Abstract: Active control of low frequency sound radiation using planar secondary sources is theoretically investigated.The primary sound field originates from a vibrating panel and the planar secondary sources are modeled as simply supported rectangular panels in an infinite baffe. The sound power of the primary and secondary panels is calculated, and then a series of formulae are derived to obtain the optimum reduction in sound power. Finally, active reduction for a number of secondary panel arrangements is examined, it is concluded that when the modal distribution of the secondary panel does coincide with that of the primary panel, one secondary panel is sufficient. Otherwise four secondary panels can guarantee considerable reduction in sound power over entire frequency range of interest.