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 which 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.
Degrees of freedom
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.
Common 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.
- 10 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.
- 11 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).
- 20 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).
- 30 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.