Active noise control in ducts in presence of standing waves. Its influenceon 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 st anding wave is formed in them. A feedforward strategy of control is the mos t 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 specia l arrangements or algorithms. A model, which demonstates that the acoustic field in the duct is formed by standing waves before and after the active c ontrol, is developed and from it, the optimal location of the secondary sou rce to avoid feedback is derived. This result is experimentally validated f or a pure tone noise in a duct, and then applied to attenuate fan noise pro pagating in the same duct. In addition, experimental work has provided some extra information about the location of the error microphone and the usefu lness of its signal to evaluate the active noise control system performance . (C) 2000 Elsevier Science Ltd. All rights reserved.