Abstract: Notwithstanding practical limitations imposed by mobile device platforms and applications, truly captivating musical instruments may be synthesized in ways that allow musically expressive performances to be captured and rendered in real-time. Synthetic musical instruments that provide a game, grading or instructional mode are described in which one or more qualities of a user's performance are assessed relative to a musical score. By providing a range of modes (from score-assisted to fully user-expressive), user interactions with synthetic musical instruments are made more engaging and tend to capture user interest over generally longer periods of time. Synthetic musical instruments are described in which force dynamics of user gestures (such as finger contact forces applied to a multi-touch sensitive display or surface and/or the temporal extent and applied pressure of sustained contact thereon) are captured and drive the digital synthesis in ways that enhance expressiveness of user performances.