The pitch period for the same speaker varies with respect to time as well as varies across speakers. This clearly illustrates the variability of speech signal.

For a given sound-unit, the formant frequency values vary slightly for the same speaker. This illustrates the variability of the vocal tract system characteristics within the same speaker.

For a given sound-unit, the formant frequency values vary slightly across different speakers. This illustrates the differences in the vocal tract system characteristics across speakers, inspite of their articulation and perception as the same sound. This observation is useful to discriminate speakers. But the formant frequencies of a given sound unit are more similar across different speakers, than across sound units within the same speaker. This observation is useful for speech recognition studies.