A simple beam-plate model is set up whose dynamic deformations are characterized by typical mid-frequency vibration behavior. The interface uncertainty effects on the vibro-acoustic response of the rib-stiffened plate system are then investigated numerically based on the beam-plate model in the first instance. It is found that the uncertainty in the number of interface degrees of freedom (DOFs) can generate more significant dynamic effects on the built-up system than on the interface locations. The effects of the interface uncertainty increase as the dynamic mismatch between the beam and plate increases. This suggests that in the case of large dynamic mismatch between ribs and plates, the uncertainty problems in the number of interface DOFs should be avoided as much as possible in order to improve the accuracy and validity of the hybrid FE/SEA method.