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to produce voice of desired loudness, thus minimizing vocal fold contact pressure. At the laryngeal level, this can be achieved by adopting a barely abducted (with the vocal folds just touching each other), thin vocal fold configura- tion (Berry et al., 2001; Zhang, 2020). This barely abducted configuration is often targeted in voice therapy (e.g., the resonant voice therapy; Verdolini-Marston et al., 1995). In voice training, register balancing between thick and thin vocal folds in singing is often promoted to minimize sub- glottal pressure and purportedly laryngeal pathologies over time (e.g., the Bel Canto technique).
Vocal fold contact pressure can also be lowered by vocal tract adjustments. For example, when targeting a desired loudness, vocal fold contact pressure can be lowered by constricting the epilarynx (the part of the upper airway immediately above the vocal folds) or increas- ing the mouth opening whenever possible. Epilaryngeal narrowing often leads to clustering of vocal tract reso- nances in the 2- to 3-kHz range, which is known as the singer’s formant, and amplifies voice harmonics in this frequency range. Increasing the mouth opening increases the efficiency of sound radiation from the mouth. Both adjustments reduce the subglottal pressure required to produce a desired loudness, thus reducing vocal fold con- tact pressure (Zhang, 2021).
Unfortunately, untrained speakers often increase vocal fold adduction when attempting to increase vocal intensity (Isshiki, 1964), especially in an emotional situation. This is particularly the case of speakers who habitually squeeze the larynx during talking. Hyperadduction of the vocal folds may also develop as an adaptive behavior in response to transient vocal fold tissue changes. Hyperadducted vocal folds are not vocally efficient, meaning that a higher sub- glottal pressure is required to produce a desired loudness than that needed for barely abducted vocal folds. Because hyperadduction is often accompanied by reduced stiffness and increased thickness in the cover layer, the risk of vocal fold injury is excessively high due to the combination of the high subglottal pressure required, tightly compressed vocal folds, and low cover layer stiffness. Tightly compressed vocal folds also have the tendency to exhibit irregular vocal fold vibration with large cycle-to-cycle variations, resulting in a rough voice quality. Whenever possible, this vocal fold configuration should be avoided in loud voice production by making the appropriate adjustments at the larynx and within the vocal tract.
Glottal Insufficiency and Adaptive Compensations
Although voice production with tightly compressed vocal folds is unhealthy, voice production with the vocal folds too far apart is also undesired. Whereas the latter vocal configuration requires the least laryngeal effort and poses the lowest risk to vocal fold injury at a low subglottal pressure, voice production is extremely inefficient due to the lack of glottal closure. Thus, attempting to talk loudly in this configuration would require excessively high subglottal pressures, resulting in a high respiratory effort and, potentially, a high vocal fold contact pressure. The produced voice is breathy in nature due to the large airflow escaping through the glottis. With the high lung volume expenditure, one may also feel short of breath and need to take another breath in the middle of an utter- ance, particularly when attempting to increase loudness. As a result, such a configuration is not ideal for conversa- tional communication or loud voice production.
However, the ability to sufficiently adduct the vocal folds may be lost or weakened due to changes in vocal fold physiology, a condition known medically as glottal insuf- ficiency. Such insufficiency may occur as a result of vocal fold paralysis or paresis due to trauma to the laryngeal nerves, vocal fold atrophy with aging, or changes in the membranous cover layer (e.g., vocal fold swelling or scar- ring). Under such conditions, one may develop adaptive vocal behaviors in an attempt to increase vocal efficiency and conserve air expenditure. This can be achieved by increasing activation of the adductory muscles to improve glottal closure if the neuromuscular mechanism is still intact. One may also adduct supraglottal struc- tures such as the false folds and epiglottis (Figure 3), as often observed in muscle tension dysphonia. Although supraglottal adduction does not improve glottal clo- sure, it may enhance source-tract interaction and thus increase vocal efficiency in addition to air conservation. Such adaptive behaviors often lead to increased laryngeal effort, vocal fatigue over time, and a strained voice qual- ity. If such adaptation persists, it may lead to long-term voice disorders.
For example, vocal fold swelling often occurs after exten- sive shouting or screaming in a sports event or giving a lecture for a longer than the normal period. Extremely high subglottal pressures and, even more so, vocal fold hyperadduction in these situations readily lead to vocal
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