Fluctuations and Perturbations of the Voice
What are fluctuations and perturbations?
The terms fluctuation and perturbation are used in voice science to describe disturbances, or changes in the output of the voice. A perturbation is generally understood to be a small, temporary change in the vocal system; a fluctuation is a more significant change and tends to indicate that the voice is somehow unstable. Here are some descriptions of common terms used to describe various kinds of vocal fluctuations and perturbations. It should be noted that these truly are descriptive terms, rather than precise scientific measurements.
Jitter: Short-term changes in Fo (pitch). 'Short-term', in this context, means within a single cycle of vibration.
Shimmer: Short-term changes in amplitude (loudness).
Vibrato: A 'cultured' or artistic fluctuation, usually introduced purposefully by performer. Vibrato involves changes in pitch of +/- 0-3%, occurring 4.5-6.5 times per second (i.e., at a rate of 4.5-6.5 Hz). The vibrato rate can vary with many factors, including vocal intensity, the vocal conditioning of the singer, pitch, and the amount of pitch changes in the music being sung.
Trill: A deliberate attempt by a singer to alternate back and forth between a given base note, and the note either a half-step or whole-step above it.
Trillo: A rapid repetition of the same note; the voice stops and starts very quickly. This is done with the cricoarytenoid muscles; the vocal folds are rapidly approximated (held close together), then separated so that no vibration can occur, then approximated again, and so on.
The above definitions point to a grey area in the field of vocology. Some variation in voicing is desirable and considered to be importantly artistically (as vibrato in singing). Too much variability can lead to unintelligibility, however, and no variability (such as computer simulated speech) produces highly unnatural phonation. Refined quantitative assessment and good clinical judgment are certainly a part of future discussions as the field of vocology evolves.
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