Formants are properties of the resonant cavities, therefore it may be expected that formant bands may move slightly during the articulation interval of a vowel. In long vowels, especially in the case of English, the articulation may change and formants will move considerably. Therefore a criterion must be used to define a vowel against a non-vowel pattern, which may be formant stability over a given time interval. Such criterion should grant that vowels are separated from approximants and glides, as these have a consonantal role (semi-consonants). Therefore frequency slopes must be under a given value to separate modulated vowels from glides and approx- imants. This is done by special units as the ones in Eq. (10) and (11), by the Dynamic Formant Tracking and Temporal Clipping blocks. The purpose of these blocks is to delimit vowels, as illustrated in Fig. 14 (top template). The resulting delimited vowels are coded in color in the bottom template of the same figure.