This paper proposes a spectro-temporal analysis based on a bank of cochlea filters in combinaison with a nonlinear operator for amplitude modulation enhancement in the medium and high frequency formants. The output of the spectro-temporal analysis is represented as a 3D image where it is possible to observe very short-term speech transitions and formant modulations. With such analysis, it is possible to obtain patterns characteristics of phonemes and transitions betwen phonemes, which can not be obtained by using other speech analysis ( FFT, LPC ) techniques. The paper presents 3D images of vowels, where the amplitude modulation of the formants is clearly visable.