Abstract. This paper presents the development of a low frequency vibration energy harvester based on the electromagnetic principle integrated with low threshold voltage diode multiplier circuitry. The harvester is designed to be modular and scalable based on a single electromagnetic generator. In this design four individual generators are arranged in series. The dimension of the device is 75mm x 140mm x140mm. Experimental results showed that a peak open circuit voltage of 1.6 V at 10 Hz and 0.8 g. Maximum power of 0.45 mW were extracted by the converter at resistive load of 5 kΩ. The power/acceleration density of the harvester is 600 μW/gcm 3 , which is the highest for any vibration energy harvester based on the electromagnetic principles reported.

A cylindrical micro-perforated panel (MPP) can be used to absorb the sound of a flow system in a circular duct of a vacuum cleaner. A cascaded cylindrical MPP is a special class type where two cylindrical MPPs are arranged in a series to improve sound attenuation. The manufacturing of MPP primarily involves the machining of micro-perforations, because small holes are not readily made using injection moulding due to the complexity of the die, flow control of the molten polymer through the small orifices and dimensional stability, making it unsuitable for mass production. This limitation can be overcome with the use of additive manufacturing (AM) technology, where the micro-perforations can be designed and manufactured, with relatively larger tolerances. Experimental validation ensures that the manufactured prototype in this study is performed according to design. Results show that the transmission loss of the model and the experimental outcomes agree. The cascaded arrangement of the cylindrical MPP results in a wider effective frequency range and an increased transmission loss. Parametric studies of the combined effects of the perforation diameter, perforation ratio and the depth of air cavity on the diameter of the duct and length ratio are conducted using a transfer matrix method. A case study is demonstrated here in the design. Moreover, an AM of cascaded cylindrical MPP is performed to attenuate peak noise at 1650 Hz, where the optimum parameters of the cascaded cylindrical MPP are obtained using a genetic algorithm. The manufactured cascaded cylindrical MPP is installed on a vacuum cleaner duct, and the measurement of sound power level shows a reduction of 4 dB(A).

This paper describes the boundary element method (BEM) and the experimental and optimisation studies conducted to understand the potential of expansion chamber coupled micro-perforated cylindrical panel (MPCP) in enhancing the acoustic attenuation for in-duct noise control issues. Owing to the complex structure of the MPCP and to achieve the correct prediction of acoustic attenuation, BEM is adopted on the basis of the Simcenter 3D software to compute the sound transmission loss (TL), As the MPCP is cylindrical in shape with numbers of sub-milimeter holes, additive manufacturing-based 3D printing is utilised for the model prototyping to reduce current design limitation and enable fast fabrication. The TL measurement-based two-load method is adopted for model validation. Subsequently, parametric studies of the MPCP concerning the perforation hole diameter, perforation ratio and depth of air space are carried out to investigate the acoustic performance. Optimisation via response surface method is used as it allows for evaluating the effects of multiple parameters as required in this study. The model validation result shows that the error between the BEM and the measured values is relatively small and shows good agreement. The R-square value is 0.89. The finding from the parametric studies shows that a wider peak attenuation can be achieved by reducing the perforation hole diameter, and one way to increase the TL amplitude is by increasing the air cavity depth. Finally, the optimised MPCP model is adopted to the commercial vacuum cleaner for verification. The sound pressure level of the vacuum cleaner is significantly attenuated within the objective frequency of 1.7 kHz.

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