In the framework of the European Hydrological Cycle in the Mediterranean Experiment project, a field campaign devoted to the study of electrical activity during storms took place in the south of France in 2012. An acoustic station composed of four microphones and four microbarometers was deployed within the coverage of a Lightning Mapping Array network. On the 26 October 2012, a thunderstorm passed just over the acoustic station. Fifty‐six natural thunder events, due to cloud‐to‐ground and intracloud flashes, were recorded. This paper studies the acoustic reconstruction, in the low frequency range from 1 to 40 Hz, of the recorded flashes and their comparison with detections from electromagnetic networks. Concurrent detections from the European Cooperation for Lightning Detection lightning location system were also used. Some case studies show clearly that acoustic signal from thunder comes from the return stroke but also from the horizontal discharges which occur inside the clouds. The huge amount of observation data leads to a statistical analysis of lightning discharges acoustically recorded. Especially, the distributions of altitudes of reconstructed acoustic detections are explored in detail. The impact of the distance to the source on these distributions is established. The capacity of the acoustic method to describe precisely the lower part of nearby cloud‐to‐ground discharges, where the Lightning Mapping Array network is not effective, is also highlighted.

Twenty-seven natural flashes of three thunderstorms which occurred during October 2012 in Southern France are acoustically reconstructed and analyzed in the [0.1 − 180]-Hz frequency bandwidth and the [0.3−20]-km distance range. A 50-m triangular array of four recalibrated microphones sampled at 500 Hz has been used for the recording. A novel method of separation of return strokes from intracloud discharges within the acoustical signal is detailed and systematically applied. It shows the possibility to separate nearby cloud-to-ground (CG) events or nearby and distant CGs of the same flash. The separation method yields a total of 36 return stroke signals and spectra, along with some intracloud signals. The combination of reconstruction, separation, and frequency analysis provides new insights on the origin of thunder infrasound, showing unambiguously that thunder infrasound originate dominantly from return strokes. Apart from the higher amplitude of CGs, no clear difference between intracloud and CG spectra is observed. No sharp frequency peaks can be put into evidence. The spectral variability with distance is highlighted, especially for the total acoustic energy and the center frequency and bandwidth. A link between acoustic energy and impulse charge moment change is also found, though only by a small number of data.The description of these two mechanisms explaining the infrasonic and audible acoustic content of thunder has been summarized by Few (1995). Regarding the audible content, the sudden core heating (25,000-30,000 K) just after the electric discharge produces a chain of strong shock waves along the lightning channel. The time dependence of pressure within the lightning channel was estimated by Orville (1968a) from the first measured time resolved optical spectra (Orville, 1968b), with peak overpressure exceeding 80 kPa. Based on a radiation thermodynamical model, Hill (1971) could simulate such values, with even higher shock amplitudes for shorter times (see his Figure 5). The different steps to switch from strong shock waves to weak shock waves and finally to acoustic waves are developed by Few (1969). Theory for self similar strong shock wave was proposed by Taylor (1950) for point sources and Lin (1954) for line sources and then numerically extended to transition and weak shock regimes by Brode (1955) for spherically symmetric shocks. The cylindrical symmetry was described by Plooster (1970). An empirical match between strong and weak shock theories has been applied to lightning by Jones et al. (1968) assuming a rectilinear channel. However, lightning channel tortuosity has been observed optically by Hill (1968), who proposed a mean value of 16 ∘ deflection of one segment to the next. Synthesizing these previous studies, Few et al. (1967) andFew (1969) assumed that the transition between strong and weak shocks takes place approximately in the same range of distances as the transition between cylindrical and spherical divergence. This distance is called relaxation radius. Indeed,

Time-reversal invariance of nonlinear acoustic wave propagation is experimentally investigated. Reversibility is studied for propagation shorter or longer than shock formation distance. In the first case, time-reversal invariance holds and a sinusoid distorted by nonlinearities during forward propagation progressively recovers its initial shape after the time-reversal operation. In the second case, reversibility is broken locally at the shock front as a time-reversal operation transforms a stable compression shock into an unstable expansion shock. Achieving experimentally the time-reversal process with a time-reversal mirror made of reversible piezoelectric transducers for very broadband signals, would require transducers with huge bandwidths. To date, such transducers remain unavailable. In order to overcome this technical limitation, we restricted ourselves in this study to one-dimensional (1D) propagation, for which an experimental ersatz of a time-reversal mirror can be used. Indeed, in a 1D case, the time-reversal operation applied on a plane wave can be mimicked for an antisymmetric wave form by a reflection of the plane wave onto a pressure-release interface.

Nanodroplets have great, promising medical applications such as contrast imaging, embolotherapy, or targeted drug delivery. Their functions can be mechanically activated by means of focused ultrasound inducing a phase change of the inner liquid known as the acoustic droplet vaporization (ADV) process. In this context, a four-phases (vapor + liquid + shell + surrounding environment) model of ADV is proposed. Attention is especially devoted to the mechanical properties of the encapsulating shell, incorporating the well-known strain-softening behavior of Mooney-Rivlin material adapted to very large deformations of soft, nearly incompressible materials. Various responses to ultrasound excitation are illustrated, depending on linear and nonlinear mechanical shell properties and acoustical excitation parameters. Different classes of ADV outcomes are exhibited, and a relevant threshold ensuring complete vaporization of the inner liquid layer is defined. The dependence of this threshold with acoustical, geometrical, and mechanical parameters is also provided.