Laboratoire d'Acoustique de l'Université du Mans

The Acoustic Black Hole (ABH) is a technique for passive vibration control that was recently developed within the Structural Dynamics and Vibroacoustics communities. From a general perspective, the ABH effect is achieved by embedding a local inhomogeneity in a thin-walled structure, typically a beam or a plate. This inhomogeneity is characterized by a variation of the geometric properties (although material variations are also possible) according to a spatial power law profile. The combination of a local stiffness reduction, due to the power law variation of the wall thickness, and of a local increase in damping, provided by the concurrent application of viscoelastic layers, gives rise to a significant reduction of the wave speed and to a remarkable enhancement of the attenuation properties. As an elastic wave travels within an ABH, its speed experiences a smooth and continuous decrease. In the ideal case, that is when the wall thickness vanishes at the ABH center, the wave speed decreases to zero. In the non-ideal case, that is when the ABH has a non-zero residual thickness at its center, the wave speed still decreases smoothly but it never vanishes. In this latter case, which is of great importance for practical applications, the ABH is typically combined with lossy media (e.g. viscoelastic layers) in order to achieve significantly enhanced structural loss factors. If the speed of an incoming wave can vanish inside the ABH, it follows that this object behaves as a wave trap that extracts elastic energy from the host medium without, in principle, ever releasing it. Several characteristic properties are generally observed in structures with embedded ABHs: significant reduction in vibration and acoustic radiation levels, low reflection coefficient at the ABH location, localized vibration and trapped modes, and existence of cut-on frequencies. Contrarily to passive vibration methods based on viscoelastic materials, the ABH was developed and applied to reduce vibrations and structure-radiated noise without increasing the total mass of the system. More recently, applications to other areas including elastic metastructures, energy harvesting, vibro-impact systems, and cochlear systems were also investigated. This review is intended to provide a comprehensive summary of the state-of-the-art of ABH technology, spanning from theoretical and numerical contributions to practical applications.

Perfect absorption is an interdisciplinary topic with a large number of applications, the challenge of which consists of broadening its inherently narrow frequency-band performance. We experimentally and analytically report perfect and broadband absorption for audible sound, by the mechanism of critical coupling, with a sub-wavelength multi-resonant scatterer (SMRS) made of a plate-resonator/closed waveguide structure. In order to introduce the role of the key parameters, we first present the case of a single resonant scatterer (SRS) made of a Helmholtz resonator/closed waveguide structure. In both cases the controlled balance between the energy leakage of the several resonances and the inherent losses of the system leads to perfect absorption peaks. In the case of the SMRS we show that systems with large inherent losses can be critically coupled using resonances with large leakage. In particular, we show that in the SMRS system, with a thickness of λ/12 and diameter of λ/7, several perfect absorption peaks overlap to produce absorption bigger than 93% for frequencies that extend over a factor of 2 in audible frequencies. The reported concepts and methodology provide guidelines for the design of broadband perfect absorbers which could contribute to solve the major issue of noise reduction.

Transition fronts, moving through solids and fluids in the form of propagating domain or phase boundaries, have recently been mimicked at the structural level in bistable architectures. What has been limited to simple one-dimensional (1D) examples is here cast into a blueprint for higher dimensions, demonstrated through 2D experiments and described by a continuum mechanical model that draws inspiration from phase transition theory in crystalline solids. Unlike materials, the presented structural analogs admit precise control of the transition wave's direction, shape, and velocity through spatially tailoring the underlying periodic network architecture (locally varying the shape or stiffness of the fundamental building blocks, and exploiting interactions of transition fronts with lattice defects such as point defects and free surfaces). The outcome is a predictable and programmable strongly nonlinear metamaterial motion with potential for, for example, propulsion in soft robotics, morphing surfaces, reconfigurable devices, mechanical logic, and controlled energy absorption.

We theoretically and experimentally report subwavelength resonant panels for low-frequency quasiperfect sound absorption including transmission by using the accumulation of cavity resonances due to the slow sound phenomenon. The subwavelength panel is composed of periodic horizontal slits loaded by identical Helmholtz resonators (HRs). Due to the presence of the HRs, the propagation inside each slit is strongly dispersive, with near-zero phase velocity close to the resonance of the HRs. In this slow sound regime, the frequencies of the cavity modes inside the slit are down-shifted and the slit behaves as a subwavelength resonator. Moreover, due to strong dispersion, the cavity resonances accumulate at the limit of the band gap below the resonance frequency of the HRs. Near this accumulation frequency, simultaneously symmetric and antisymmetric quasicritical coupling can be achieved. In this way, using only monopolar resonators quasiperfect absorption can be obtained in a material including transmission.

In this paper, we propose a method for nonlinear system (NLS) identification using a swept-sine input signal and based on nonlinear convolution. The method uses a nonlinear model, namely, the nonparametric generalized polynomial Hammerstein model made of power series associated with linear filters. Simulation results show that the method identifies the nonlinear model of the system under test and estimates the linear filters of the unknown NLS. The method has also been tested on a real-world system: an audio limiter. Once the nonlinear model of the limiter is identified, a test signal can be regenerated to compare the outputs of both the real-world system and its nonlinear model. The results show good agreement between both model-based and real-world system outputs.

This paper discusses on a quantitative comparison of the performances of different advanced algorithms for phase data de-noising. In order to quantify the performances, several criteria are proposed: the gain in the signal-to-noise ratio, the Q index, the standard deviation of the phase error, and the signal to distortion ratio. The proposed methodology to investigate de-noising algorithms is based on the use of a realistic simulation of noise-corrupted phase data. A database including 25 fringe patterns divided into 5 patterns and 5 different signal-to-noise ratios was generated to evaluate the selected de-noising algorithms. A total of 34 algorithms divided into different families were evaluated. Quantitative appraisal leads to ranking within the considered criteria. A fairly good correlation between the signal-to-noise ratio gain and the quality index has been observed. There exists an anti-correlation between the phase error and the quality index which indicates that the phase errors are mainly structural distortions in the fringe pattern. Experimental results are thoroughly discussed in the paper.

[EN] Vortex beams, characterized by a collimated wavefront with a phase dislocation at their principal axis, have found practical applications to construct acoustic tweezers for particle trapping and manipulation, or underwater communications. However, the natural diffraction of the wavefront limits the size of the vortex. This result in vortices whose bright core is larger than the wavelength, limiting their use for practical applications such as long-range underwater communications. In this work, we synthesize a vortex beam of sub-wavelength size at a distance beyond Rayleigh diffraction length using the nonlinear mixing of two confocal, high-frequency and detuned vortex beams of different topological charges. By using the nonlinear mixing of two confocal vortices, it was generated a low-frequency (1 kHz) focused vortex beam of integer topological charge whose distance between magnitude maxima is about 18 times smaller than its wavelength at a distance about 3 times the Rayleigh diffraction length. Sub-wavelength vortices emerge as a result of the spatiotemporal interference of two primary vortex beams due to the conservation of angular momentum during nonlinear wave-mixing. This mechanism opens new paths to design directive parametric antennas for vortex transceivers or particle manipulation systems at scales well below the diffraction limit.

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<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper deals with the calculation of the force and the stiffness between two ring permanent magnets whose polarization is radial. Such a configuration corresponds to a passive magnetic bearing. The magnetic force exerted between ring permanent magnets is determined by using the Coulombian model. The expressions obtained are semianalytical and we show that it is not possible to find an exact analytical expression of the force between two ring permanent magnets. Then, thanks to these semianalytical calculations, the ring dimensions are optimized in order to have a great force or a great stiffness. Moreover, we show that the relative position of the rings for which the force is the strongest depends on the air gap dimension. This result is new because the curvature effect is taken into account in this paper. We can say that such semianalytical expressions are more precise than the numerical evaluation of the magnetic forces obtained with the finite-element method. Moreover, semianalytical expressions have a low computational cost whereas the finite-element method has a high one. Thereby, as shown in this paper, such calculations allow an easy optimization of quadripolar lenses or devices using permanent magnets. </para>

The fifth-generation (5G) of cellular networks and beyond requires massive connectivity, high data rates, and low latency. Millimeter-wave (mmWave) communications is a key 5G enabling technology to meet these requirements thanks to its technical potentials that can be integrated with other 5G enablers such as ultra-dense networks (UDNs) and massive multiple-input-multiple-output (massive MIMO) systems. However, some technical challenges, which are mainly related to specific characteristics of mmWave propagation, must be addressed. All the aforementioned points will be discussed in this article before presenting the different existing architectures of massive MIMO mmWave systems. This survey mainly aims at presenting a comprehensive state-of-the-art review of the channel estimation techniques associated with the different mmWave system architectures. Subsequently, we will provide a comparison among existing solutions in terms of their respective benefits and shortcomings. Finally, some open directions of research are discussed, and challenges that wait to be met are pointed out.

The absorption properties of a metaporous material made of non-resonant simple shape three-dimensional rigid inclusions (cube, cylinder, sphere, cone, and ring torus) embedded in a rigidly backed rigid-frame porous material are studied. A nearly total absorption can be obtained for a frequency lower than the quarter-wavelength resonance frequency due to the excitation of a trapped mode. To be correctly excited, this mode requires a filling fraction larger in three-dimensions than in two-dimensions for purely convex (cube, cylinder, sphere, and cone) shapes. At long wavelengths compared to the spatial period, a cube is found to be the best purely convex inclusion shape to embed in a cubic unit cell, while the embedment of a sphere or a cone cannot lead to an optimal absorption for some porous material properties and dimensions of the unit cell. At a fixed position of purely convex shape inclusion barycenter, the absorption coefficient only depends on the filling fraction and does not depend on the shape below the Bragg frequency arising from the interaction between the inclusion and its image with respect to the rigid backing. The influence of the incidence angle and of the material properties, namely, the flow resistivity is also shown. The results of the modeling are validated experimentally in the case of cubic and cylindrical inclusions.

We present exact three-dimensional semi-analytical ex- pressions of the force exerted between two coaxial thick coils with rect- angular cross-sections. Then, we present a semi-analytical formulation of their mutual inductance. For this purpose, we have to calculate six and seven integrations for calculating the force and the mutual induc- tance respectively. After mathematical manipulations, we can obtain semi-analytical formulations based on only two integrations. It is to be noted that such integrals can be evaluated numerically as they are smooth and derivable. Then, we compare our results with the flla- ment and the flnite element methods. All the results are in excellent agreement.

Abstract Rationale It is currently unclear which patients with obstructive sleep apnea (OSA) are at increased cardiovascular risk. Objective To investigate the value of pulse wave amplitude drops (PWADs), reflecting sympathetic activations and vasoreactivity, as a biomarker of cardiovascular risk in OSA. Methods PWADs were derived from pulse oximetry–based photoplethysmography signals in three prospective cohorts: HypnoLaus (N = 1,941), the Pays-de-la-Loire Sleep Cohort (PLSC; N = 6,367), and “Impact of Sleep Apnea syndrome in the evolution of Acute Coronary syndrome. Effect of intervention with CPAP” (ISAACC) (N = 692). The PWAD index was the number of PWADs (&amp;gt;30%) per hour during sleep. All participants were divided into subgroups according to the presence or absence of OSA (defined as ≥15 or more events per hour or &amp;lt;15/h, respectively, on the apnea–hypopnea index) and the median PWAD index. Primary outcome was the incidence of composite cardiovascular events. Measurements and Main Results Using Cox models adjusted for cardiovascular risk factors (hazard ratio; HR [95% confidence interval]), patients with a low PWAD index and OSA had a higher incidence of cardiovascular events compared with the high-PWAD and OSA group and those without OSA in the HypnoLaus cohort (HR, 2.16 [1.07–4.34], P = 0.031; and 2.35 [1.12–4.93], P = 0.024) and in the PLSC (1.36 [1.13–1.63], P = 0.001; and 1.44 [1.06–1.94], P = 0.019), respectively. In the ISAACC cohort, the low-PWAD and OSA untreated group had a higher cardiovascular event recurrence rate than that of the no-OSA group (2.03 [1.08–3.81], P = 0.028). In the PLSC and HypnoLaus cohorts, every increase of 10 events per hour in the continuous PWAD index was negatively associated with incident cardiovascular events exclusively in patients with OSA (HR, 0.85 [0.73–0.99], P = 0.031; and HR, 0.91 [0.86–0.96], P &amp;lt; 0.001, respectively). This association was not significant in the no-OSA group and the ISAACC cohort. Conclusions In patients with OSA, a low PWAD index reflecting poor autonomic and vascular reactivity was independently associated with a higher cardiovascular risk.

Here, a new spectrum sensing method, called mean-to-square extreme eigenvalue (MSEE), is proposed. Considering a multiple antenna communication system, the proposal is drawn from the arithmetic-to-geometric mean (AGM) algorithm using only the smallest and the largest eigenvalues of the covariance matrix of the received signal. The aim of MSEE is to avoid the heavy computational costs of AGM method. First, based on the random matrix theory, a theoretical development to set the threshold of the proposal is provided. Then, the validity of the expression is verified by simulations. Finally, simulation results show an interesting performance of MSEE compared with several spectrum sensing methods in the literature.

This paper concerns the ultrasonic characterization of human cancellous bone samples by solving the inverse problem using experimental transmitted signals. The ultrasonic propagation in cancellous bone is modeled using the Biot theory modified by the Johnson et al. model for viscous exchange between fluid and structure. The sensitivity of the Young modulus and the Poisson ratio of the skeletal frame is studied showing their effect on the fast and slow wave forms. The inverse problem is solved numerically by the least squares method. Five parameters are inverted: the porosity, tortuosity, viscous characteristic length, Young modulus, and Poisson ratio of the skeletal frame. The minimization of the discrepancy between experiment and theory is made in the time domain. The inverse problem is shown to be well posed, and its solution to be unique. Experimental results for slow and fast waves transmitted through human cancellous bone samples are given and compared with theoretical predictions.

Many models for the prediction of the acoustical properties of porous media require non-acoustical parameters few of which are directly measurable. One popular prediction model by Johnson, Champoux, Allard, and Lafarge [J. Appl. Phys. 70(4), 1975-1979 (1991)] (459 citations, Scopus, April 2019) requires six non-acoustical parameters. This paper proves that the use of more than three parameters in the Johnson-Champoux-Allard-Lafarge model is not necessary at all. Here the authors present theoretical and experimental evidence that the acoustical impedance of a range of porous media with pore size distribution close to log-normal (granular, fibrous, and foams) can be predicted through the knowledge of the porosity, median pore size, and standard deviation in the pore size only. A unique feature of this paper is that it effectively halves the number of parameters required to predict the acoustical properties of porous media very accurately. The significance of this paper is that it proposes an unambiguous relationship between the pore microstructure and key acoustical properties of porous media with log-normal pore size distribution. This unique model is well suited for using acoustical data for measuring and inverting key non-acoustical properties of a wider range of porous media used in a range of applications which are not necessarily acoustic.

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Topological band theory strongly relies on prototypical lattice models with particular symmetries. We report here on a theoretical and experimental work on acoustic waveguides that are directly mapped to the one-dimensional Su-Schrieffer-Heeger model. Starting from the continuous two-dimensional wave equation, we use a combination of monomode approximation and the condition of equal-length tube segments to arrive at the wanted chiral symmetric discrete equations. It is shown that open or closed boundary conditions lead automatically to the existence of topological edge modes. We illustrate by graphical construction how the edge modes appear naturally owing to a quarter-wavelength condition and the conservation of flux. Furthermore, the transparent chirality of our system, which is ensured by simple geometrical constraints allows us to study chiral disorder numerically and experimentally. Our experimental results in the audible regime demonstrate the predicted robustness of the topological edge modes.

This article presents a numerical optimization procedure of continuous gradient porous layer properties to achieve perfect absorption under normal incidence. This design tool is applied on a graded porous medium composed of a periodic arrangement of ordered unit cells allowing one to link the effective acoustic properties to its geometry. The best microgeometry continuous gradient providing the optimal acoustic reflection and/or transmission is designed via a nonlinear conjugate gradient algorithm. The acoustic performances of the so-designed continuous graded material are discussed with respect to the optimized homogeneous, i.e., nongraded and monotonically graded material. The numerical results show a shifting of the perfect absorption peak to lower frequencies or a widening of the perfect absorption frequency range for graded materials when compared to uniform ones. The results are validated experimentally on 3D-printed samples, therefore, confirming the relevance of such a gradient along with the efficiency of the control of the entire design process.

Abstract—This paper presents an improvement of the calculation of the magnetic field components created by ring permanent magnets. The three-dimensional approach taken is based on the Coulombian Model. Moreover, the magnetic field components are calculated without using the vector potential or the scalar potential. It is noted that all the expressions given in this paper take into account the magnetic pole volume density for ring permanent magnets radially magnetized. We show that this volume density must be taken into account for calculating precisely the magnetic field components in the near-field or the far-field. Then, this paper presents the component switch theorem that can be used between infinite parallelepiped magnets whose cross-section is a square. This theorem implies that the magnetic field components created by an infinite parallelepiped magnet can be deducted from the ones created by the same parallelepiped magnet with a perpendicular magnetization. Then, we discuss the validity of this theorem for axisymmetric problems (ring permanent magnets). Indeed, axisymmetric problems dealing with ring permanent magnets are often treated with a 2D approach. The results presented in this paper clearly show that the two-dimensional studies dealing with the optimization of ring permanent magnet dimensions cannot be treated with the same precisions as 3D studies. 1.