We theoretically report on an innovative and practical acoustic energy harvester based on a defected acoustic metamaterial (AMM) with piezoelectric material. The idea is to create suitable resonant defects in an AMM to confine the strain energy originating from an acoustic incidence. This scavenged energy is converted into electrical energy by attaching a structured piezoelectric material into the defect area of the AMM. We show an acoustic energy harvester based on a meta-structure capable of producing electrical power from an acoustic pressure. Numerical simulations are provided to analyze and elucidate the principles and the performances of the proposed system. A maximum output voltage of 1.3 V and a power density of 0.54 μW/cm3 are obtained at a frequency of 2257.5 Hz. The proposed concept should have broad applications on energy harvesting as well as on low-frequency sound isolation, since this system acts as both acoustic insulator and energy harvester.

The metascreen-based acoustic passive phased array provides anew degree of freedom formanipu-latingacoustic waves due to their fascinating properties, such as afully shifting phase, keeping impedance matching, and holding subwavelength spatial resolution. We develop acoustic theories to analyze the transmission/reflection spectra and the refracted pressure fields of a metascreen composed of elements with four Helmholtz resonators (HRs) in series and a straight pipe. We find that these propertiesare also valid under oblique incidence with large angles, with the underlying physics stemming from the hybrid resonances between the HRs and the straight pipe. By imposing thedesired phase profiles, the refracted fields can be tailored in ananomalous yet controllable manner. In particular, two typesof negative refractionare exhibited,based on two distinct mechanisms: one is formed fromclassical diffraction theory and the other is dominated by the periodicity of the metascreen. Positive (normal) and negative refractions can be converted by simply changing the incident angle, with the coexistence of two typesof refractionin a certain range of incident angles. 1 2 . The symbols p i , p r , and p t represent the obliquely incident, reflected and transmitted sound waves, respectively. (b) A single HR is illustrated for the purpose of the corrected acoustic impedance of the cavity. (c) A single HR is located in front of a hard boundary (dashed line) to obtain the corrected acoustic radiation impedance at the junction between the neck and the straight pipe.New J. Phys. 18 (2016) 043024 Y Li et al

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We theoretically and numerically report on an innovative acoustic energy harvester based on acoustic multilateral metasurfaces and a piezoelectric bimorph. The coiling-up-space concept realized by labyrinthine units is applied to achieve the desired phase profiles for the acoustic focusing and energy confinement. The acoustic energy confined by the metasurfaces from a point source is converted into electrical energy by a structured piezoelectric bimorph. Numerical simulations and theoretical analysis evidenced that the output voltage and power drastically increase with the sides of the multilateral metasurface energy harvesting system. Maximum output voltage and power 52 and 407 times higher than those under the case without metasurfaces are achieved with enclosed multilateral metasurface design.

An advanced concept of reflective acoustic focusing based on an ultrathin metasurface is numerically and analytically investigated. We propose a designed reflective metasurface with a thickness of λ/15, with λ being wavelength, composed of locally resonant Helmholtz-like elements which discretely realize the 2π phase shift. The theoretical design based on the generalized Snell's law is numerically achieved by the proposed ultrathin metasurface. Numerical simulations and theoretical analysis have converged to a good consensus and validated the ultrathin reflective metasurface concept for acoustic focusing. Furthermore, another reflective metasurface with sub-wavelength thickness (λ/8) and based on the coiling-up-space concept constructed by three-dimensional (3D) labyrinthine elements is investigated and compared to the ultrathin one. Despite both metasurfaces illustrating equivalent good performances for acoustic focusing, the ultrathin one demonstrates its superiority with thinner thickness, simpler design, and easier fabrication, which would greatly facilitate its real implementation in relevant applications.

Based on the perturbation theory and Bernoulli equation, equations of aspherical oscillation of two interacting bubbles are derived. This system is then used for the numerical investigation of the deformation of the two bubbles' surfaces in a spherical ultrasound field in liquid. We find that the details of the aspherical oscillation of two bubbles are shown by the analysis of 𝑎2(𝑡) and 𝑏2(𝑡) that describe the surface deformation of bubbles 1 and 2, respectively.

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The driving parametric regions in frequency-amplitude space and the optimal parameters for single-bubble sonoluminescence (SBSL) in alcohol aqueous solutions are studied systematically by taking measurements of the spectrum and bubble dynamics. The experimental results show that with an increase in alcohol concentration, the region shrinks and shifts. The optimized parameters differ for alcohol solutions having different concentrations, and SBSL driven by fixed parameters dims quickly and is even destroyed immediately with the addition of a small amount of alcohol to pure water. Furthermore, it is seen that the intensity of optimized SBSL decreases as the alcohol concentration increases. The corresponding measurements of the dynamics of the optimized SBSL bubble show that the maximum bubble radius at an alcohol concentration of 1.04 mM is only half that for pure water. Meanwhile, the optimized driving amplitude acquired by direct measurement and that obtained by fitting the radius-time curves with the Rayleigh-Plesset equation both decrease by 12% in the same comparison. Therefore, a decrease in the driving acoustic pressure may be an important reason for the decrease in the optimized SBSL intensity, which should help clarify SBSL mechanisms in alcohol aqueous solutions.