A system for reducing aerodynamic noise of a supplementary wing of an aircraft is provided. The supplementary wing is hinged to a main wing and is extendable when a gap region between the main wing and the supplementary wing is opened. The system includes a separating surface which can be moved into the gap region when the supplementary wing is extended and which extends at least partially along a separation flow line between a vortex flow region and a gap flow for the air flowing in the gap region. The separating surface is an n-stable surface which through an actuator device can be moved between at least one of the stable states and at least one additional state.