AbstractModal filter concept is introduced for structural vibration analysis and control. The modal filter is a means to extract the modal coordinates of each mode from the system outputs by mapping the response vector from the physical space to the modal space. This concept was used originally to deal with the spill‐over problem in the control of a continuous system. We have chosen to optimize some semi‐distributed piezo‐electric layers by minimizing the influence of unwanted modes on the sensors and actuators. Confirmation of our computation was obtained by an experimental set‐up of optimal active modal control on a thin composite plate. Thus, experimental optimal control in the independent modal space of this plate has been possible without any effect of spill‐over. The efficiency and the robustness of this procedure was demonstrated by our experimental