Vincent Gibiat

Non-destructive imaging using the time domain topological energy method

The time domain topological gradient has recently been presented as a promising technique for imaging of complex structures [N. Dominguez, et al., Wave Motion 42 (1) (2005) 31-52], showing results obtained on simulated data. The method relies on the adequate combination of two computed ultrasonic fields, one forward and one adjoint. The adjoint field, that carries the information about the defects, is analogous to a time reversal operation. The forward field plays the role of a photographic developer. The great interest of the method is that the time reversal operation is not done experimentally but numerically, allowing implementation of low cost systems. The expected low sensitivity of the method to the presence of measurement or structure noise had been discussed in a former work. This paper shows results obtained on experimental data in the context of non-destructive testing, showing the efficiency of the method even in complex configuration such that composite material testing.

Fast topological imaging

Mathematical optimization methods based on the topological sensitivity analysis have been used to develop innovative ultrasonic imaging methods. With a single illumination of the medium, they have proved experimentally to yield a lateral resolution comparable to classical multiple-illumination techniques. As these methods are based on the numerical simulations of two wave fields, they require extensive computation. A time-domain finite-difference scheme is usually used for that purpose. This paper presents the development of an experimental imaging method based on the topological sensitivity. The numerical cost is reduced by replacing the numerical simulations by simple mathematical operations between the radiation patterns of the arrays transducers and the frequency-domain signals to be emitted. These radiation patterns are preliminary computed once and for all. They were obtained with a finite element model for the anisotropic elastodynamic case and with semi-analytical integrations for the acoustic case. Experimental results are presented for a composite material sample and for a prefractal network immersed in water. A lateral resolution below 2.5 times the wavelength is obtained with a single plane wave illumination. The method is also applied with multiple illuminations, so that objects hidden in complex media can be investigated.

Selective focusing through target identification and experimental acoustic signature extraction: Numerical experiments

Using transducer arrays and appropriate emission delays allow to focus acoustic waves at a chosen location in a medium. The focusing spatial accuracy depends on the accurate knowledge of its acoustic properties. When those properties are unknown, methods based on the Time-Reversal principle allow accurate focusing. Still, these methods are either intrusive (an active source has to be introduced at the target location first), either blind (the target cannot be selected in the presence of several objects.) The purpose of the present work is to achieve non-invasive accurate focusing on a selected target using inaccurate acoustic properties for the investigated medium. Potential applications are for instance noninvasive surgery based on High Intensity Focused Ultrasound (HIFU). Numerical experiments are presented and demonstrate accurate focusing on a previously designated target located in an unknown heterogeneous medium.

Temporal response of a simplified bidimensional numerical model of the cochlea

Within the framework of a study related to bone conduction, numerical simulations have been performed in the time domain, with the aim of comparing the cochlear partition displacement in the case of different places of stimulation. An oversimplified 2D model of the cochlea is used. It is first excited with pulses centered on various audible‐range frequencies with a localisation of the source which is analogous to the position of the oval window. Secondly, new sets of calculations introduce different localisations and/or spatial extensions of the sources. An analogy with seismology being adequate to simulate the solid‐fluid (cochlear partition‐perilymph) coupling, a finite difference numerical simulation based upon the Virieux scheme for elastic waves propagation has been used. The movement of the simplified basilar membrane is observable when excited via air or bone conduction. Results of the propagation of a single pulse within the model will be presented and discussed through information available in li...

Super‐resolution imaging of active sound and vibrational sources using a time‐reversal sink

Theory and experiments of super-resolution focusing using a time-reversal sink have been investigated in high-frequency regime [Rosny and Fink, Phys.Rev.Lett. 89] and in audible range [Bavu, Besnainou, Gibiat, Rosny and Fink, Act.Acust., 93]. This technique, generalized to the case acoustic and vibrational imaging of active sources, allows super-resolution imaging and provides a new method of characterization of active sources in a known background medium. This imaging technique involves a measurement in the background medium using an array, and the simulation of the backpropagating-field in a fictive medium. An ideal numerical time-reversal sink (NumTRAS) is then used to refine results and obtain high-contrast, high-resolution imaging of initial sources. The algorithm has been validated in parallel supercomputer simulations, in both vibrational and acoustics fields and has been used to detect active vibrational sources in a clamped Mindlin plate and active sound sources in an anechoic room. All results show high-resolution imaging capabilities when compared with classical time-reversal backpropagation. NumTRAS provides an alternative to other imaging and source detection techniques, such as acoustic holography and beamforming. Beyond the applications of acoustic and vibrational non-destructive evaluation of industrial structures, NumTRAS has applications in evaluation of musical structures and is being tested to detect and characterize moving sources.

Experimental determination of the stiffness of a saxophone reed by means of the evaluation of the reactive power

A simple model for a saxophone or a clarinet reed—one pure spring with stiffness k—is acceptable for frequencies beneath the resonance frequency of the reed. When the instrument sustains a harmonic tone, Eq. (1) of the previous paper holds and the right‐hand side quantities can be experimentally determined in order to evaluate k or k/S2. Experiments were driven on a saxophone so as not to encounter large uncertainties due to the weakness of even harmonics and corresponding minima of the input impedance of a cylindrical pipe. The reed stiffness is derived from separate measurements of the input impedance (TMTC method) and of the internal pressure spectrum when the saxophone is played by a musician or blown by an artificial system. Combined with an evaluation of the reed area, results on reed stiffness under playing conditions are in good agreement with compliance values measured on isolated reeds by other authors. The reactive power balance between the reed and the pipe also sheds a new light on the playin...

Quasiperiodicity and bifurcations in wolf tones

The wolf tone, often obtained on the lower bowed stringed instrument, is studied with an experimental setup that mimics the true instrument with the help of a digital bow [Causse and Weinreich, Proceed. 13th ICA, Belgrade 1989]. In this experiment, the resonant frequency of the bridge can be adjusted so that it is low enough for a good coupling with the fundamental frequency of the string, while keeping the bridge as rigid as possible. With this experimental setup, verification of the well‐known result that the wolf tone depends strongly on the pressure and speed of the bow and on the bowing point of the string has been made. By changing bow speed and pressure, by bifurcations after the normal periodic sound (Helmholtz motion), various quasiperiodic tones built on two or three basic frequencies have easily been obtained. Other possible scenarios related to bifurcation theory are indicated by observations of more complex signals.