This thesis is focused on theoretical and experimental studies regarding semi-active control of the noise and vibration response of a thin cylindrical shell using electromagnetic transducers. The motivation of this research is the increasing interest in low cost, low energy consumption and low weight practical solution to the noise and vibration problems encountered in transportation vehicles such as cruise ships, trains, aircraft and cars.
Initially, a fully coupled structural-acoustic model of a cylindrical shell and acoustic enclosure, based on the Modal-Interaction-Model, is presented, which derives flexural displacement from a modal expansions of the in vacuo flexural modes of a simply supported cylinder and the interior acoustic pressure field from a modal expansion of acoustic natural modes for the rigidly-walled cylindrical cavity. An energy formulation is adopted to describe the flexural and acoustic response of the system subject to a rain-on-the-roof stochastic excitation. A systematic convergence analysis aimed at finding the natural modes that should be included in the modal expansion for the sound pressure response of the cylindrical enclosure and for the flexural response of the cylindrical wall is presented.
Then, the effects of classical mechanical fixed Tuned Vibration Absorbers (TVA) are assessed with a low-frequency simulation study. The tuning criteria of the classical fixed TVA are first recalled and a reduced structural-TVA model that considers only one natural structural mode was used to derive general guidelines regarding the positions of vibration absorbers on cylindrical structures.
The last part of the thesis is devoted to the time-varying shunted electromagnetic absorbers. An electro-mechanical analogy study, which lead to the design of the RL shunt circuit, is presented. The shunted electromagnetic absorber is operated in the sweeping mode, in which the values of shunt elements are continuously varied as to harmonically vary the stiffness and damping properties of the absorber so that its mechanical fundamental natural frequency is continuously swept in a broad frequency range which correspond to the frequency range of interest whereas its mechanical damping is continuously adapted to maximise the vibration absorption from the hosting structure. In this operation mode the time-varying shunted electromagnetic absorber produces a broadband control of the cylinder flexural response and of the interior noise without need of tuning and system identification of the structure.
