Abstract:
There are many different kinds of energy in the surrounding environment and there is a great demand for renewable energy in life. Ubiquitous sound source in the environment could be harvested as a renewable energy source through certain energy conversion mechanism. The purpose of this thesis is to investigate the capability of acoustic energy harvesting under the action of nonlinear force by using electret material. In the literature, quarter-wavelength-resonator based on sound energy harvesting only has one resonance. This project uses the membrane-coupled quarter-wavelength resonator to exploit the multiple resonances for more efficient energy harvesting. In the experiment, the proposed membrane-coupled quarter-wavelength resonator is used to amplify the sound pressure at two frequencies. The electret material is then used to convert kinetic energy of the membrane excited by sound into electricity. The experiment using this proposed structure shows two resonances, but the positions of these two resonances are difficult to adjust. In order to make the two resonances tunable, permanent magnets are used which can not only implement the adjustment of two resonances but also introduce the effect of nonlinear force on bandwidth for sound energy harvesting. The linear experiment was first tested to investigate the best performance of electric power generation with different circular membranes. The effect of stiffness by changing the tension that applied on membrane is demonstrated in the experiment. The experiment shows that the higher stiffness of membrane has higher natural frequency. Also, the membrane with medium degree of tension is found to possess best energy harvesting capability. The nonlinear experiment showed that the larger section area of membrane could exhibit an evident nonlinear phenomenon around resonances given a relatively large incident sound pressure. Nonlinear experiment also investigated the effect of magnetic force on energy harvesting, bandwidth and the tuning of resonances. An obvious change was when nonlinear magnet force acted on the circular membrane, there was a huge decrease of voltage output and bandwidth of resonance. The natural frequency of the membrane is adjusted because the magnet force changed the tension of the membrane. Although the results indicate that the magnet reduced the power and bandwidth, the magnet is still very helpful for adjusting the frequency position of the resonance.