1. A method of speech analysis comprising: representing a length of speech as a temporal sequence of frames, with each frame representing speech sounds at one of a succession of brief time periods; analyzing each frame of speech to obtain a plurality of spectral parameters, each of which represents the energy at one of a series of different frequency bands; finding the difference between the energy of a given spectral parameter of a given frame and the energy, in a nearby frame, of a spectral parameter associated with an energy band which is close to, but different than, the frequency band represented by said given spectral parameter; and using that difference to calculate a slope parameter which provides an indication of the extent to which the frequency of the acoustic energy in the part of the spectrum represented by said given spectral parameter is going up or going down.