Fluctuations in the voice can be caused by various phenomena occurring at all points in the production of speech or singing. Let us examine categories of the causes:
Neurological Causes
Muscular contractions of various sorts are needed for voice production; among these are sustained adduction (closing) of the vocal folds and cricothyroid activity to lower the larynx. As with all muscular activity, a certain amount of natural shaking, or tremor, is present; this tremor can become more pronounced if the muscles involved are fatigued or overworked. For example, think of how your muscles tend to shake after doing heavy weightlifting or a lot of precise work.
Various diseases, such as Parkinson’s disease, can also cause large abnormal tremors in various muscles, and, in turn, in the voice. Certain frequencies of tremors occur quite often:
- a very slow 1-2 Hz fluctuation referred to as wow;
- 4-6 Hz, referred to as vocal tremor;
- 10-12 Hz, called vocal flutter.
The terms wow and flutter are borrowed from recording industry terminology for the same kinds of phenomena in other sound sources.
Biomechanical Causes
Another source of fluctuations in vocal output is inherent in the nature of the larynx itself; it’s made up of human tissue, covered with a thin layer of mucus. Like all living tissue, it has various irregularities and variations in its composition, which inevitably cause the vocal folds to never vibrate exactly the same way twice. Each cycle of vibration will vary in all kinds of small ways, and these variations will affect the vocal output.
A second biomechanical cause of fluctuations is the irregular flow of blood through the vocal folds. With each contraction of the heart, more blood is sent into the vocal folds, causing them to expand slightly in volume, change their shape, and become slightly stiffer. These effects rapidly dissipate, of course, and are then renewed with the next pump of blood from the heart.
Lastly, the various vocal articulators, such as the tongue, soft palate, and jaw, are continually moving during speech and singing, in order to create vowels and consonants. These articulators are connected to the larynx in various ways, and thus can affect vocal fold vibration. For instance, if the jaw is opened wide, it can press downward on the larynx, and any forward movement of the tongue pulls the hyoid bone forward and upward, thus elevating the larynx.
Aerodynamic Causes
Instability in the flow of air through the glottis is yet another source of fluctuations. This airflow can become turbulent and unstable in much the same way as the flow of water from a partially-blocked garden hose; this turbulence will create a breathy-sounding voice.
Deposits of food or beverages in the vocal tract can cause a ‘rattle’ in the voice, since they will vibrate at their own frequencies, and can be dislodged and move around in the vocal tract during phonation.
Acoustical Causes
The shaping of the vocal tract for various consonants, such as [b], [d] and [g], causes the vocal tract to be occluded, or blocked, for brief periods of time. This can halt phonation or cause register changes for the very brief period of time for which the occlusion persists.
Another acoustical source arises from the fact that some of the sound pressures from the vocal output get reflected from the vocal tract downward, back toward the glottis, where they exert force on the still-vibrating vocal folds and can interfere with normal vibration.