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International audience; This paper deals with the design of transaural systems in usual rooms, whose response has a strong influence on sound reproduction. The paper proposes to select configurations for the best perceptual rendering. However, realistic perceptive experiments cannot deal with the many possible room and loudspeaker configurations. Therefore, the authors propose to assess them using objective scores that are extrapolated from the results of perceptive tests assessing a suitable selection of rooms and loudspeaker configurations. This extrapolation then allows comparison of a much larger set of combinations, leading to the conclusion that close-to-ears configurations allow reduction of the room influence, leading to a good perceived fidelityeven inside usual rooms. Closer loudspeakers are, however, likely to be more sensitive to listener position, so the robustness of loudspeaker configurations to listener misplacement were investigated. A suitable objective score, again based on a perceptive test, led to the surprising conclusion that some close-to-ears configurations are also robust to listener position.

International audience; Head-Related Transfer Functions (HRTF) characterize the scattering of an incident wave by a listener's head. Measurements of these HRTF is a complex process: the required number of positions is generally high, and the measurement setup could induce a specific scattering (not intended). Especially in the proximity of the listener the source distance has an in influence, and the total number of measurement positions could be superior to a thousand. Measurements of dummy heads are easier than measurements of real listeners, because of the simplified control of the position. However, measurement of dummy heads is challenging, several measurements campaign were done in the literature but not directly comparable because of the variability of experimental setups. In this paper, we present the HRTF measurements of five dummy heads using the same experimental setup. In total, 794 source positions mainly in the half-sphere of positive elevations were measured. These dummy heads were all built to approximate an "average listener", and then should be similar. One of these dummy head was a rigid sphere allowing a comparison to the analytical simulation and deduce an estimation of measurement errors.

The goal of this experimental research is to determine the potential of pure acoustic detection for hostile fire indication for helicopters, while guarantying a very low true false alarm rate in the case of high helicopter speed, large helicopter bullet distance range and small calibres. An integration into a multi-sensor system, could provide a fully embedded, defence-feature technology capable of immediately alerting a helicopter crew that even a small projectile has closely passed by their machine, and provide an indication of the bullet trajectory. This article summarizes the design, the test in real-flight situation, and the performance results of an experimental acoustic array demonstrator developed for this purpose and evaluated for a referential of situations of 4 small arms firings with calibres from 5.56 mm to 12.7 mm and a total of 592 firings  52 flight exposure situations (altitude, hover, speed, orientation). The detection system is an experimental planar, centimetric, acoustic array recording unit designed for low sensitivity to laminar and turbulent air flow, rear pressure and vibrations and high acoustic selectivity. This is combined with a transient signal post-processing solver that is implemented as a deterministic cost minimization function. The conclusion of the research is a strong confirmation of the feasibility of an under-helicopter acoustic detection of small calibres hostile firings as well as the proposal of a general performance law for the capability of such a system to detect small calibres, including passing-by distances of up to several hundreds of m and speeds of up to 120 kn.

Proceedings of the ICA 2019 and EAA Euroregio : 23rd International Congress on Acoustics, integrating 4th EAA Euroregio 2019 : 9-13 September 2019, Aachen, Germany / proceedings editor: Martin Ochmann, Michael Vorländer, Janina Fels 23rd International Congress on Acoustics, integrating 4th EAA Euroregio 2019, ICA 2019, Aachen, Germany, 9 Sep 2019 - 13 Sep 2019; Aachen (2019)., Published by Aachen

International audience; An acoustic velocity sensor is described. It uses a small volume of ionized gas as sensing element to estimate the velocity of air particles, especially their oscillations caused by an acoustic perturbation. The generation of charged particles is performed by a negative point-to-plane discharge, the plane anode being split into two electrically insulated parts. The principle of this velocity sensor is based on the measurement of the radial deflection of the charged particles which flow between the point and the two half-planes, this deflection being caused by the motion of the air particles resulting from the acoustic field. An electroacoustic model of this velocity sensor is proposed, based on the fluctuations of the current density distribution over the plane electrodes. An experimental setup is developed using a waveguide. It allows to compare simultaneously the acoustic velocity deduced from the corona discharge sensor on one side, and that resulting from the two-microphone meth od on the other side. This paper also proposes a parametric study to quantify the influence of the electrical and geometrical parameters of the discharge on this acoustic sensor.

International audience; De nombreux travaux ont mis en évidence l’intérêt des absorbeurs non linéaires pour la réduction des vibrations. En particulier, il a été montré qu’une membrane en grande déformation s’apparente à un système à raideur cubique à 1 degré de liberté qui peut être couplé à un système acoustique linéaire primaire et donner lieu à un transfert d’énergie irréversible (Targeted Energy Transfer) appelé pompage acoustique.Ce dispositif présente un potentiel d’application à basse fréquence mais l’intérêt pratique pour la réduction du bruit dans une pièce résonante était jusqu’à présent limité. En effet, dans cette configuration la face arrière de la membrane, placée en paroi, réémet un bruit indésirable vers l’extérieur.Nous présentons un système basé sur les mêmes principes mais qui s’affranchit de ces difficultés pratiques. Il s’agit d’un absorbeur non linéaire de type membrane, à raideur cubique, doublement couplé au système primaire. Pour une excitation harmonique, l’idée consiste à appliquer des pressions déphasées de chaque côté de la membrane. Ainsi la membrane est isolée de l’extérieur et la somme des pressions appliquées sur les deux faces de la membrane peut être différente de la pression dans la pièce résonante, avec un impact sur le seuil de déclenchement.L’analyse du système couplé se fait avec les outils classiques de la dynamique non-linéaire (équilibrage harmonique). Nous présentons un modèle et des résultats expérimentaux montrant une atténuation significative pour la première résonance d’une salle, dans une configuration représentative d’une application industrielle.

A nonlinear vibroacoustic absorber (Nonlinear Energy Sink: NES), involving a clamped thin membrane made in Latex, is assessed in the acoustic domain. This NES is here considered as an one-port acoustic system, analyzed at low frequencies and for increasing excitation levels. This dynamic and frequency range requires a suitable experimental technique, which is presented first. It involves a specific impedance tube able to deal with samples of sufficient size, and reaching high sound levels with a guaranteed linear response thank's to a specific acoustic source. The identification method presented here requires a single pressure measurement, and is calibrated from a set of known acoustic loads. The NES reflection coefficient is then estimated at increasing source levels, showing its strong level dependency. This is presented as a mean to understand energy dissipation. The results of the experimental tests are first compared to a nonlinear viscoelastic model of the membrane absorber. In a second step, a family of one degree of freedom models, treated as equivalent Helmholtz resonators is identified from the measurements, allowing a parametric description of the NES behavior over a wide range of levels.

National audience; Il existe de nombreuses techniques d'identification du champ rayonné par une source de bruit, destinées à en localiser le mécanisme ou à en déterminer le niveau et la répartition spatiale. L'approche présentée ici met l'accent sur la réduction du nombre de points de mesures et la possibilité de les effectuer dans un environnement acoustique ordinaire. Elle permet alors de reconstruire en 3D la répartition du champ qui aurait été mesuré en environnement anéchoïque ou semi-anéchoïque, donc d'en déduire la puissance et la directivité, d'effectuer des post-traitements plus spécifiques, ou d'inclure la source mesurée dans des simulations. Le principe mis en oeuvre est d'identifier la décomposition du rayonnement sur une série de termes, comme pour l’holographie de champ proche (NAH) oo les méthodes de séparation de champ (FSM), mais ici la série est celle des modes de rayonnement calculés d'après la forme générale de l'objet mesuré - ce qui réduit considérablement le nombre de termes à identifier.

International audience; Noise 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.

International audience; In order to examine the perceptual dimensions used by listeners to differentiate sounds, dissimilarity judgments are analyzed with a multidimensional scaling technique. The paired comparison method is often recommended as the standard method for collecting judgments of dissimilarity between audio stimuli. An alternative method is presented here. On each trial, listeners are asked to select which stimulus among three stimuli is the most similar to a reference stimulus. The method was tested wuth simulations and an actual listening test. Across trials, every stimulus was used in turn as the reference stimulus. The analysis of simulated data indicated that, the number of stimuli did not influence the estimation of dissimilarity, and that 20 simulated listeners were sufficient for recovering continuous dissimilarity values. The listening test conducted to evaluate the musical restitution by 12 loudspeakers led to two perceptual dimensions similar to those obtained in a paired comparison experiment.

International audience

National audience; Cette étude expérimentale est réalisée dans un tube de Kundt court où des mesures de pression sont effectuées au moyen de quatre microphones. La plage fréquentielle va de 10 à 200 Hz pour de forts niveaux allant jusqu’à 300 Pa. Une méthode qui permet de déterminer l’impédance et le coefficient de réflexion d’une terminaison inconnue est présentée. Cette méthode nécessite la connaissance de quatre terminaisons inconnues et d’au moins une mesure de pression. Une validation des impédances estimées est effectuée avec plusieurs microphones puis une terminaison inconnue. Le coefficient de réflexion d’une membrane visco-élastique non linéaire est déterminé. Le comportement d’un absorbeur non linéaire est alors mis en évidence.

National audience; La localisation et la reconstruction de sources sonores (problème acoustique inverse) est une problématique commune à de nombreuses applications industrielles. Dans le cadre de mesures in situ, il est préférable d’utiliser des méthodes légères, adaptées à des formes de sources complexes, utilisant un nombre de capteurs aussi faible que possible et à des positions arbitraires proches de l’objet à mesurer. Une etude récente simulant un tel problème inverse (la reconstruction de la pression rayonnée par un transformateur en champ lointain à partir de mesures en champ proche) a montré que l’utilisation d’une décomposition du champ acoustique en “modes de rayonnement” peut être une méthode prometteuse. Cette approche consiste à mesurer la pression à des positions arbitraires et à utiliser ces mesures pour estimer les coefficient d’une série de solutions indépendantes (“modes de rayonnement”) qui décrivent le rayonnement en champ lointain d’une déformée quelconque avec une précision connue, en utilisant un nombre de termes minimal.Cette méthode est utilisée ici pour identifier le rayonnement d’un haut-parleur monté sur une enceinte close à partir de mesures de pression acoustique effectuées en chambre anéchoïque en un nombre restreint de points. Le calcul du champ rayonné à partir de la décomposition identifiée est ensuite comparé aux mesures, aux points ayant servi à l’identification et en d’autres points. Les résultats obtenus par la méthode des “modes de rayonnement” sont également confrontés à ceux obtenus par une méthode d’imagerie ayant fait ses preuves : l’holographie de champ proche (NAH).

National audience; Depuis plusieurs années se développe un intérêt croissant pour la caractérisation des sources basse fréquence qui sont de plus en plus répandues notamment dans les zones urbaines. L'analyse du rayonnement de ces sources doit souvent être réalisée dans une salle anéchoïque afin de les isoler de leur environnement. Or, toutes ces salles possèdent une fréquence de coupure, souvent entre 80 et 100 Hz, en dessous de laquelle les mesures ne sont plus anéchoïques. Une autre difficulté des mesures aux basses fréquences est due aux dimensions des salles anéchoïques qui imposent de travailler en champ proche. L'approche présentée consiste à tirer parti de la complémentarité entre les matériaux absorbants et les techniques de contrôle actif de bruit, qui ont de tr es bonnes performances dans les basses fréquences. Ceci permet de concevoir des solutions hybrides combinant une couche absorbante et un ensemble de microphones-sources-contrôleurs. Cette technique est centrée sur le critère d'anéchoïcité, et non d'absorption, afin de respecter la forme spatiale des fronts d'ondes. Elle consiste à contrôler la pression diffractée par les parois de la salle. Nous présentons des résultats de simulation de contrôle actif du champ diffracté et des résultats expérimentaux obtenus pour diverses configurations de salle anéchoïque (échelle réduite et échelle 1). L'objectif de cette présentation est de comparer les performances d'une mesure normalisée entre une salle anéchoïque classique et une salle équipée d'un système de contrôle actif.

The performance of cochlear implant (CI) listeners is limited by several factors among which the lack of spatial selectivity of the electrical stimulation. Recently, many studies have explored the use of multipolar strategies where several electrodes are stimulated simultaneously to focus the electrical field in a restricted region of the cochlea. Objective These strategies are based on several assumptions concerning the electrical properties of the inner ear that need validation. The first, often implicit, assumption is that the medium is purely resistive and that the current waveforms produced by several electrodes sum linearly. The second assumption relates to the estimation of the contribution of each electrode to the overall electrical field. These individual contributions are usually obtained by stimulating each electrode and measuring the resulting voltage with the other inactive electrodes (i.e. the impedance matrix). However, measuring the voltage on active electrodes (i.e. the diagonal of the matrix) is not straightforward because of the polarization of the electrode-fluid interface. In existing multipolar strategies, the diagonal terms of the matrix are therefore inferred using linear extrapolation from measurements made at neighboring electrodes. Approach In experiment 1, several impedance measurements were carried out in vitro and in eight CI users using sinusoidal and pulsatile waveforms to test the resistivity and linearity hypotheses. In experiment 2, we used an equivalent electrical model including a constant phase element in order to isolate the polarization component of the contact impedance. Main results In experiment 1, high-resolution voltage recordings (1.1 MHz sampling) showed the resistivity assumption to be valid at 46.4 kHz, the highest frequency tested. However, these measures also revealed the presence of parasitic capacitive effects at high frequency that could be deleterious to multipolar strategies. Experiment 2 showed that the electrical model provides a better account of the high-resolution impedance measurements than previous approaches in the CI field that used resistor-capacitance circuit models. Significance These results validate the main hypotheses underlying the use of multipolar stimulation but also suggest possible modifications to their implementation, including the use of an impedance model and the modification of the electrical pulse waveform.

Les risques de desintegration de l’Union vont s’accentuer si nous ne nous montrons pas capables de redonner sens a notre projet d’alliance des Europeens en regenerant l’idee et la volonte de bâtir une communaute sociale et politique. La redefinition des biens communs que nous voulons partager fera appel a un nouvel humanisme dans une Europe ouverte au monde. Ses alliances ont repose dans le passe sur une vision du monde euro-centree et dominatrice. Elles devront desormais reposer sur une acceptation de la pluralite du monde et incarner des cooperations et des solidarites nouvelles. C’est une revolution copernicienne. Pour se montrer capable de renouveler l’idee kantienne de paix perpetuelle, l’Europe doit faire l’epreuve du regard des autres et construire avec eux des liens d’egal a egal. Pour aller vers un ordre mondial pacifique et solidaire, il faut avancer vers une alliance des peuples beaucoup plus vaste que celle des Europeens, qui n’aura ni centre ni peripheries.

This paper evaluates the potential of the field separation method (FSM) for performing subwoofer measurements in a small test room with poor absorbing properties, as is commonly available. The FSM requires the knowledge of both acoustic pressure and velocity fields on a closed surface surrounding the tested source. Pressures and velocities, measured using a p-p probe on a half-sphere mesh, are collected under various conditions: in a room with variable reverberation time (6.4-0.6 s) and with four measurement half-sphere radii. The measured data are expanded on spherical harmonics, separating outward and inward propagation. The pressure field reflected by walls of the surrounding room is then subtracted from the measured field to estimate the pressure field that would have been radiated under free-field conditions. Theoretical frequency response of the subwoofer is computed using an analytical formulation derived from an extended Thiele and Small model of the membrane motion, coupled to a boundary element model for computing the radiated pressure while taking into account the actual subwoofer geometry. Measurement and simulation results show a good agreement. The effects of the measurement distance, the measurement point number, and the room reverberation time on the separation process are then discussed.

This paper presents a convenient and cost-effective experimental tool for measuring the mobility characteristics of a mechanical structure. The objective is to demonstrate that the point mobility measurement can be performed using only an electrodynamic inertial exciter. Unlike previous work based on voice coil actuators, no load cell or accelerometer is needed. Instead, it is theoretically shown that the mobility characteristics of the structure can be estimated from variations in the electrical input impedance of the actuator fixed onto it, provided that the electromechanical parameters of the actuator are known. The proof of concept is made experimentally using a cheap commercially available actuator on a simply supported plate, leading to a good dynamic range from 100 Hz to 1 kHz. The methodology to assess the basic parameters of the actuator is also given. Measured data are compared to a standard shaker testing and the strengths and weaknesses of the sensorless mobility measuring device are discussed. It is believed that this sensorless mobility measuring device can be a convenient experimental tool to determine the dynamic characteristics of a wide range of mechanical structures.

A method is described for monitoring a single delamination in a laminated composite plate. The dynamic response of the structure is obtained via the electromechanical impedance of a piezoelectric transducer cemented onto the structure. We focus here on the coupling with bending modes using thin devices which can be embedded in the material during the manufacturing process. A model including the delamination as well as the transducer is presented. The geometrical parameters of the delaminated zone are identified by comparing the modal parameters calculated from impedance measurements with those obtained using our model. Upon applying this method of identification to a test laminated beam, the length and position of several delaminations were estimated to within 15%.

International audience

International audience

The efficiency of passive methods for noise reduction decreases at low frequency. In this frequency range active structural acoustic control (ASAC) can reduce sound radiation of structures by controlling their vibration usually using piezoelectric or electrodynamic transducers. Besides its global effect on the structure, a bending nearfield is observed, depending on the actuator type. In some ASAC strategies as in the virtual impedance approach, actuators and sensors are collocated. Although it does not affect the radiation of the structure, the local bending strongly modifies the transfer function between transducers. A method is proposed in order to identify and compensate it. The global part of the vibratory field is extracted by cancelling the contribution of higher order modes approximated as a stiffness. After compensation, the signal from the sensor is much more representative of the radiation and nearly independent of the nature and the size of the actuator.

International audience

National audience

Although laser Doppler velocimetry (LDV) has been used for measurements in fluid mechanics and even high-level acoustics for a number of years, much uncertainty remains concerning its ability to measure the velocities encountered in the field of more usual linear acoustics. Specific aspects of the acoustic velocity have to be taken into account in order to achieve measurements, namely to deal with low levels or high frequencies, which requires some optimization of both the optics, the mechanical supports, and the signal processing. This paper presents a LDV set-up dedicated to acoustics in air, and presents the performances obtained using commercially available hardware. It also illustrates the main limitations, especially concerning the signal processing, and presents a new approach for the signal processing, optimized for a sine wave forced excitation of the acoustic field.

This paper proposes a new method to locate high speed moving sound sources based on a bilinear time-frequency transformation. A microphone array, used as a directional sensor, is optimised in order to focus on the linear part of the frequency modulation due to the Doppler effect. Simulations are carried out to quantify the accuracy of the localizations. A comparison is made with the currently used dedopplerization method. Finally, both methods have been tested on acoustic sources of a high speed train (TGV) in normal operating conditions.

This is a wake-up call. It points to the most imminent critical issues for the European Union’s peaceful survival and suggest a new vision. It is addressed to the leaders of the European Union.The authors distance themselves from the host of tenuous condemnations of the European project.The publication is based on a recent The Global Round Table discussion and takes the outcome of the 2014 European Parliament election into account.The authors and supporters of the wake-up call include experienced national leaders at the highest level, civil society leaders and academicians.

National audience

International audience; Acoustic communication is widespread in animals. According to the sensory drive hypothesis [Endler JA (1993) Philos Trans R Soc Lond B Biol Sci 340(1292):215-225], communication signals and perceptual systems have coevolved. A clear illustration of this is the evolution of the tetrapod middle ear, adapted to life on land. Here we report the discovery of a bone conduction-mediated stimulation of the ear by wave propagation in Sechellophryne gardineri, one of the world's smallest terrestrial tetrapods, which lacks a middle ear yet produces acoustic signals. Based on X-ray synchrotron holotomography, we measured the biomechanical properties of the otic tissues and modeled the acoustic propagation. Our models show how bone conduction enhanced by the resonating role of the mouth allows these seemingly deaf frogs to communicate effectively without a middle ear.

International audience; The acoustic pressure sensor described in this paper uses a small volume of ionized gas (plasma) as sensing element to receive energy from surrounding gas (air) set into oscillations by an acoustic disturbance. The generation of the ionized gas is performed by negative point-to-plane corona discharges. The passage of a pressure disturbance through this gas disturbs the flow of the charged particles between the electrodes, and provokes a current variation of the electrical system. This current variation is directly related to the acoustic pressure. An electroacoustic model of this plasma microphone is proposed. From the current-voltage relation associated with corona discharges, this model is based on the variations of the threshold voltage and the mobility of ions with pressure and temperature of the surrounding gas. An experimental setup is developed, it simultaneously allows one to compare the acoustic pressure deduced from the corona discharge sensor with that resulting from the two-microphone method in a standing wave tube. This paper also proposes a parametric study to quantify the influence of the electrical and geometrical parameters of the discharge on the sensitivity of the plasma microphone.

International audience

Hybrid active/passive absorbers have proven to be efficient over a large frequency range. The next step consists in building up a system which can exhibit good absorption and insulation properties. To simulate such hybrid cell, active and passive behaviors of an electroacoustic loudspeaker have been modelized by using a one-dimensional approach. The rear acoustic load at the back of the membrane has been taken into account to obtain a reliable model. The proposed model has been validated with measurements performed in a 7 cm diameter tube. Then, a hybrid cell composed of a porous plate and a small thickness loudspeaker has been designed and numerically tested. It is shown that, when driving the loudspeaker for total absorption, the transmission losses are suppressed at lower frequencies. To overcome this problem, a dual actuator cell is designed to deal with both absorption and transmission. Simulations shows that this solution can lead to good results. It is also shown that interaction of the loudspeakers can be significantly reduced by using directive sources, thus lowering supplying voltages and condition number of the matrix inversion required by the control process.

An experiment has been set up to study multimodal acoustic propagation inside a cylindrical duct in presence of a turbulent mean flow. This paper describes the preliminary work which has been found necessary for assembling the experiment together with first measurement results. In order to set up this experimental facility, a high level acoustic source was developed to generate higher propagating modes in the presence of mean flow. A microphonic antenna was designed for detecting the propagating modes. LDV measurements were performed and synchronous detection was used to extract both the mean flow and the acoustic components of the particle velocity. Results of aeraulic measurements are presented. Then, results of acoustic velocity measurements are compared to results obtained from the microphonic antenna.

This work deals with the energy pumping phenomenon for acoustical applications. The concept of energy pumping is to passively reduce the vibrations of a primary system by attaching to it an essentially nonlinear damped oscillator also named Nonlinear Energy Sink (NES) creating a strongly nonlinear coupling which localizes and dissipates the vibrational energy. In the context of acoustics, a vibroacoustic coupling is used. In an earlier work, we showed experimentally that a loudspeaker used as a Suspended Piston (SP) working outside its range of linearity can be used as a NES. In this work, the performance and efficiency of a SP NES is studied numerically and experimentally. The considered acoustic medium is a resonant pipe. The coupling between the pipe and the NES is ensured acoustically by a small acoustic compliance (the air in a coupling box). Various observed aspects of energy pumping are presented: behavior under sinusoidal forcing, pumping threshold, resonance capture and transient response. As a SP NES technology permits an easy control of the moving mass of the NES, the effect of this parameter is also studied.

International audience; This paper presents experimental results showing that a loudspeaker used as a suspended piston working outside its range of linearity can also be used as a nonlinear acoustic absorber. The main advantage of this technology of absorber is the perspective to adjust independently the device parameters (mass, nonlinear stiffness and damping) according to the operational conditions. To achieve this purpose, quasi-static and dynamic tests have been performed on three types of commercial devices (one with structural modifications), in order to define the constructive characteristics that it should present. An experimental setup has been developed using a one-dimensional acoustic linear system coupled through a box (acting as a weak spring) to a loudspeaker used as a suspended piston acting as an essentially nonlinear oscillator. The tests carried out on the whole vibro-acoustic system have showed the occurrence of the acoustic TET from the acoustic media to the suspended piston and demonstrated the efficiency of this new kind of absorber at low frequencies over a wide frequency range. Moreover, the experimental analyses conducted with different NES masses have confirmed that it is possible to optimize the noise absorption with respect to the excitation level of the acoustic resonator.

This article presents the experimental implementation and results of a hybrid passive/active absorber (smart foam) made up from the combination of a passive absorbent (foam) and a curved polyvinylidene fluoride (PVDF) film actuator bonded to the rear surface of the foam. Various smart foam prototypes were built and tested in active absorption experiments conducted in an impedance tube under plane wave propagation condition at frequencies between 100 and 1500 Hz. Three control cases were tested. The first case used a fixed controller derived in the frequency domain from estimations of the primary disturbance at a directive microphone position in the tube and the transfer function between the control PVDF and the directive microphone. The two other cases used an adaptive time-domain feedforward controller to absorb either a single-frequency incident wave or a broadband incident wave. The non-linearity of the smart foams and the causality constraint were identified to be important factors influencing active control performance. The effectiveness of the various smart foam prototypes is discussed in terms of the active and passive absorption coefficients as well as the control voltage of the PVDF actuator normalized by the incident sound pressure.

Recent developments in the theory for acoustical properties of porous materials with rigid frames introduced two characteristic dimensions related to the high-frequency behaviour of the viscous forces and the thermal exchanges in addition to the classical parameters of porosity, flow resistivity and tortuosity. These characteristic dimensions are used with recent expressions of the effective mass and the compressibility of the air in porous materials to predict the surface impedance in the high-frequency range. Measurements of the surface impedance performed in a free field are used to evaluate the accuracy of the model. The same model is generalised in the context of the Biot theory to the case of elastic framed materials. The effect of the frame wave on the surface impedance of a layer of fibrous material, and on the transmission through a fibrous material bonded onto a plate is studied with this new model.

National audience; Dans le cadre d'un projet de contrôle actif de la transmission sonore au travers d'une cloison, notre travail vise à identifier les principales difficultés liées à l'utilisation d'un nombre nécessairement limité de microphones de contrôle et de sources acoustiques, ainsi qu'à leur grande proximité avec la cloison. Une première partie présente un modèle permettant de simuler l'influence d'un petit nombre de sources ponctuelles sur la transmission au travers d'une plaque. Ce modèle met en évidence la grande sensibilité du dispositif à sa géométrie, ce qui conduit en général à un contrôle très localisé du champ incident, et donc à des réductions minimes de la transmission sur une large gamme de fréquence. Les principales causes des limites mises en évidence par le modèle sont alors analysées, en particulier à la lumière de mesures effectuées sur une maquette. Ainsi, des cartes du champ de pression mesuré à la surface de la maquette montrent que le nombre et la position des points de contrôle sont des paramètres essentiels lorsque l'on désire obtenir une zone de contrôle la plus étendue possible. Ces résultats permettent d'identifier quelques pistes pour optimiser les performances d'un dispositif réaliste, et notamment de déterminer des caractéristiques que devraient présenter les transducteurs à utiliser dans un tel dispositif.

This paper deals with the application of the concept of targeted energy transfer to the field of acoustics, providing a new approach to passive sound control in the low frequency domain, where no efficient dissipative mechanism exists. The targeted energy transfer, also called energy pumping, is a phenomenon that we observe by combining a pure nonlinear oscillator with a linear primary system. It corresponds to an almost irreversible transfer of vibration energy from the linear system to the auxiliary nonlinear one, where the energy is finally dissipated. In this study, an experimental set-up has been developed using the air inside a tube as the acoustic linear system, a thin circular visco-elastic membrane as an essentially cubic oscillator and the air inside a box as a weak coupling between those two elements. In this paper, which mainly deals with experimental results, it is shown that several regimes exist under sinusoidal forcing, corresponding to the different nonlinear normal modes of the system. One of these regimes is the quasi-periodic energy pumping regime. The targeted energy transfer phenomenon is also visible on the free oscillations of the system. Indeed, above an initial excitation threshold, the sound extinction in the tube follows a quasi-linear decrease that is much faster than the usual exponential one. During this linear decrease, the energy of the acoustic medium is irreversibly transferred to the membrane and then damped into this element called nonlinear energy sink. We present also the frequency responses of the system which shows a clipping of the original resonance peak of the acoustic medium and we finally demonstrate the ability of the nonlinear absorber to operate in a large frequency band, tuning itself to any linear system.

This work concerns the theoretical analysis and synthesis of sound fields by a compact spherical loudspeaker array. Such an electroacoustic device consists of several transducers mounted on a sphere-like structure, which are driven independently in order to achieve non-uniform directivity patterns. The control strategy usually adopted is to provide the array with some preprogrammed basic directivities corresponding to spherical harmonic functions. Thus, an arbitrary radiation pattern can be approximately achieved by changing the gains associated with these basic directivities. Here, a different approach based on the acoustic radiation modes of the array is proposed. Unlike spherical harmonics, radiation modes constitute a finite set of vectors that spans a subspace on which any radiation pattern the array is able to reproduce can be projected. Furthermore, radiation modes radiate sound energy independently. Since the eigenvalue analysis that must be carried out in order to obtain the modes leads also to their radiation efficiencies, the low frequency constraint in the directivity synthesis by a spherical array is naturally evaluated. Finally, it is useless to drive inefficient radiation modes. Therefore, the radiation mode approach leads to a reduced number of active channels, and to minimum source voltages for a given target directivity pattern.

Sound directivity control is made possible by a compact array of independent loudspeakers operating at the same frequency range. The drivers are usually distributed over a sphere-like frame according to a Platonic solid geometry to obtain a highly symmetrical configuration. The radiation pattern of spherical loudspeaker arrays has been predicted from the surface velocity pattern by approximating the drivers membranes as rigid vibrating spherical caps, although a rigorous assessment of this model has not been provided so far. Many aspects concerning compact array electromechanics remain unclear, such as the effects on the acoustical performance of the drivers interaction inside the array cavity, or the fact that voltages rather than velocities are controlled in practice. This work presents a detailed investigation of the electromechanical behavior of spherical loudspeaker arrays. Simulation results are shown to agree with laser vibrometer measurements and experimental sound power data obtained for a 12-driver spherical array prototype at low frequencies, whereas the non-rigid body motion and the first cavity eigenfrequency yield a discrepancy between theoretical and experimental results at high frequencies. Finally, although the internal acoustic coupling affects the drivers vibration in the low-frequency range, it does not play an important role on the radiated sound power.

This study investigated the dimensions underlying perceived differences between loudspeakers. Listeners compared the sound reproduction of 12 loudspeakers in a room, using three musical excerpts. For the loudspeakers to be compared one just after the other in exactly the same conditions, the sounds radiated by the loudspeakers were recorded in a listening room, and the recorded sounds were submitted to paired comparisons using headphones. The resulting perceptual dissimilarities were analyzed by using a multidimensional scaling technique, revealing two main perceptual dimensions used by listeners to discriminate the loudspeakers. These dimensions were identical for the three musical excerpts. As the signals heard by listeners were directly accessible, they were used to define acoustical attributes describing the perceptual dimensions. Instead of arbitrarily choosing one acoustical analysis to define these attributes, several analyses were compared. The temporal, spectral, and time-frequency domains were investigated, and different auditory models were tested. These auditory models allowed the best description of the differences perceived by listeners, and were used to define two acoustical attributes describing our perceptual dimensions: the bass/treble balance and the medium emergence.

Energy pumping corresponds to a particular vibratory regime of a non-linear system coupled to a linear primary structure. Its non-linear behaviour allows to reduce vibrations of the primary structure, and is thus a new passive vibration control technique. This phenomenon has mainly been studied in mechanical engineering, so we transposed its principle to noise control in an acoustic medium. The presentation will focus on results about acoustic energy pumping that we observed both experimentally and numerically, in the time and frequency domains. These results highlight two main points : physically, energy pumping corresponds to an irreversible transfer of energy from the primary system to the non-linear absorber, and practically, its effects are a noise level limitation in the acoustic medium in permanent regime and a much faster sound extinction in transient regime. [NO PDF]