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Modal Filtered-x LMS algorithm is an adaptive algorithm to reduce global level of noise in vibro-acoustic cavities like car, aeroplane, room and other transportation equipment etc. The global level of noise is quantified by global acoustic potential energy of a vibro-acoustic cavity. For a strongly coupled vibro-acoustic cavity, global acoustic potential energy can be written in terms of modal amplitudes of coupled modes of a vibro-acoustic cavity whereas for a weakly coupled vibro-acoustic cavity, global acoustic potential energy can be written in terms of modal amplitudes of rigid walled acoustic modes. In the M-FxLMS algorithm, the weights of the adaptive filter are updated to minimise global acoustic potential energy expressed in the modal domain. This formulation evolves the concept of modal secondary paths and modal filtered reference signals. Modal secondary paths are modal impulse responses with respect to a particular secondary source. Modal impulse response depicts the variation of modal amplitude of a particular mode when a secondary source is driven by an impulse. The modal filtered reference signals are obtained by convolution of reference signal with modal secondary paths. The modal secondary path and modal filtered reference signal in M-FxLMS algorithm are analogous to physical secondary path and filtered reference signal in the conventional FxLMS algorithm. This paper investigates the identification techniques of modal secondary paths. Two types of secondary sources can be used to control global level of noise: acoustic and structural. This paper discusses the behaviour of modal secondary paths corresponding to acoustic and structural secondary sources for a strongly coupled cavity and a weakly coupled cavity. A numerical study is conducted in a car-like vibro-acoustic cavity. The variation of modal secondary paths corresponding to an acoustic control source and a structural control source are presented. Further, the performance of M-FxLMS algorithm with only a structural control source and with both an acoustic and a structural control sources is studied. Both an acoustic disturbance acting inside the cavity and a structural disturbance acting on the flexible panel of the cavity are considered. The study is conducted at three frequencies: a cavity controlled resonance, a panel controlled resonance and an off-resonant frequency.
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