Simply stated, Fo is controlled by:
To fully understand the mechanisms of pitch control, it is helpful to first review the tissue composition of the vocal folds and to recall that different tissues types have unique biomechanical properties. The epithelium, the thin skin that covers the vocal fold, is a loose material that can't hold much tension. However, deeper vocal fold tissues - collagen, elastin and muscle - exhibit some resistance to stretching, but can become quite stiff.
During soft talking or falsetto singing, only the highly pliable cover of the vocal fold - the epithelium and underlying fatty tissue - is vibrating. Because only the properties of this superficial layer must be considered, scientists can describe Fo control with a relatively simple model: the Cover Model of Fo Control. However, during speech at a normal loudness or chest and head singing, deeper layers of the vocal folds are set into vibration. Because of the involvement of multiple layers, a more complicated model of the vocal folds must be used: the Body-Cover Model of Fo Control.
Roles of the TA and CT Muscles
Also, researchers have found that a rise in Fo is generally obtained by an increase in TA activity so long as CT activity is not near its maximum. The following animation demonstrates how the CT and TA muscles move in an excised human larynx.
The relationship between vocal fold length and Fo is non-linear. In other words, incremental increases in vocal fold length do not produce similar step-wise increases in Fo. You can test this phonemenon yourself. Stretch a small rubber band between your thumb and forefinger. The elasticity of a rubber band is similar to that of vocal fold tissue. Pluck it while holding it loosely (little tension in the rubber). Now, stretch the rubber band a little more; do you hear a discernable rise in pitch? Probably not. Next, elongate the rubber band to near its limit. You should hear a significant rise in pitch. Scientists can quantitatively predict the upward shift in Fo, using this relationship: Fo will rise only if the square root of stress increases with length more than the length itself increases. Equation 8.7 expresses this relationship quantitatively.
Cover Model of Fo control
A quantitative analysis of the cover model reveals that a positive change in TA muscle activity always causes a decrease in Fo. Conversely, increasing CT activity increases Fo. The differential action between the TA and CT muscles is also apparent in the vocal strain equation (8.6). In essence, the TA and CT have opposing effects on the change of length.
Body-Cover Model of Fo control
It is probably beyond the scope of most vocologists to memorize the complexities of different, but adjacent, tissues vibrating according to their biomechanical characteristics. However, the ability to quantify the relationship between amount of muscle in vibration with stress and vocal fold length to predict Fo is essential to scientists modeling various types of phonation.
Effect of Lung Pressure on Fo
Thus, humans have a number of strategies for controlling Fo:
Not surprisingly, speakers and singers are interested in optimal combinations of these strategies for Fo control. Let's compare muscle activation plots (of the thyroarytenoid muscle and cricothyroid muscle) from a non-singer and a trained singer:
Interestingly, the graphs are quite similar. Both the trained and the untrained vocalist tended to use the TA and CT muscles equally, as seen by the dashed diagonal lines running at 45-degree angles through the graphs.
Finally, there are also some physical limitations of the system, for example, the size of an individual's cricothyroid space. How does yours measure up? Keep your chin level and relax your neck muscles. Locate your larynx with your fingertips. It should be fairly easy to palplate the thyroid notch at the top of the larynx (the upper edge of the Adam's apple) at the top of larynx to locate the length of the thyroid cartilage. Move your fingertip downward. Is the crycoid cartilage close in proximity to the bottom of the thryoid cartilage? If so, there may be limited room to rotate the cricoid cartilage toward the thyroid cartilage. Clearly, this is a relative measurement, so it might be an interesting experiment to feel not only your own, but the cricothyroid space among your VERY CLOSE friends.