The Modal Identification Approach to Multimodal Acoustic Cancellation in a Long Duct.

The cancellation of a multimodal acoustic signal within an infinitely long rectangular duct is examined. A rigorous theoretical base that is founded in duct acoustics is developed from a modal identification viewpoint. The performance of an ANC system is presented as a function of the volume velocity strengths of the cancelling loudspeakers. Conclusions regarding the number and placement of both cancelling loudspeakers and performance microphones are drawn. A multimodal AANC system is proposed and tested through computer simulation. The adaptation policy that governs the updating of the cancelling loudspeaker volume velocity strengths is discussed in terms of a step-size policy and a direction policy. Three step -size policies--constant, absolute-error, and squared-error --and three direction policies--random, same-as-last, and bunny-hop--are proposed and compared.