Normal Patterns or "Modes" of Vibration
In the tutorial describing models
of oscillation, we explored the mechanism of
oscillation in the vocal folds, including the Bernoulli
effect and how oscillation can be sustained. But there's
more to the story - the details about the patterns in
the folds vibrate.
The wavelike motion of the
vocal folds during oscillation is described
scientifically in terms of 'normal modes'
of vibration. Each mode is a distinct way
in which the various parts of the vocal fold
can move during one vibrational cycle.
A simple, rigid mass-spring system has a limited number of possible modes of
vibration, depending on how many masses are involved and how many directions
they are free to move in (degrees of freedom).
By contrast, soft tissue,
such as that which makes up the vocal folds,
has infinite degrees of freedom, and thus,
an infinite number of possible 'modes' of
vibration. However, in practice, a small
number of dominant vibratory patterns tend
to appear over and over again.
modes of vibration for the human voice
It should be noted that no single mode is likely to represent the entire vibratory
pattern of the vocal folds at a given time. Different parts of the vocal folds
tend to vibrate in different ways simultaneously. Study of these patterns is
complicated by the fact that in a live subject, the vibration of the folds
can only be observed from above (using a laryngoscope). Thus, the observer
sees the top surface of the vocal folds, but cannot directly observe the lower
parts of the folds while they are vibrating.
The various modes of
vibration are each given a numeric label
consisting of two integers. The first integer
represents the number of lengthwise divisions
in the movement of the folds; the second
integer is the number of vertical (cross-sectional)
divisions in the movement. The four most
common modes observed in the human voice
are listed below, in order from the most
to the least common.
The patterns of vibration
are perhaps more easily seen than described
in words. Therefore, an animation for each
of the four modes may be viewed. These animations,
created by Dr. David Berry, who worked extensively
with vocal fold modeling at The University
of Iowa, are in mpg format and require a
media player to run.
mode: In this mode, amplitude
is at its maximum in the center of
the fold, and decreases gradually toward
the end points. This is the simplest
of all the modes.
mode: Adds vertical 'flexion',
or movement, in addition to the lengthwise
vibratory patterns seen in the other
three modes listed. In the 11 mode,
there is a half-wavelength pattern
of vibration in both the horizontal
and vertical plane (i.e., both lengthwise
along the folds and vertically in the
'thickness' of the fold).
mode: 20 mode: Here, the length
of the fold is 'split' in two; each
half vibrates as in the '10' mode above.
Note that the center of the vocal fold
is not vibrating at all (zero amplitude).
mode: : Again, similar to 10
and 20 above, but now, the vocal fold
is divided into three parts, each of
which vibrates as in the '10' mode.