Transitional phonemes are a class of voiced sounds that do not have stationary vocal tract shapes. The quasi- periodicity of the voiced sounds is viewable in the time domain but not as well in the frequency domain. This is because of the varying shape of the vocal tract which also result in very loosely defined formants.
There are two main classes of transitional phonemes. The first class is the diphthongs. These spectral envelopes of these sounds show transitional formants and the time domain plots show the periodicity due to the voiced nature of these sounds.
The second class is the semi-vowels.
This class is divided into two categories: glides and liquds.
Liquids exhibit an increasing frequency in the time domain which can
also be seen in the spectrograms of the words. A trait
exhibited by the liquids is the presence of anti-resonances in the
frequency domain. This explains why their smooth spectral
envelopes are "pulled down" towards zero at times. The glides
are also quasi-periodic in the time domain but do not exhibit anit-resonances.