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.