Modalys is a sound synthesis software developed at Ircam for research and musical applications. This software allows one to build virtual instruments based on physical models to obtain the most entire range of expressive variations in the instrument in response to intuitive controls. An instrument, as a complex structure, is described by the mechanical/acoustical interaction of its components (strings, tubes, resonators, soundboard,...). Some new research have been done recently to extend the sound prediction to three-dimensional objects with the help of numerical methods. In particular, theoretical and numerical treatment of the unilateral and frictionless dynamic contact between two arbitrary elastic bodies was studied and highlighted by simulations implemented in Modalys.