| Active noise control (ANC) is a noise reduction technique which generates secondary controllable sound to act with the primary noise source or field to reduce the original sound pressure level. The advantages of the ANC are smaller in size, lighter in weight and easier to control compared to the traditional passive noise control method, therefore the ANC system is widely used in many noise control applications especially at low frequencies.This thesis aims at the configuration optimization of the physical system in ANC of the transformer. Firstly, a simplified model is built to simulate the noise radiation from the transformer. The distribution of the control sources and error senses is discussed as well as their effects on the performance of the ANC system. Then the genetic algorithm is employed for the placement opitimization of the control sources and the error sensors. Better noise reduction is achieved after the position optimization.In addition, the performance of the active control system near two reflecting surfaces is investigated as the transformer is usually surrounded by the walls in practical application. The point source is used to present the transformer for simplification. Analytical sound power expressions of a point source and those of two point sources near two surfaces are derived. The output power of the active control system with two surfaces is also calculated, and the performance of the active noise control system with two reflecting surfaces is compared to that with one surface. The optimal position of the control source is explored and the reason is explained from the viewpoint of the sound pressure distribution.Finally, the effect of the reflecting surface on the number requirement of the control sources in multichannel active noise control system is studied in this thesis. Less control sources is required in half space than in the free field in order to achieve a specific noise reduction after optimal configuration. Physical explaination from the perspective of the distribution of the sound field is performed. |
