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Mechanical frequency up-conversion is a technique whereby a high-frequency oscillation is induced from a low-frequency source and aims to address inherent problems in low frequency vibration energy harvesting. This work presents a novel 1:2:6 internal resonances based frequency up-converting harvester in which the operation frequency is 6 times up-converted. Compared to traditional conceptual mechanisms (e.g. mechanical impact, mechanical plucking, impulsive acceleration), the proposed harvester can operate at much lower acceleration level, the missing of sound during the frequency conversion also reduces the energy consumption. Experimental measurements and theoretical simulations demonstrate that the proposed design can collect energy from ultra-low frequency (< 5 Hz) vibration sources. In addition, the harvester shows very high performance compared to the current state-of-the-art devices, a maximum 2.07 mW of average power is obtained from the fabricated prototype at the excitation frequency and level of 1.71 Hz & 0.19 g, respectively. [ABSTRACT FROM AUTHOR]

This paper proposes a modified theory for synthesizing the acoustic impedance of an absorbing panel by considering the interference among parallel absorbers. The absorbing panel comprises periodically distributed absorbers with different acoustic characteristics, and the periodic distribution allows the impedance of the panel to be characterized by that of a unit cell. However, at frequencies between the resonances of the absorbers in the unit cell, the unit-cell impedance given by traditional theory deviates significantly from that given by a finite-element model. Inspecting the flow field near the surface of the unit cell reveals that the out-of-phase flow plays an important role in the interaction among the parallel absorbers and induces the deviation. A modified theory is proposed by multiplying the original resultant impedance by a factor that considers the interaction. The modified theory is verified by numerical results for several typical absorbing panels with different patterns of unit cells and different geometrical parameters of absorbers, and experimental validation is also carried out. As further evidence for the correctness and universality of the modified theoretical model, a comparison is presented with the mutual-radiation-impedance theoretical model based on a Helmholtz resonator array panel. The results of validation on different absorbing panels and the comparison with the mutual-radiation-impedance method show that the modified theoretical model is better at predicting the absorption coefficient than is the traditional theory. [ABSTRACT FROM AUTHOR]

Piezoelectric fibers of composition 0.55Pb(Ni1/3Nb2/3)O3–0.45Pb(Zr0.3Ti0.7)O3 were fabricated by extrusion of Pb(Nb,Ni)O3–Pb(Zr,Ti)O3 (PNN–PZT) powder around a platinum wire. Their properties are compared with the same bulk material properties. Fiber’s ceramic component exhibits a d31 of -145 pC/N and a lateral coupling factor of -0.22. The bulk material properties are much higher with a d31 of -404 pC/N and a lateral coupling factor of -0.38. The origin of this discrepancy was investigated by different means: microstructure (grain size and perovskite phase ratio), density, and polarization limitations. The polarization is the main limitation. Due to the radial electric field, a radial polarization gradient is obtained that limits the polarization on the outer edge. The calculation of the expected properties are then found to be k31=-0.28 (homogenization of the properties’ gradient). The remaining discrepancy is then assumed to be due to a density problem that can explain the 30% property loss. Density effect on piezoelectric properties of PNN-PZT bulk ceramics is presented to validate this assumption. [ABSTRACT FROM AUTHOR]

The acoustic black hole (ABH) effect is realized in thin plate structures with a decreasing thickness according to a power-law function, and offers potential applications for structure vibration damping enhancement and free-field noise radiation suppression. In this paper, a wavenumber domain method (WNDM) is proposed for the analysis of vibro-acoustic coupling and internal noise reduction mechanism of a pentahedral cavity enclosed by a flexible plate with a two-dimensional ABH indentation, subject to a point force excitation. The system response of the ABH plate-cavity is computed by a validated finite element model. The relationship between the space-averaged sound energy inside the cavity and the spectra of the structural displacement and the acoustic mode of the cavity is established. This allows revealing a dual physical mechanism behind the observed noise reduction: amplitude reduction and mismatching between the wavenumber spectra of the plate displacement and the acoustic field, which results in a weakened vibro-acoustic coupling. An additional configuration with an ABH embedded in an irregular pentagonal wall of the cavity is examined. Despite the increasing complexity in the geometry of the coupling interface and its coupling with the cavity, numerical analyses confirm the generality of the observed physical phenomena and the applicability of the proposed WNDM to more complex system configurations. [ABSTRACT FROM AUTHOR]