This paper describes a special perceptually based spectral transformation which takes into account basic properties of the auditory system. The main feature of this spectral transformation which has been especially designed as preprocessing stage for an automatic speech recognition system is the possibility of simple recursive calculation. Aurally adequate spectrum analysis is performed by a modified Fourier-t-Transformation which allows the adjustment of time and frequency resolution to the frequency dependency of the human ear. Psychoacoustic data is the basis for these resolution capabilities. Keywords: Speech Signal Preprocessing, Auditorily Based Spectral Transformation, Fourier-t-Transformation, auditory model