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The formants of speech sounds are usually attributed to resonances of the vocal tract. Formant frequencies are usually estimated by inspection of spectrograms or by automated techniques such as linear prediction. In this paper we measure the frequencies of the first two resonances of the vocal tract directly, in real time, using acoustic impedance spectrometry. The vocal tract is excited by a carefully calibrated, broad band, acoustic current signal applied outside the lips while the subject is speaking. The sound pressure response is analysed to give the resonant frequencies. We compare this new method (Real-time Acoustic Vocal tract Excitation or RAVE) with linear prediction and we report the vocal tract resonances for eleven vowels of Australian English. We also report preliminary results of using feedback from vocal tract excitation as a speech trainer, and its effect on improving the pronunciation of foreign vowel sounds by monolingual anglophones.