Design of a broadband ultra-large area acoustic cloak based on a fluid medium Tunable cylindrical shell as an element in acoustic metamaterial J. Acoust. Soc. Am. 136, 1601 (2014); 10.1121/1.4894723Omnidirectional acoustic absorber with a porous core and a metamaterial matching layera)
A phononic crystal (PC) slab made of a single metallic phase is shown, theoretically and experimentally, to display perfect negative index matching and focusing capability when surrounded with water. The proposed PC slab is a centimeter scale hollow metallic foam-like structure in which acoustic energy is mediated via the metal lattice. The negative index property arises from an isolated branch of the dispersion curves corresponding to a mode that can be coupled to incident acoustic waves in surrounding water. This band also intercepts the water sound line at a frequency in the ultrasonic range. The metallic structure is consequently a candidate for the negative refraction of incident longitudinal waves. V
A wide range of mesoscopic phononic materials can exhibit frequency bands where transmission is forbidden, i.e. band gaps. Three different mechanisms for their origin can be distinguished, namely Bragg, hybridization and weak elastic coupling effects. Characteristic properties of gaps of different origins are investigated and compared, for a 3D crystal of tungsten carbide beads in water, a 2D crystal of nylon rods in water, and a 3D opal-like structure of weakly sintered aluminum beads. For the second type of crystal, it is shown that Bragg and hybridization gaps can be overlapped, allowing the study of the interaction between these two mechanisms. Atypical dispersion characteristics are demonstrated near the resonance frequency
The properties of a one-dimensional phononic crystal made of identical piezoelectric elements separated by thin metallic electrodes connected to the ground are studied theoretically for cases where the locations of the electrical connections change as a function of time with a specific speed. This spatiotemporal modulation of the electrical boundary conditions results in significant non-linear effects that are evidenced numerically. The interaction between an incident harmonic longitudinal wave and the time-dependent phononic crystal is shown to lead to frequency splitting analogous to Brillouin scattering. Moreover, the boundaries of the Bragg bandgaps are strongly affected, and for some specific modulation speed, one-way wave propagation can be achieved. Published by AIP Publishing.
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