Clinicians and singers are used to viewing vocal fold vibration from a superior endoscopic view. However, both vocal fold vibration and vocal control are three-dimensional. Although it is hidden from the superior view, the vocal fold medial surface shape in the vertical dimension plays an important part in determining both the vibratory pattern as viewed from above and the produced voice. 

The earliest discussion of the importance of medial surface shape was probably by van den Berg (1968). Based on x-ray tracings of the human larynx, he argued that vertical thickness plays an important role in controlling voice registers, with chest voices produced with thick folds and falsetto with thin folds. Findings from recent computational studies (reviewed in Zhang, 2023) support van den Berg’s argument. These studies showed that vertical thickness plays a dominant role in determining the duration of glottal closure during phonation, and the produced voice quality is sensitive to subtle changes in medial surface shape. The effect of vocal fold approximation is in comparison much smaller, except near phonation onset. 

Thus, thickness control is essential to voice quality modulation. Vocal fry and chest-like voices are likely produced with thicker vocal folds, whereas head- or falsetto-like voices are produced with thinner folds. Because thickness co-varies with stiffness in humans (Zhang, 2016), it is not surprising that different voice qualities may be more easily produced at different pitch ranges. For example, vocal fry and chest-like voices are more easily produced at lower pitches whereas head- or falsetto-like voices are more easily produced at higher pitches (Henrich, 2006; Herbst, 2020). Since male vocal folds are often thicker than female vocal folds, male voices are often more easily produced in the chest register than female voices.

It is reasonable to expect that all else being equal, speakers with thicker vocal folds would be better able to maintain sufficient thickness and produce strong high-frequency harmonics at high pitches than speakers with thinner vocal folds. Whether male or female, one may be endowed with naturally thick vocal folds and will be able to more easily produce a fuller voice at high notes. In contrast, speakers with naturally thin vocal folds will be able to produce a lighter clear voice more easily at low notes. In humans, medial surface thickness is primarily controlled by actions of the thyroarytenoid, cricothyroid, and extrinsic laryngeal muscles (see Zhang, 2023, for a detailed review). Thus, one could also nurture vocal fold versatility through exercises targeting these muscles, particularly the thyroarytenoid muscles, to strengthen isolated control of the vocal folds from thick to thin. Such an approach has been successful in training singers to produce desired voice quality at different pitch ranges (e.g., Austin, 2005; Steinhauer et al., 2017). 

This importance of medial surface shape indicates that clinical evaluation of vocal fold adduction should consider both vocal fold approximation in the horizontal plane and changes in medial surface shape in the vertical dimension. For conditions of glottal insufficiency, clinical intervention that targets restoring both the desired glottal gap (medialization) and optimal medial surface shape is more likely to produce optimal voice outcomes (Zhang, 2022). 

In summary, voice control is three-dimensional and voice research should pay more attention to changes in medial surface shape. Future studies in a large population of both normal speakers and patients are needed to better characterize the three-dimensional medial surface shape, its variability across speakers, changes throughout the life span, and how it is impacted by voice disorders and clinical interventions. More discussions can be found in Zhang (2023).

References

Austin, S. F. (2005). Treasure” chest”-a physiological and pedagogical review of the low mechanism. Journal of Singing-The Official Journal of the National Association of Teachers of Singing, 61(3), 241-251. 

Henrich, D. N. (2006). Mirroring the voice from Garcia to the present day: Some insights into singing voice registers. Logopedics Phoniatrics Vocology, 31(1), 3-14. 

Herbst, C. T. (2020). Registers-The Snake Pit of Voice Pedagogy Part 1: Proprioception, perception, and laryngeal mechanisms. Journal of Singing, 77(2), 175-190. 

Steinhauer, K., McDonald, M. M., & Estill, J. (2017). The Estill voice model: Theory & translation. Estill Voice International. 

Van den Berg, J. W. (1968). “Register problems,” Ann. NY Acad. Sci. 155(1), 129–134.

Zhang, Z. (2016). Mechanics of human voice production and control, J. Acoust. Soc. Am., 140(4), 2614–2635.

Zhang, Z. (2022). Contribution of undesired medial surface shape to suboptimal voice outcome after medialization laryngoplasty, J. Voice, in press. https://doi.org/10.1016/j.jvoice.2022.03.010

Zhang, Z. (2023). Vocal fold vertical thickness in human voice production and control: A review, J. Voice, in press. https://doi.org/10.1016/j.jvoice.2023.02.021