Featured Application: Active double-panel sound insulation structure can be applied as potential noise control technic in aviation and navigation to actively control the low frequency sound transmission into the cabin.Abstract: Physical mechanisms of active control of sound transmission through orthogonally two ribs stiffened double-panel structure are investigated. This is the continued work of the single rib stiffened case. For the orthogonally two ribs stiffened case, four different cluster mode groups can be coupled with each other, due to the interlaced coupling effects of the horizontal and vertical ribs. One cavity mode can couple with and transmit sound energy to any type of base plate mode of the radiating ribbed plate. Consequently, the main differences of the control mechanism, when compared with the single ribbed case, lie in two aspects. One is that a novel mechanism appears. That is, suppressing and rearranging the cavity modes simultaneously achieves the suppression of the base plate modes. The other is that rearrangement of the cavity modes to rearrange the base plate modes for achieving sound radiation cancellation almost does not appear. The reason is that all types of cavity mode can couple with any one of the base plate modes due to the coupling effects of the two ribs. There is only a need to rearrange several important cavity modes to achieve suppressing the base plate mode of the radiating ribbed plate. Appl. Sci. 2019, 9, 3286 2 of 17 pay close attention to distinctly explore the elementary physical nature of active control. However, the double-layer cabin walls are usually the classical orthogonally n-ribbed plate. In order to improve the engineering relevance of the analytical research, this study further presents a more practical structure which refers to the orthogonally two ribs stiffened double-panel system. Hence, this research is continued from the work in Ref. [8]. The aim of this paper is to further explore the active control mechanisms of such a structure, which may support engineering application of this technique in the future. The orthogonal ribbed case is only concerned with one horizontal and one vertical rib. Although such a structure is not in accord with the engineering applications, it seems to be more practical. The purpose of this is to simplify the investigation. Understanding the physical nature of active control in orthogonal ribbed cases may provide better understanding of complicated mechanisms involved in an orthogonal n-ribbed case [9].Many techniques [10][11][12][13][14][15] have been proposed to accurately predict free vibration of the ribbed plate. Some other techniques [2,16] have been also applied to model complicated double-panel structures composed of rib stiffened plates. These contributions deepen the comprehending of the vibration characteristics of the rib stiffened plate and rule of sound energy transmission through these structures, which offers assistance for noise and vibration control engineering. One new modeling approach proposed recently i...