Time reversal imaging for sensor networks with optimal compensation in time.

Using extensive numerical simulations, several distributed sensor imaging algorithms for localized damage in a structure are analyzed. Given a configuration of ultrasonic transducers, a full response matrix for the healthy structure is assumed known. It is used as a basis for comparison with the response matrix that is recorded when there is damage. Numerical simulations are done with the wave equation in two dimensions. The healthy structure contains many scatterers. The aim is to image point-like defects with several regularly distributed sensors. Because of the complexity of the environment, the recorded traces have a lot of delay spread and travel time migration does not work so well. Instead, the traces are back propagated numerically assuming that there is some knowledge of the background. Since the time at which the back propagated field will focus on the defects is unknown, the Shannon entropy or the bounded variation norm of the image is computed and the time where it is minimal is picked. This imaging method performs well because it produces a tight image near the location of the defects at the time of refocusing. When there are several defects, the singular value decomposition of the response matrix is also carried out.

Time-domain simulation of a guitar: model and method.

This paper presents a three-dimensional time-domain numerical model of the vibration and acoustic radiation from a guitar. The model involves the transverse displacement of the string excited by a force pulse, the flexural motion of the soundboard, and the sound radiation. A specific spectral method is used for solving the Kirchhoff-Love's dynamic top plate model for a damped, heterogeneous orthotropic material. The air-plate interaction is solved with a fictitious domain method, and a conservative scheme is used for the time discretization. Frequency analysis is performed on the simulated sound pressure and plate velocity waveforms in order to evaluate quantitatively the transfer of energy through the various components of the coupled system: from the string to the soundboard and from the soundboard to the air. The effects of some structural changes in soundboard thickness and cavity volume on the produced sounds are presented and discussed. Simulations of the same guitar in three different cases are also performed: "in vacuo," in air with a perfectly rigid top plate, and in air with an elastic top plate. This allows comparisons between structural, acoustic, and structural-acoustic modes of the instrument. Finally, attention is paid to the evolution with time of the spatial pressure field. This shows, in particular, the complex evolution of the directivity pattern in the near field of the instrument, especially during the attack.