No abstract

A new sound energy harvesting system that comprises an acoustic metastructure and a double-clamped piezoelectric bimorph plate is presented. A subwavelength-scale acoustic metastructure is specially designed by utilizing a doubly coiled-up configuration for the strong confinement of the sound energy inside the acoustic metamaterial cavity in the low frequency range. The amplified sound pressure in the acoustic metamaterial cavity then substantially increases the vibratory motions of the piezoelectric bimorph plate. For the purpose of generating more electricity from the sound, the resonant frequency of the acoustic metamaterial cavity is tuned to the fundamental frequency of the piezoelectric bimorph plate. The numerical and experiment results show that the proposed sound energy harvesting system increase a sound pressure level (SPL) by up to ∼16 dB, and yields an output voltage that is 6.32 times higher than that of the conventional piezoelectric energy harvesting plate. For the incident SPL of 100 dB, the maximum output power is measured as approximately 0.345 μW at a resonant frequency of 600 Hz.

Various mathematical beam models have been proposed for the efficient analysis of a piezoelectric energy harvester (PEH) and carrying out parameter study but there appears no beam model suitable for a PEH of a moderate width-to-length aspect ratio with a distributed tip mass, and so, moderate width-to-length aspect ratios and distribution effects of a tip mass over a finite length will be mainly focused on in the present beam analysis. To deal with a wide range of aspect ratios, the material coefficients appearing in the constitutive equations of a PEH beam will be interpolated by those of the limiting plane-strain and plane-stress conditions. The key idea in the interpolation is to derive the interpolation parameter analytically by using the fundamental frequency of a cantilevered beam of moderate aspect ratios. To deal with the distribution effects of a tip mass over a finite length, the use of a set of polynomial deflection shape functions is proposed in the assumed mode approach. The equations to predict the electrical outputs based on the proposed enhanced beam model are explicitly expressed in template forms, so one can calculate the outputs easily from the forms. The validity and accuracy were checked for unimorph and bimorph PEHs by comparing the results from the developed beam model, the conventional beam model, and a three-dimensional finite element model. The comparisons showed substantial improvements by the developed model in predicting the electrical outputs.

We present the design, including an experimental demonstration, of an acoustic metamaterial panel aimed at reducing incoming broadband noise in the audible frequency range while allowing an incoming fluid to pass through the holes formed in the panel. The noise reduction performance of the proposed acoustic metamaterial panel is attributed to an array of annular cavities enclosing the fluid passage holes. The acoustic behavior of the acoustic metamaterial panel is theoretically analyzed by the transfer matrix method, and an equivalent acoustic impedance of each annular cavity is included with its effective length in the derived transfer matrix. The effective bulk modulus for the acoustic metamaterial panel is then extracted from the transmission and reflection coefficients by using the retrieval method. It is shown that the frequency range of the negative effective bulk modulus coincides with the stop band of the acoustic metamaterial panel. The underlying physical mechanism for the negative effective bulk modulus is attributed to the out-of-phase motion of vibrating particles in adjacent annular cavities. A calculated transmission coefficient curve of the acoustic metamaterial panel is shown to be in good agreement with the measured one. The findings presented in this work should be useful in the design of a holey soundproof panel.

A small-form-factor piezoelectric vibration energy harvester using a resonant frequency-down conversion While environmental vibrations are usually in the range of a few hundred Hertz, small-form-factor piezoelectric vibration energy harvesters will have higher resonant frequencies due to the structural size effect. To address this issue, we propose a resonant frequency-down conversion based on the theory of dynamic vibration absorber for the design of a small-form-factor piezoelectric vibration energy harvester. The proposed energy harvester consists of two frequency-tuned elastic components for lowering the first resonant frequency of an integrated system but is so configured that an energy harvesting beam component is inverted with respect to the other supporting beam component for a small form factor. Furthermore, in order to change the unwanted modal characteristic of small separation of resonant frequencies, as is the case with an inverted configuration, a proof mass on the supporting beam component is slightly shifted toward a second proof mass on the tip of the energy harvesting beam component. The proposed small-form-factor design capability was experimentally verified using a fabricated prototype with an occupation volume of 20 × 39 × 6.9 mm 3