Laboratory of Mechanics and Acoustics

We investigate the behaviour of metainterfaces composed of three-dimensional subwavelength Helmholtz resonators (HRs), that are open at both ends and may have distinct neck geometries, using a homogenized model derived from a three-scale asymptotic approach. This model reduces such metainterfaces to homogenized boundary conditions that incorporate a resonant pressure field, providing a continuous representation of the discrete pressure field within the cavities that constitute the metasurface. Notable special cases include mirror- symmetric metainterfaces and metasurfaces that operate solely in reflection. The model, developed in the time domain, is validated and discussed in the harmonic regime through comparisons with direct numerical simulations.

The hallmark of topological phases in periodic materials is the existence of localized modes on the edges. These modes benefit from topological protection, that renders them immune to backscattering over defects or disorder. However, in the context of classical waves such as in photonics or acoustics, total immunity to backscattering has never been achieved so far. In this presentation, I will show how to obtain not only perfect transmission but also invisbility of topological edge waves by exploiting the combination of chiral and mirror symmetry. This guarantees a unit transmission coefficient with a zero phase shift for edge waves scattering across symmetry preserving defects or disorder.

In this work we investigate Anderson localization in random acoustic structures through the localization landscape theory. We discuss its limitations for acoustic waves and introduce modifications to recover useful information on localized modes.

Porous media represent a vast class of materials, encountered in, i.e., geology, medicine and acoustics. Knowledge of the porous microstructure is necessary for the use of the different models describing the acoustical behavior of porous media. The focus of this work is put on inverse identification methods, that allow the retrieval of several microstructural parameters of a porous material. The inverse method is based on the measurement of an acoustic field transmitted and/or reflected by the porous material, and is recast as a Bayesian inference problem. This allows for an uncertainty quantification of the identified parameters, and model selection, in case of competing models for the acoustic representation of a porous material. Different materials are tested in impedance tubes, and their properties are inferred using different acoustical models. Samples of multiple thicknesses are backed with airgaps of different length, which allows for a densification of the test matrix for a given material: this ultimately improves knowledge on the porous microstructure properties, described by posterior probability density functions in a Bayesian context.

International audience

International audience

This study is dedicated to the development and experimental validation of an active acoustic anechoic method to reproduce, in an enclosed space, free field conditions, that is without reflections from the walls.The use of an anechoic room is a classical solution but it is not sufficient at low frequencies, in particular, below the cutoff frequency of the room which is directly related to the thickness of the absorbent material on the walls. In order to combine passive and active noise reduction techniques, the goal of this study is to control the field scattered by the walls in this frequency range using active techniques. A specific difficulty with this active control problem concerns the measurement of the diffracted field which is not a directly accessible physical quantity. In this work, a strategy for estimating the diffracted pressure is developed from measurements of total pressure on a surface surrounding the source to be characterized. Active control techniques are then used to drive an array of actuators to minimize the estimated diffracted field. The active anechoic method used here consists in identifying in an precalibration step the mathematical operator which relates the diffracted pressure to the total pressures. At the active control stage, the operator is then used as a filter to be applied on the total pressure measurements. For this first experimentin real size, the goal is to control the diffracted field by the reflecting wall of a semi-anechoic room with 9 secondary sources in the frequency range 40-200 Hz. The sources are situated on the reflecting wall. The diffracted pressure is first estimated from measurements from 16 microphones located in a plane parallel to the reflecting wall. Several control strategies are applied and evaluated to characterize the acoustic radiation of a low frequency source.

International audience