: Transfer functions for digital sound synthesis based on physical models have recently been presented. The method transforms a continuous model for the vibrating body, given by a partial dierential equation (PDE), into a multidimensional (MD) transfer function model (TFM). The TFM takes initial and boundary conditions, as well as excitation functions into account. It also treats the physical eects modeled by the PDE exactly. The algorithms obtained after discretization of the TFM preserve the inherent physical stability and are suitable for real-time implementations on digital signal processors. A recently presented example of a linear transversal oscillating tightened string with frequency dependent loss terms is extended here to a nonlinear vibration model. The nonlinearity of the string vibration is caused by an output dependent tension modulation. Because of the nonlinearity we only obtain an implicit equation instead of a TFM derived in the linear case. The nonlinear string mod..