Abstract: The active control mechanism of the structure-borne noise within an irregular enclosure is analyzed by comparing the variations of the amplitudes and the phases of the modal coordinates of two panels and interior sound pressure before and after control with two control performance functions. Results obtained demonstrate that at most disturbance frequencies, the sound pressure level within the enclosure is reduced by rearranging the modes of the two panels. When the disturbance frequency is merely one of the natural frequencies of panel a, the control voltage is applied to reduce the dominant mode of panel a and to adjust the mode arrangement of panel b respectively with the result of the decrease of sound pressure level within the enclosure.