Vocal fold tissue composition and characteristics

To fully understand the mechanisms of pitch control, it's 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.

Figure 2. Cross-section through vocal fold

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 pitch control with a relatively simple model: the so-called Cover Model. 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.

Roles of the cricothyroid and thyroarytenoid muscles

The tension of the vocal fold is what mainly determines of pitch control. Just as a guitar string or a rubber band can be stretched to produce a higher pitch when plucked, increased stress on vocal fold fibers produces an increase in pitch. The actions of the cricothyroid (CT) and thyroarytenoid (TA) muscles shorten or lengthen the vocal folds. It may be helpful to remember:

It's also important to understand that these muscles can work independently of one another in regulating pitch. A number of muscle activation studies conclude that speakers and singers – particularly trained vocalists – tend to balance CT and TA muscle activity, utilizing neither to its maximum potential. Researchers theorize that humans naturally are inclined to use all or none of a particular muscle. Training can help vocalists use just part of a particular laryngeal muscle's capability.

Also, researchers have found that a rise in pitch 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.

Figure 3. Cross-section through vocal fold

The relationship between vocal fold length and pitch is non-linear. In other words, incremental increases in vocal fold length do not produce similar step-wise increases in pitch. You can test this phenomenon 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 discernible 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 pitch, using this relationship: pitch will rise only if the square root of stress increases with length more than the length itself increases.

Cover model of pitch control

The key concept of the cover model is this: in falsetto or soft phonation when vibrational amplitudes are small, oscillation is primarily taking place in the cover. The tension of the vocal folds is determined by length only. Scientists continue to study how much control singers and speakers have in regulating the depth of vibration of the cover. They hypothesize that the vocal ligament (the area between the epithelium and the TA muscle) may play a major role in absorbing stress, leaving the superficial layers loose for vibration at high fundamental frequencies.

Body-cover model of pitch

The body-cover model involves more means of pitch control than does the cover model (where CT does everything). These means or variables of control are 1) the depth or amount of tissue vibrating (into the vocal fold or away from the surface), and 2) the activation of the TA muscle. As the depth of vibration increases, TA activation has a greater positive correlation with pitch, and of course, if the TA isn't activated, the body-cover model is no different than the cover model.

Effect of lung pressure on pitch

As more air is pushed from the lungs past the vocal folds, pitch usually rises. This can easily be demonstrated. Make an 'ah' sound at a low comfortable pitch, then press with your fingertips inwardly just below the sternum. Do you hear the sharp rise in pitch? This phenomenon is more noticeable at lower frequencies (when the vocal fold lengths are shorter) than at higher pitches.

Thus, humans have a number of strategies for controlling pitch:

Not surprisingly, speakers and singers are interested in optimal combinations of these strategies for pitch control. Let's compare muscle activation plots (of the thyroarytenoid muscle and cricothyroid muscle) from a non-singer and a trained singer (Figure 1).

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 palpate 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 cricoid cartilage close in proximity to the bottom of the thyroid cartilage? If so, there may be limited room to rotate the cricoid cartilage toward the thyroid cartilage.