This paper is concerned with feedback vibration control of a lightly damped flexiblestructure that has a large number of well-separated modes. A single active electricaldynamic absorber is used to reduce a particular single vibration mode selectively ormultiple modes simultaneously. The absorber is realized electrically by feeding back thestructural acceleration at one position to a collocated piezoceramic patch actuator via acontroller consisting of one or several second order lowpass filters. A simple analyticalmethod is presented to design a modal control filter that is optimal in that it maximallyflattens the mobility frequency response of the target mode, as well as robust in that itworks within a prescribed maximum control spillover of 2 dB at all frequencies.Experiments are conducted with a free–free beam to demonstrate its ability to controlany single mode optimally and robustly. It is also shown that an active absorberwith multiple such filters can effectively control multiple modes simultaneously.