GENERAL PHYSICS – ACOUSTIC WAVES ACOUSTIC INTERACTION BETWEEN TWO CLOSE ELASTIC SPHERICAL SHELLS �

The acoustic scattering by two close thin spherical shells in water is studied. At low frequency, the resonant behavior of one shell is due to the propagation of the A-wave. The energy of this wave lies mainly in the surrounding water, so that it can easily couple two spheres, provided they are close enough. As the backscattered form function of the system composed of the two spheres is compared to that of one single sphere, a strong resonant coupling is exhibited in the low frequency region. Key words: acoustics, multiple scattering, elastic spheres, resonant coupling. 1. ACOUSTIC SCATTERING BY TWO SPHERES Let us suppose a harmonic plane wave incident on a system of two spheres embedded in water. This wave propagates in the (x, z) plane, under angle α with respect to the Oz axis. Fig. 1 shows the geometry of the problem. The observer P has coordinates (r, θ, ϕ) in the main coordinates system centered on O. Supposing an e –iωt time dependence of all acoustic fields that will be omitted throughout the paper, the displacement potential of the incident plane wave at point P may be written as ϕ = eik⋅r = eikrsin αsin θcosϕeikr cosαcosθ = with n inc standing fo

The coherent shear wave in suspensions

This is an Open Access Article. It is published by IOP under the Creative Commons Attribution 3.0 Unported Licence (CC BY). Full details of this licence are available at: http://creativecommons.org/licenses/by/3.0/ This paper was presented at the 16th Anglo-French Physical Acoustics Conference (AFPAC)We consider a disordered suspension of spherical silica particles in water. For a particle size of a few hundred nanometres and concentration (volume fraction) around 0.15 to 0.2, experiments conducted in the MHz range have shown that the non ideal nature of water must be taken into account for the "longitudinal" coherent wave attenuation to be understood, because of wave conversions, from longitudinal to shear and then back to longitudinal, occurring at each pair of scattering events. We are interested here in the properties of the "shear" coherent wave that are given by the expansion of its squared wavenumber, around that in the absence of particles, in powers of the concentration. At 1 MHz and a particle radius of 0.05 m, we show that convergence of the modal series involved in that expansion may be reached after three terms: we use ten terms subsequently. We study the evolution of both the effective shear velocity and attenuation with concentration, as well as that of the effective shear viscosity deduced therefrom

Conversion de modes de Lamb à l'extrémité d'une plaque immergée

This paper deals with an experimental study of the conversion of an incident Lamb wave on the extremity of an immersed plate. This wave gives rise to one or several different Lamb waves propagating in the opposite direction. The converted waves are observed only after a distance from the end of the plate. Non guided waves are also observed backward in the plate. An interpretation is proposed, based on the description of Lamb waves by the Debye series

Reflection and transmission at low concentration by a depth-varying random distribution of cylinders in a fluid slab-like region

International audienceThis paper reports a study of the backscattering behavior of a solid layer containing randomly spaced spherical cavities in the long wavelength limit. The motivation for the work arises from a need to model the responses of porous composite materials in ultrasonic NDE procedures. A comparison is made between models based on a summation over discrete scatterers, which show interesting emergent properties, and an integral formulation based on an ensemble average, and with a simple slab effective medium approximation. The similarities and differences between these three models are demonstrated. A simple quantitative criterion is established which sets the maximum frequency at which ensemble average or equivalent homogeneous medium models can represent echo signal generation in a porous layer for given interpore spacing, or equivalently, given pore size and concentration

ÉTUDE EXPÉRIMENTALE DE LA DIFFUSION D'ONDES DE LAMB À L'EXTRÉMITÉ D'UNE PLAQUE IMMERGÉE

Nous étudions exprimentalement, pour une plaque immergée dans l'eau, la conversion en bout de plaque d'une onde de Lamb incidente en une onde de volume diffusée dans l'eau. L'amplitude de l'onde de volume est tracée en fonction de l'angle d'observation, pour différents modes de Lamb (symétriques et antisymétriques). Les différences caractéristiques entre les courbes associées à des ondes de Lamb symétriques et celles associées à des ondes de Lamb antisymétriques sont soulignées. La mise en évidence de ces différences nous conduit à proposer un modèle théorique simple permettant de rendre compte des principales propriétés des courbes expérimentales.We present an experimental study of the radiation of Lamb waves at the end section of an aluminum plate imbedded in water. The amplitude of the scattered bulk wave in water is plotted, for different incident Lamb waves (symetric ones and antisymetric ones), versus the angle of diffraction. The caracteristic differences between the scattering phenomenon of a symetric Lamb wave and an antisymetric one are outlined. A simple model is proposed in order to predict most of the properties of the experimental curves obtained