Active noise control in large circular duct using an error sensors plane

Active Noise Control (ANC) of higher order modes in a circular duct is studied. To overcome the limitations of a modal approach to locate the error sensors, an alternative strategy, referred to as the error sensor plane concept, is proposed. The basic objective of the error sensor plane concept is to create a quiet cross-section in the duct so that the noise from the primary source cannot propagate over this section. To create this quiet cross-section, a network of error microphones is located in the cross-section of the duct. To determine the required number of error microphones and their location in the quiet cross-section, a model simulating a multi-channel ANC system for a circular duct is developed. Using this model, it was deduced that to achieve good control performance, the maximum distance between adjacent error sensors had to be minimized, as well as the distance between the error sensors and the duct wall. To determine the optimal location of the error sensors, the k mean algorithm is implemented. A scale model was used in a laboratory setup to verify the efficiency of the proposed strategy to control high order modes in a circular duct. The obtained results demonstrate that, for the control to be effective, the maximum distance between each error sensor, and the limit of its zone of influence should be less or equal to 1/3 of the wavelength for the frequency considered, i.e. when D max/ λ cf < 1/3.