The Systems of the Human Voice: An Integrative View
Vocal tract linguistic actions integrate and depend on three anatomical and physiological systems: The respiratory system, the laryngeal system, and the supralaryngeal articulatoxy system. Human vocal tract gestures and the voice that these produce are the result of the coupling of phonation with resonance (laryngeal vibration), and vocal tract articulation. Figure 2.4 shows in schematic form the three systems involved in the production of vocal tract gestures.
The thr ee systems are under the control of the nervous system, including high- level cortical control, as well as brain stem reflexes that protect the airways from external stimuli and foreign objects. Let us start with phonation.
The production of the human voice depends on the generation of a flow of air from or into the lungs. Respiration is the foundation of voice production. To be sustained, phonation requires the flow of a constant volume of air. Lung capacity, chest wall compliance, pharyngeal, nasal, and oral anatomy, and the speaker’s psychological condition are all recruited to this end. In addition, very fine-grained control of the laryngeal musculature is required to produce glottal closure together with fine-grained control of the elasticity and tension of the vocal folds. Phonation depends on highly specialised muscular control of particular
vectors of laryngeal action, depending on the muscles recmited, as well as very fine-grained timing of onset of muscle contraction, and the degree of recruitment and fade in phonation.
The vocal tract is a biophysical system that is animated by pressure gradients in the production of the speaker’s vocal tract gestures. The process of phonation is initiated by the alternation of the egressive action of deflating the lungs to compress air and the digressive action of dilating the lungs in order to create a flow of air into the lungs. The rhythmic alternation of these two actions initiates the flow of air that is expelled from the lungs via the glottis, thereby lowering the pressure across the larynx. The larynx is the upper part of the windpipe. It consists of the vocal folds, the glottis, the epiglottis, the thyroid cartilage, the arytenoid cartilage, and the cricoid cartilage. Figure 2.5 shows the anatomical structure of the larynx.
The larynx is a valve. It has two basic functions. First, it protects the lungs by blocking the entry of foreign objects into them. Second, it allows people to block their airways in order to increase intra-abdominal pressure that is important in the ejection of objects, childbirth, and lifting heavy weights.
Figure 2.5 Anatomical structure of the larynx. [Source: https://coimnons.wikimedia.org/ wiki/File:Larynx_and_nearby_structures; downloaded 13 July 2020.]
If the pressure of the airflow across the larynx is sufficiently lowered, a critical threshold is crossed that triggers the vibration of the vocal folds. Phonation is the process of modifying the flow of air from the lungs through a closed glottis. The oscillation of the vocal folds functions to modulate the flow and pressure of the air passing thr ough the larynx. The flow of air causes the vocal cords and ligaments to vibrate. Voice pitch is modulated by the size, length, and tension of the vocal cords and ligaments. The posterior ends of the vocal cords and ligaments are connected to the arytenoid cartilages, located along the upper end of the cricoid cartilage. The muscles of the larynx cause movements of the thyroid cartilage and the arytenoid cartilage. These movements vary the tension exerted on the vocal folds, in the process altering the pitch of the voice.