To what degree of voice perturbation are jitter measurements valid? A novel approach with synthesized vowels and visuo-perceptual pattern recognition

Objective measurement of the severity of dysphonia typically requires signal processing algorithms applied to acoustic recordings. Since Lieberman (1963) introduced the concept of perturbation analysis in the area of voice, the best-known acoustic parameter in clinical practice is conventional jitter. However, jitter measurements have some critical limitations. According to a widely accepted guideline, in sustained vowels of dysphonic voices, only perturbation measures less than about 5% are reliable: this is related to period extraction methods. This limit of 5% deserves critical analysis, certainly when there are indications that some acoustic analysis programs can be applied to quite irregular voices such as substitution voices. The present experiment demonstrates that – on signals of synthesized deviant voices (sustained vowel) with moderate additive noise – different raters are able to visually identify in a very consistent way the period durations of successive cycles up to values of about 13% jitter. Furthermore, even for higher values – over 30% – the jitter % computed with the period values rated by visual perception is, for some of the raters, very comparable to the real value. This suggests that improved acoustic programs using more reliable algorithms could validly transgress the traditional limit of 5% if they demonstrate the correspondence of their computations with the true jitter values. This is now made possible by synthesizers generating artificial deviant voices that cannot be distinguished from true dysphonia, and in which the jitter put in is exactly known.