The acoustic measurement of the resonances in the space between vocal folds and lips, the vocal tract, allows a non-invasive, objective analysis of the spectral energy distribution for different articulatory cases. Whereas a conventional LPC analysis is successful only when applied to more or less stationary voice signals, an external excitation with subsequent measurement of the vocal tract impedance at the mouth can give reliable results when the voice signal is not stable or even missing. This contribution adresses the problem how to compare results from impedance measurements with LPC measurements. For a set of normal speakers, both measurements have been performed simultaneously. Based upon the evaluation of the LPC curves, similar values for resonance frequency, amplitude and bandwidth were derived. In a study of 81 normal speakers the results from the evaluations are compared.