Frequency modulation characteristics of sustained /a/ sung in vocal vibrato

Frequency modulation characteristics of sustained vowel phonations in vocal vibrato were investigated. Eight male singers produced sustained /a/ in vibrato at low-, middle-, and high-pitch levels with comfortable loudness. The recorded voice samples were digitized and analyzed by a program yielding a plot of fundamental frequencies (Fo) of individual fundamental cycles. Modulation frequency, extent, rates of Fo increase and decrease, and modulation jitter and modulation shimmer were measured for individual modulation cycles. Central tendency and variability of these measures, intercorrelations among these measures, and temporal patterns of frequency modulations were investigated. Results indicated (1) significant effects of pitch levels on modulation frequency, (2) more regularity in modulation frequency than extent, (3) predominantly linear temporal patterns of frequency modulation, and (4) faster Fo increase than decrease. Implications of these findings for the nature of underlying mechanisms of frequency control in vocal vibrato are discussed.