Active control of amplitude or frequency modulated sounds in a duct

Modulated disturbances are often produced by noise sources in rotating machinery such as gears, bearings and fans. Such disturbances carry frequency side-bands, which interact with the main components (such as gear mesh or blade passage frequencies and their harmonics) to influence the perception of sound. Traditional active control schemes tend to attenuate the mean-squared error, while yielding a residual spectrum with more dominant side-band structures that could degrade sound quality. In this paper, we study the side-band phenomenon by applying active control to amplitude or frequency modulated sounds in a duct. An attempt is first made to attenuate these sounds using the conventional FX-LMS algorithm. This algorithm creates a residual noise spectrum where frequency side-bands become more prevalent. Next, the narrowband adaptive noise equalizer (ANE) is employed since it allows independent gain control at each disturbance frequency known a priori, even if they are closely spaced. The ANE algorithm is based on the adaptive notch filter concept, and thus complete cancellation at all disturbance frequencies is also possible. Indeed, superior control of the residual spectral shape is observed. A few areas of research that could make improvements to available algorithms, in the context of modulated noise disturbances, are suggested.