Andrea Colombi

This study introduces the first three-dimensional spectral-element method (SEM) simulation of ultrasonic wave propagation in the head of a bottlenose dolphin (Tursiops truncatus). Unlike traditional finite-element methods (FEM), which struggle with high-frequency simulations because of costly linear-system inversions and slower convergence, SEM offers exponential convergence and efficient parallel computation. Using computed tomography data, we developed a detailed hexahedral mesh capturing key anatomical features, such as acoustic fats and lower jaws. Our simulations of plane and spherical waves confirm SEM's effectiveness for ultrasonic time-domain modeling. This approach opens new avenues for marine-bioacoustic research, including echolocation, the biophysics of hearing and sound generation, and assessments of anthropogenic underwater noise. By overcoming FEM's limitations, SEM provides a powerful scalable tool to test hypotheses in dolphin bioacoustics, with significant implications for conservation and understanding marine-mammal auditory systems under increasing environmental challenges.

We experimentally implement a one-directional virtual geometric periodicity in an elastic metamaterial. First, unwanted boundary reflections at the domain ends are cancelled through the iterative injection of the polarity-reversed reflected wave field. The resulting boundless experimental state allows for a much better analysis of the influence of metamaterials on the propagating wave field. Subsequently, the propagating wave field exiting on one end of the structure is reintroduced at the opposite end, creating a virtual geometric periodicity in one propagation direction. We find that the experimentally observed band gap converges to the analytical solution through the introduction of the virtual periodicity. The established work flow introduces an approach to the experimental investigation and validation of metamaterial prototypes in the presence of strongly dispersive wave propagation and internal scattering. The fully data-driven ad hoc treatment of boundary conditions in metamaterial experimentation with arbitrary mechanical properties enables reflection suppression, virtual periodicity, and the introduction of more general fictitious boundaries.

In this work, we experimentally and numerically investigate the propagation and attenuation of vertically polarized surface waves in an unconsolidated granular medium equipped with small-scale metabarriers of different depths, i.e., arrays composed of one, two, and three embedded layers of sub-wavelength resonators. Our findings reveal how such a multi-layer arrangement strongly affects the attenuation of the surface wave motion within and after the barrier. When the surface waves collide with the barriers, the wavefront is back-scattered and steered downward underneath the oscillators. Due to the stiffness gradient of the granular medium, part of the wavefield is then rerouted to the surface level after overcoming the resonant array. Overall, the in-depth insertion of additional layers of resonators leads to a greater and broader band wave attenuation when compared to the single layer case.

We experimentally demonstrate that a rainbow-based metamaterial, created by a graded array of resonant rods attached to an elastic beam, operates as a mechanical delay-line by slowing down surface elastic waves to take advantage of wave interaction with resonance. Experiments demonstrate that the rainbow effect reduces the amplitude of the propagating wave in the host structure. At the same time, it dramatically increases both the period of interaction between the waves and the resonators and the wavefield amplitude in the rod endowed with the harvester. Increased energy is thus fed into the resonators over time: we show the enhanced energy harvesting capabilities of this system.

In this paper, we describe the manifestation of localized states of a diffuse elastic wavefield inside a two-dimensional metamaterial made of a collection of vertical long beams glued to a thin plate. Through mesoscopic physics, we demonstrate that localized states arise due to multiwave interactions at the beam-plate attachment when the beams act as coupled resonators for both compressional and flexural resonances on the metasurface. In practice, when the compressional resonance of the beams clamps the plate on a large frequency band gap, thus preventing wave propagation inside the metamaterial area, the flexural resonance of the same beams permits evanescent waves to be diffused on a narrow frequency band between clusters of neighboring beams. This experiment physically highlights a tight-binding-like coupling in the localized regime for this two-dimensional metamaterial.