Green's function can be retrieved from cross-correlation of a diffuse field generated by noise sources. Today, this important result is the fundamental of several passive imaging techniques. The aim of this paper is to establish the suitability of these methods to detect and locate a defect in a reverberant elastic plate. The relations between the noise cross-correlation function over a few number of noise sources and the imaginary part of the Green's function are derived and numerically validated. Then we show through numerical experiments that this technique is exploitable for defect detection and localization in a differential mode, despite a non-perfect estimation of the Green's functions. Finally, a filtering technique based on the singular value decomposition is shown to improve the detection.

Passive listening methodology has been shown to be a practical and effective method for passive structural health monitoring. In this work, this approach is applied experimentally to monitor the occurrence of defects in thin aluminum plates. A correlation matrix is estimated from noise vibrations recorded on a transducer array. A defect is localized by applying a beamforming algorithm to the difference between the correlation matrices obtained with and without the defect. We successfully detect defects for different kinds of noise sources. Moreover, we show that this technique is robust to detect massive inclusions, holes, and cracks. With a vibrometer, we observe that the fidelity of the estimated transient responses strongly depends on the number of uncorrelated noise sources. Finally, we show that the defect is successfully localized even if the noise source distribution is not uniform, provided that it remains spatially stationary between the states with and without defect. A simple theoretical framework is proposed to interpret these results.

A statistical reverberation model based on the average distribution of wave packets propagating in a finite plate is described and it is shown that by fitting experimental data to theoretical expressions, either structural or source-related features can be extracted with very few sensors. An original method to localise an impulse acoustic source in a reverberant plate is proposed and discussed as a possible application of this principle. The reverberated wave packets (coda), which are ordinarily discarded in classical localisation techniques, are used here to estimate an energy-related term characteristic of the source. Thereafter, the energies of the early wave packets measured on three sensors are enough to estimate the source location (whereas a minimum of four sensors is required in classical localisation techniques). After detailing the conditions of application of the method, several examples of experimental source localisation are given

The influence of liquid-flow up to 7 mm/s is examined on transmission spectra of phononic crystals, revealing a potential use for slow liquid-flow measurement techniques. It is known that transmission of ultrasound through a phononic crystal is determined by its periodicity and depends on the material characteristics of the crystal's constituents. Here, the crystal consists of metal rods with the space in between filled with water. Previous studies have assumed still water in the crystal, and here, we consider flowing liquid. First, the crystal bandgaps are investigated in still water, and the results of transmission experiments are compared with theoretical band structures obtained with the finite element method. Then, changes in transmission spectra are investigated for different speeds of liquid flow. Two situations are investigated: a crystal is placed with a principal symmetry axis in the flow direction ($\Gamma$ X) and then at an angle ($\Gamma$ M). The good stability of the bandgap structure of the transmission spectrum for both directions is observed, which may be of importance for the application of phononic crystals as acoustic filters in an environment of flowing liquid. Minor transmission amplitude changes on the other hand reveal a possibility for slow liquid flow measurements.