'onde du signal d'excitation. L'exemple présenté ici est celui d'une tranchée. Le déplacement est calculé par une méthode d'équations intégrales (ou éléments finis de frontière). Dans les deux cas, les problèmes étudiés sont bidimensionnels et l'excitation est harmonique.The aim of this paper is the study of the propagation of vibrating waves in a stratified medium. First, the propagation medium is represented by a constant depth layer of viscoelastic material. The evaluation of the displacement is based upon the theory of distributions and a spatial Fourier transform. Then the displacement is computed for the same model of layer in presence of a trench, by using a boundary element method. In both cases, the problem considered is two-dimensional and the excitation is periodic

PROPAGATION D'ONDES DANS UN SOL STRATIFIÉ AVEC OBSTACLE

L'étude présentée ici a débuté dans le cadre d'un appel d'offre du Ministère chargé de l'Environnement /1/. Son but était l'évaluation de l'atténuation des vibrations due à la présence d'obstacles dans le sol. L'aspect que nous avons étudié plus particulièrement est la propagation dans une couche de sol, pour des problèmes bidimensionnels et harmoniques. Dans une première partie, le milieu de propagation est modélisé par une couche de matériau viscoélastique, homogène, d'épaisseur constante. Le déplacement est calculé à l'aide de la théorie des distributions et d'une transformation de Fourier spatiale. On suppose ensuite que la couche contient un obstacle de dimensions non négligeables par rapport aux longueurs d'onde du signal d'excitation. Le déplacement est calculé par une méthode d'équations intégrales (ou éléments finis de frontière). L'efficacité de l'obstacle est obtenue comme le rapport des amplitudes du déplacement en présence et en l'absence de l'obstacle.The aim of the study described here is the propagation of vibrating waves in a stratified medium. First, the propagation medium is represented by a constant depth layer of viscoelastic material. The evaluation of the displacement is based upon the theory of distributions and a spatial Fourier transform. Then the displacement is computed for the same model of layer in the presence of a trench, by using a boundary element method. In both cases, the problem considered is two-dimensional and the excitation is periodic. The application of this study is the evaluation of the effect of an obstacle on the propagation of the vibrations

An Ultrasonic Testbench for Emulating the Degradation of Sonar Performance in Fluctuating Media

International audienceA scaled experimental protocol, in a water tank, is proposed to mimic the effects of medium heterogeneities on underwater acoustic propagation. The procedure consists in transmitting an ultrasonic wave (f=2.25 MHz) through a RAndom Faced Acoustic Lens (RAFAL) in order to induce a spatially fluctuating sound pressure field. A scaling process based on a dimensional analysis ensures the representativeness of the experiment, when compared to the fluctuations of a few kHz continuous waves in the ocean. The regimes of saturation and unsaturation are explored by tuning the statistical parameters of the RAFAL's output face, the signal frequency and the propagation distance in the water tank. The control and reproducibility of the measurements is therefore ensured. The influence of volume effects is especially studied, since the experiments were conducted in free field conditions. Results in terms of mutual coherence function (second-order moment) are presented ; a good agreement is found between the values of radii of coherence calculated using experimental and simulation data, as well as theoretical results. The evaluation of the complex pressure distribution highlights the relevance of the experimental scheme, since behaviors typical of the regimes of fluctuations (saturation and unsaturation) are found

Auditory Evaluation of Sound Signals Radiated by a Vibrating Surface

International audienceThis paper presents a combination of vibroacoustic and psychoacoustic studies of sounds radiated by a vibrating structure. The calculated sound field is the sound pressure radiated by a baffled thin-plate structure immersed in a fluid, on the surface of which the acceleration is given. Various configurations are selected for the time and space functions of the acceleration variable, each configuration leading to a particular acoustic signal (a low-frequency tone complex in our case). These signals are then transformed into sound files, which are used as test signals in psychoacoustic experiments for assessing their perceptual attributes and quality. Two experiments were run. In the first one, the unpleasantness of a series of signals at different levels was measured by direct estimation and compared with their calculated loudness and sharpness using Zwicker's model. The same measurements were repeated with the signals set to the same maximum amplitude. In the second experiment, the pleasantness of another series of sounds at equal loudness was measured, as well as dissimilarity and preference on pairs of these sounds. An MDS analysis was run to extract auditory attributes that could account for the perceived differences between sounds and correlate with the estimated pleasantness. The results from the first experiment show that pleasantness is always highly (and negatively) correlated with loudness. The same holds for sharpness, when sounds are played at the same maximum amplitude. The second experiment shows that the perceptual attributes revealed by the MDS analysis are related to pitch and timbre, the latter being highly correlated with pleasantness. Overall, this study confirms the interest of extending vibroacoustic studies to a more complete "psychomechanical" investigation of the whole process of sound generation. It is suggested that such investigations may apply to product sound quality and to active or passive noise control, by providing psychoacoustic feedback to the design of the vibrating structures or of the noise-control systems

Moments des signaux sonores en milieu marin aléatoire modélisation des fluctuations par un bruit blanc Gaussien extension au cas du bruit blanc large bande

Le problème de l'évaluation de l'intensité et de la cohérence du signal sonore reçu par les systèmes de détection sous-marine est résolu en milieu déterministe. Dans cette étude on introduit une composante aléatoire dans la célérité du milieu modélisant les fluctuations aléatoires du milieu marin. Pour caractériser ces effets du milieu sur la propagation sonore, l'estimation des moments du champ sonore est nécessaire. On rappelle les équations de propagation des moments du champ sonore en milieu aléatoire en présence d'un bruit blanc gaussien (perturbations du milieu marin dues aux ondes internes): équations de propagation paraboliques des moments du champ sonore obtenues par Tatarskii. L'obtention de ces équations peut être généralisée dans le formalisme de Ito [1]. De plus alors, l'application du théorème limite de Papanicolaou-Kohler permet d'affaiblir les hypothèses sur l'indice de réfraction et d'estimer le domaine de validité des équations paraboliques des moments

Research of corrective techniques using a testbench emulating the degradation of performance in random oceans

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Active control of the field scattered by the rigid wall of a semi-anechoic room—Simulations and full-scale off-line experiment

International audienceThis paper is dedicated to an experimental implementation of active anechoicity control illustrating previous theoretical work. The aim of this experimental is to cancel the pressure scattered by the single rigid wall of a semi-anechoic room, turning the latter into a fully anechoic one over a given frequency range. The proposed method is presented in two steps: estimation of the scattered pressure and its control. Two numerical simulations are presented in order to show the feasibility of the method. An experimental campaign is presented, where the active control system leads to a reduction of the field scattered by the rigid wall throughout a significant part of the measuring volume inside the room. Although these results are based on post-processing of measurements, and not real-time computation, they confirm the possibility to extend the frequency range of a small facility toward lower frequencies, by adding to its passive lining a set of ordinary sources and pressure microphones