We propose methods to track natural variations

in the characteristics of the vocal-tract system from speech

signals. We are especially interested in the cases where these

characteristics vary over time, as happens in dynamic sounds

such as consonant-vowel transitions. We show that the selection

of appropriate analysis segments is crucial in these methods, and

we propose a selection based on estimated instants of significant

excitation. These instants are obtained by a method based on

the average group-delay property of minimum-phase signals.

In voiced speech, they correspond to the instants of glottal

closure. The vocal-tract system is characterized by its formant

parameters, which are extracted from the analysis segments.

Because the segments are always at the same relative position

in each pitch period, in voiced speech the extracted formants are

consistent across successive pitch periods. We demonstrate the

results of the analysis for several difficult cases of speech signals.