Active noise control in ducts in presence of standing waves. Its influence on feedback effect

Much of the theoretical and experimental work which has been carried out in the area of active noise control in ducts has neglected the fact that a standing wave is formed in them. A feedforward strategy of control is the most widely used in ducts. Nevertheless, it presents a feedback problem, which can make the system unstable by corrupting the reference signal. Although there are some different solutions to this effect, most of them need special arrangements or algorithms. A model, which demonstates that the acoustic field in the duct is formed by standing waves before and after the active control, is developed and from it, the optimal location of the secondary source to avoid feedback is derived. This result is experimentally validated for a pure tone noise in a duct, and then applied to attenuate fan noise propagating in the same duct. In addition, experimental work has provided some extra information about the location of the error microphone and the usefulness of its signal to evaluate the active noise control system performance.