An investigation of optimal power absorption as a noise control technique

A method is presented that is aimed at reducing the sound power radiated by a vibrating structure using a feedback control system based on optimal structural power absorption. This control strategy is intended for applications where a coherent measurement of the disturbance is not readily available and where the use of feedback control is necessary. The method would apply, for example, in the case of noise inside the aircraft that is generated by the turbulent boundary layer on the outer skin. Current feedback control strategies that are designed to reduce radiated sound power include active structural acoustic control and radiation modal control. Such techniques are designed using a-priori knowledge of the acoustic radiation characteristics of the structure together with structural sensors as control inputs. While the analytical and experimental results obtained to date for both techniques appear promising, a considerable drawback in both cases is the complexity of the controller design that results from the necessity of knowing the acoustic radiation characteristics of the structure. This may be difficult to obtain in practice.; In work described here, an optimal power absorbing control system is proposed which relies on only local vibration information. Such a system can be effective in reducing a structure's radiated sound power. The optimal controller provides a causally constrained impedance match between the control system and structure, maximizing the power flow between the two systems. The result of the impedance match is a reduction in the total energy in the structure and corresponding reduction in sound power radiation. The controller described is a single-input single-output (SISO) system that requires only local vibration information at the position of the co-located control sensor and actuator. The form of the controller is derived from the solution of the Wiener-Hopf equation.