This book developed out of many honest and open discussions between the authors in a family kitchen. The authors discussed terminology, philosophy, application, and future study. Both authors respect their colleagues’ past, present, and future contributions across many disciplines. These disciplines include voice-performing professionals, teachers and voice coaches, speech-language pathologists, physicians, and numerous researchers and scientists, all of whom have paved—and continue to pave—the way toward a deeper understanding of and broader applications in Vocology.

Our goal was to translate the science and its applications for these valuable professionals, but as we collaborated, we discovered that our greater desire was to share our insights with the public—those who value their own voice as the great acoustic carrier of speech and song (Titze & Verdolini Abbott, 2012). When the voice is free, it improves our health (Titze & Johnson [Eds.], 2025) and our quality of life (Kapsner-Smith et al., 2015).

The first chapter of the book highlights the history of semi-occluded airways in vocalization across human and animal species. Semi-occluded airways observed in human development include infant cries, humming, and raspberry vocalizations (Kuhl & Meltzoff, 1996), while species such as frogs and doves also occlude the mouth or beak when vocalizing throughout their evolution (Riede et al., 2016). This chapter demonstrates heuristic approaches humans have taken to understand the benefits of semi-occluded vocal tract (SOVT) exercises (i.e., humming, lip trills, vocalization through straws, and other devices).

The second chapter details experimental approaches used to quantify critical shape changes of the airways during SOVT exercises, while chapter three shows that airway resistances for steady airflow can be adjusted to maximize aerodynamic and acoustic power transfer from the larynx to the vocal tract. In general, glottal airflow resistance needs to be decreased while some supraglottal resistance needs to be increased to achieve maximized power transfer (Titze, 2021).

The book also reveals that an important part of caring for the vocal instrument is performing vocal warm-ups before singing, teaching, or public speaking. As with any exercise, a vocal warm-up allows stretching of the tissues, which helps improve tensile strength and prevent injury. SOVT exercises are ideally suited for warm-ups and cool-downs before and after periods of high vocal demand. These exercises are helpful for resetting optimal configurations after muscle and tissue fatigue or extreme or exceptional voice use, requiring less cognitive effort and placing less demand on time and tissue.

An optimal airway configuration that resembles the midpoint of extreme positions invokes the equilibrium postulates discussed in Vocology: The Science and Practice of Voice Habilitation (Titze & Verdolini Abbott, 2012). Embracing the first four postulates in vocalization provides greater flexibility and allows for more options in coordinating multiple subsystems.

The SOVT methods presented in the book support improvements in glottal closure for bowed or hypo-adducted vocal folds, as reported in some studies, while other studies have found similar benefits for hyper-adducted or pressed vocal folds, indicating that the correction is not unidirectional. Rather, the system tends toward an optimal setting. This posture or setting may help prevent vocal trauma, injury, and lesion formation (Guzman et al., 2017, 2018; Yamasaki et al., 2017).

While treatments for voice disorders include medication and surgery, behavioral therapy is frequently used either as a primary treatment or as an adjunct to medical and surgical intervention. Although many warm-up and therapy exercises are successful without semi-occlusions, it is observed throughout the book that most successful voice therapy methods contain some aspect of SOVT exercises.

Over time, methods evolve, and the language used to describe them becomes clearer through improved conceptualization and further experimentation. As an example, we call attention to the still-unresolved issue of hyperfunction in vocalization. A narrowing of the larynx canal or epilarynx, as described in multiple papers, raises the question of excessive use of constrictor muscles as compensation for vocal fold dysfunction. This narrowing may contribute to what some describe as muscle tension dysphonia. However, narrowing the larynx canal also increases vocal tract impedance for maximum power transfer, which is a desired effect.

It is important to recognize that this narrowing should coincide with lengthening and expansion of the remainder of the vocal tract to support muscular flexibility. It remains unclear to what degree the larynx canal can be actively narrowed without introducing unwanted ventricular or aryepiglottic vibration or tension. This remains a challenge for future research and clinical practice. However, new CT images presented in the book clearly show expansion of the vocal tract, along with lengthening and re-posturing of the vocal folds, during vocalization through a 3 mm straw (Cox & Titze, 2023).

The final chapter provides a brief overview of the successes of various SOVT methods, and an extensive bibliography reflects that research and exploration in this area are ongoing. We encourage readers to understand the full history and development of SOVT methods within a broad context of human and animal vocalization. The scientific results for SOVT exercises are summarized by both authors in an excerpt taken directly from the recapitulation section at the end of the final chapter:

“(1) A general widening of the airways occurs passively, without the need for a specific thought process or target. The upper portion of each vocal fold is spread apart, proportional to the steady pressure built up in the supraglottal airway behind the semi-occlusion.

(2) As a result, the medial surfaces of the vocal folds can become more parallel, with balanced top and bottom adduction, facilitated by optimal cricothyroid and thyroarytenoid muscle activations; phonation threshold pressure is lowered with parallel and slightly separated vocal fold surfaces.

(3) Vocal fold vibrational amplitude and collision forces are reduced, allowing lung pressure and fundamental frequency to be taken higher during pitch glides.

(4) Maximum power is transferred from the source to the airway with lower glottal resistance and higher airway resistance.

(5) Acoustically, the first resonance frequency is lowered to around 200 Hz, meaning that many harmonics can benefit from vocal tract acoustic inertance across a range of 400–4500 Hz. Reinforcement of harmonics through vocal tract resonances beyond the first resonance helps support mixed registration and avoid register instabilities.

(6) High-frequency emphasis through source–airway interaction counteracts the high-frequency de-emphasis that occurs when vocal fold collision alone is relied upon. So-called ‘pressed voice,’ or hyper-adduction of the vocal folds, is thereby minimized.”
(Cox & Titze, 2023, p. 121)

Finally, the first author provides an Appendix offering a holistic overview of how to implement SOVT principles and methods within an inclusive treatment philosophy. The Appendix also demonstrates how SOVT methods can be incorporated into a voice evaluation. This is particularly valuable, as oral pressures obtained from semi-occlusions using straws are measurable and adaptable (Titze, Maxfield, & Cox, 2022) and can supplement information obtained during a stroboscopic voice evaluation to aid functional interpretation.

To gain a comprehensive picture of the limitations that may disrupt or impede a stable and reliable voicing carrier, the first author illustrates how SOVT methods can be applied to extremes of pitch and loudness. Through this approach, at least eight limitations are identified that may compromise the voicing carrier. The first author also suggests methods for revealing and treating these limitations. As efficiency improves vocal function (results) while reducing vocal effort and trauma (cost) (Kapsner-Smith et al., 2015; Guzman et al., 2017, 2018; Yamasaki et al., 2017), the authors convey that SOVT methods allow greater freedom in vocal production when limitations are present. As this efficiency is realized as an asset in training, the book ultimately seeks to enhance understanding of why we believe, “Voice Is FREE after SOVT.”

References

1. Titze, I. R., & Verdolini Abbott, K. V. (2012). Vocology: The Science and Practice of Voice Habilitation. National Center for Voice and Speech.
2. Titze, I. R., & Johnson, E. C. (Eds.). (2025). Sing and Shout for Health: Scientific Insights and Future Directions for Vocology. National Center for Voice and Speech.
3. Kapsner-Smith, M. R., Hunter, E. J., Kirkham, K., Cox, K., & Titze, I. R. (2015). A randomized controlled trial of two semi-occluded vocal tract voice therapy protocols. Journal of Speech, Language, and Hearing Research, 58(3), 535–549.
4. Kuhl, P. K., & Meltzoff, A. N. (1996). Infant vocalizations in response to speech: Vocal imitation and developmental change. The Journal of the Acoustical Society of America, 100(4 Pt 1), 2425–2438.
5. Riede, T., Eliason, C. M., Miller, E. H., Goller, F., & Clarke, J. A. (2016). Coos, booms, and hoots: The evolution of closed-mouth vocal behavior in birds. Evolution, 70(8), 1734–1746.
6. Titze, I. R. (2021). Regulation of laryngeal resistance and maximum power transfer with semi-occluded airway vocalization. The Journal of the Acoustical Society of America, 149(6), 4106.
7. Guzman, M., Laukkanen, A. M., Traser, L., Geneid, A., Richter, B., Muñoz, D., & Echternach, M. (2017). The influence of water resistance therapy on vocal fold vibration: A high-speed digital imaging study. Logopedics, Phoniatrics, Vocology, 42(3), 99–107.
8. Guzman, M., Saldivar, P., Pérez, R., & Muñoz, D. (2018). Aerodynamic, electroglottographic, and acoustic outcomes after tube phonation in water in elderly subjects. Folia Phoniatrica et Logopaedica, 70(3–4), 149–155.
9. Yamasaki, R., Murano, E. Z., Gebrim, E., Hachiya, A., Montagnoli, A., Behlau, M., & Tsuji, D. (2017). Vocal tract adjustments of dysphonic and non-dysphonic women pre- and post-flexible resonance tube in water exercise: A quantitative MRI study. Journal of Voice, 31(4), 442–454.
10. Cox, K. T., & Titze, I. R. (2023). Voice Is FREE after SOVT: Semi-Occluded Vocal Tract Principles, Methods, and Training. National Center for Voice and Speech.
11. Titze, I. R., Maxfield, L., & Cox, K. T. (2022). Optimizing diameter, length, and water immersion in flow-resistant tube vocalization. Journal of Voice. Nov 7: S0892-1997(22)00306-X.

HOW TO CITE