This study is concerned with reducing the tonal noise emitted by electric ducted fans which serve as the propulsion system for a variety of unmanned aerial vehicles (UAVs). The underlying concept is utilizing the shroud pressure fluctuation caused by the rotating blades to drive the over-the-rotor (OTR) acoustic treatment and generates a secondary sound field that cancels the primary ducted fan sound field. Experiment shows a considerable destructive effect can be made by meticulous tuning of the strength and phase of the secondary source. In view of the space constraints of a real electric ducted fan, the acous-tic treatment is reconfigured as a labyrinth-type acoustic metamaterial that reduces the structure size while maintaining the noise reduction performance at the same time. An advantage of the proposed tech-nique is that each tone can be dealt with by a specifically designed acoustic treatment occupying a small patch on the fan shroud. Hence, a compact multi-tonal noise control solution is possible by circumferen-tially deploying several OTR acoustic treatments.