Non-monotonic spectral transitions between successive phonemes

In fluent speech one would expect that the transition between two successive acoustic targets would exhibit either a monotonic increasing or a monotonic decreasing trajectory in the time domain of each spectral parameter. Closer examination reveals that this is not always the case. This study investigates the existence of non-monotonic trajectories found in the acoustic spectral domain at the transition between some successive phonemes. This non-monotonic behavior consists of transitional regions that exhibit either increasing and then decreasing or decreasing and then increasing trajectories for some acoustic spectral parameters. Using the superposition principle, the training part of the TIMIT acoustic-phonetic database is used to build 2471 diphone trajectory models, based on the 61 symbols used in the database for phonetic transcription. Various non-monotonic trajectories are found in some of these models for some spectral representations including the linear prediction coding (LPC) parameters and the mel-frequency cepstral coefficients (MFCC). In some cases these spectral peaks or valleys between phonemes have significant amplitudes reaching approximately the standard deviation at the center, positions of the adjacent phonemes. These non-monotonic trajectories cannot be explained by the Stevens quantal theory of speech in which abrupt changes appear at transitions between two acoustic targets.