In order to control the noise in the cavity, a careful analysis of the vibration of the plate and sound field in the cavity are necessary. The deformation of the flat plate was approximated by the orthogonal polynomials and inserted into the vibration equation of the plate and the velocity potential equation in the cavity. As the boundary conditions, the velocity of the plate is assumed to be equal to the velocity of the adjacent particles to the plate. For the partially opened cavity structure, the above results were superimposed on the sound transmitted through the cutout. The experimental system was constructed as a rectangular cavity having a 1-mm-thick steel plate, which transmits the external sound. In the case of the flat plate having a cut-out, the theoretical study appeared to agree with the experiment. While the area of cutout is less than 1% to the whole surface area of the flat plate, no significant change was observed in the vibration characteristics of it. For the case of a small cut-out, the acoustic modes show no significant difference for the case of no cutout, in spite of the large difference of the sound-pressure level.