Wave Field Synthesis (WFS) provides a means for reproducing 3D sound fields over an extended area. Beyond conventional audio reproduction applications, present research at IRCAM involves augmenting the realism of concert-based applications in which real musicians will be interacting on stage with virtual sources reproduced by WFS. The stake of such a situation is to create virtual sound sources which behave as closely as possible to real sound sources, in order to obtain a natural balance between real and virtual sources. The goal of this article is to point out physical differences between real sound sources and WFS reproduced sources situated at the same position, considering successively the sound field associated to the direct sound of the virtual source and its interaction with the room. Methods for taking into account and compensating these differences are proposed.