Active Control of Radiated Sound from Ducts

The noise radiated by duct and pipe systems is modeled in the laboratory using a circular duct driven by a speaker at one end. Active control is achieved using a control speaker located adjacent to the open end of the duct. The objective of the investigation was to minimize the total sound power radiated by the duct and a single secondary source. However, the adaptive algorithm used by the controller for this investigation seeks to cancel the acoustic pressure only at a single location; that is, the location of the “error” microphone. Analytical studies predict that in order for the total sound power radiated by primary and secondary sources to be minimized, a single secondary source must radiate sound which is of approximately equal magnitude and opposite phase to the noise source and the error microphone must be placed somewhere in the plane of minimum pressure of an ideal acoustic dipole. These analytical results were verified in part, for the case of the secondary source facing the same direction as the primary source in the plane of the pipe outlet. Other cases were studied where the control speaker was located outside the plane of the duct outlet. The performance improved for these alternative orientations for a wide range of error microphone positions. Measured sound power with and without active control is shown for a range of frequencies and error microphone locations for three configurations of the duct and control source.