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How humans control pitch

What's the difference between fundamental frequency and pitch? These terms are often used interchangeably, although there is a subtle difference. Pitch is a perceptual measure; in other words, pitch must be heard and measured by ears connected to a brain. Fundamental frequency, by definition, does not have to be sensorially perceived. Rather it is a measurement of sound wave's base frequency that can be quantified by an instrument such as a spectrograph. Thus, fundamental frequency - or Fo - is the more accurate scientific term and will be used throughout this section.

Simply stated, Fo is controlled by:

  • a change in length of the vocal folds;
  • a change in the stress of the tissues of the vocal folds.
Tissue Composition & Characteristics
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
The tension of the vocal fold is the primary determinant of Fo 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 Fo. Recall from your background in vocal anatomy that the actions of the cricothyroid (CT) and thyroarytenoid (TA) muscles shorten or lengthen the vocal folds. It may be helpful to remember:

  • The TA muscles shorten the vocal folds;
  • The CT muscles lengthen the vocal folds by pulling the thyroid cartilage forward.
It is also important to understand that these muscles can work independently of one another in regulating Fo. 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 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
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.

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
The body-cover model involves more means of Fo 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 Fo, and of course, if the TA isn't activated, the body-cover model is no different than the cover model. These means of controlling Fo are all found in Equation 8.8.

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
As more air is pushed from the lungs past the vocal folds, Fo 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 Fo:

  • changes in activity of the TA muscle
  • changes in activity of the CT muscle
  • increasing or decreasing amounts of air from the lungs

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.   

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