1. A non-intrusive method of assessing the quality of a first signal carrying speech, said method comprising the steps of: analyzing said signal carrying speech to generate output parameters according to a spectral representation imperfect vocal tract model capable of generating coefficients that can parametrically represent both speech and distortion signal elements, and weighting the output parameters according to a network definition function to generate an output derived from the weighted output parameters, the network definition function being generated using a trainable process, using well conditioned and/or ill-conditioned samples of a test signal, modeled by imperfect the vocal tract model.