Active-Noise-Control (ANC) is a method for reducing unwanted so-called primary sound by destructive interference with sound generated by so-called secondary sources (loudspeakers). The latter are specifically designed to cancel the primary noise. The design requirements of ANC-systems are related to the theory of reducing the sound power of a primary source by monopole secondary sources.
In this paper the primary source is a vibrating simply supported plate which is radiating sound into free field. The loudspeakers are placed in front of the plate. The optimal volume velocities for the sound reduction are calculated by minimizing the radiated sound intensity in front of each monopole source in order to minimize the total radiated sound power of the primary and secondary sources. The analytical calculations are carried out with one and two loudspeakers. The optimal position of the secondary sources in front of the plate is studied in consideration of a high sound reduction.