We propose a system for controlling in real time the localisation of sound sources. The system, called MidiSpace, is a real time spatializer of Midi music. We raise the issue of which interface is the most adapted for using MidiSpace. Two interfaces are proposed: a 2D interface for controlling the position of sound sources with a global view of the musical setup, and a 3D/VRML interface for moving the listener's avatar. We report on the design of these interfaces and their respective advantages, and conclude on the need for a mixed interface for spatialization. Active Listening We believe that listening environments of the future can be greatly enhanced by integrating relevant models of musical perception into musical listening devices, provided we can develop appropriate software technology to exploit them. This is the basis of the research conducted on "Active listening" at Sony Computer Science Laboratory, Paris. Active Listening refers to the idea that listeners can ...