A technique for controlling noise transmitted into the interior of a cavity involves use of point force actuators on the boundary structures. This paper is an analytical study of the nature of the control mechanism. A rectangular panel–cavity system has been used as a model for this study. The test panel was selected so that the resonance frequencies are well separated. The responses of the structural vibration and the cavity sound field are examined under both controlled and uncontrolled conditions. Results obtained demonstrate that there are two different control mechanisms. If the system response is dominated by panel-controlled modes, sound energy in the cavity is minimized by suppressing the panel modes that radiate into the cavity. If the system response is dominated by cavity-controlled modes, the control force is used to change the panel velocity distribution and thus adjust the radiation of each panel mode. In this latter case, the real part of the sound power radiation into the cavity is minimized, but there may be an increased local reactive intensity field and increased panel-vibration level.