An important feature of the piano simulations used in this project is its ability to account for reproducing the effects of large string motion (See References [12] and [14]). This is particularly relevant for those historic instruments where low tension induce large amplitude of the strings. When subjected to large amplitude, the transverse motion of the string is coupled to its longitudinal motion, and the tension is modulated due to the continuous change of length. This coupling has several consequences: one of the most spectacular is the presence, in the spectrum of soundboard motion and acoustic pressure, of additional frequency peaks which are not predicted by the linear theory of strings. These frequencies are commonly designated as the "phantom partials", and they are part of the timbral properties of piano tones.

Taking such geometrical nonlinearities of strings into account requires the use of a rather complex model on nonlinear strings which, in turn, necessitates the development of specific numerical techniques for solving the equations [Reference [12]). More recently, it was found that the temporal evolution of the string motion also depend on their boundary conditions. The usual "zig-zag" end condition, for example, induce a coupling between the vertical and horizontal polarization of the strings (Reference [20]). Simulation of this coupling is still a work in progress.