Some waveform and spectral features of vowel roughness

Acoustic wave-period variation (jitter) and acoustic wave-amplitude variation (shimmer) associated with vowel phonations representing a range of vocal roughness were investigated. Twenty normal-speaking adult males phonated each of the vowels /u/, /i/, /v/, /a/, and /ae/, first normally and then with simulated abnormal vocal roughness. Twenty hoarse adult males, each presenting a medically diagnosed laryngeal pathology, also produced each of the five test vowels. To provide a measurable presentation of the frequency and amplitude variations of interest, each recorded vowel was band-pass filtered to isolate the fundamental frequency component. Relations of the jitter and shimmer indices (obtained from the filtered vowel waves) to acoustic spectral noise levels and to roughness ratings for the vowel phonations were studied. The findings supported the hypothesis that increases in vowel acoustic wave variability (estimated by period or amplitude variation or both) are associated with increases in vowel spectral noise levels and perceived vowel roughness. The findings also suggested, for most of the vowels studied, that cyclic peak amplitude variation may provide a better index of perceived roughness than cyclic period variation. Vowel spectral noise levels, however, may provide a more clinically useful indicant of vowel roughness than the waveform variability indices derived from the filtering procedure employed in this study.