In the main discussion in section 11.9 I avoided anatomical jargon as much as possible, but this section gives a brief account of the anatomy of the human vocal tract. Figure 1 shows a labelled diagram of the main components. Air from the lungs enters at the bottom via the trachea, then passes through the larynx to join the esophagus (leading to the stomach). Proceeding upwards, the air then flows through the pharynx into the oral and nasal cavities. The epiglottis is a flap of soft tissue which closes downwards during swallowing to block the larynx and prevent food or drink entering it.
During speech or singing, most sounds originate in the larynx. Two diagrams of the larynx from the classic “Grey’s Anatomy” [1] are shown in Figs. 2 and 3. The larynx is the structure that creates the “Adam’s apple”, often particularly prominent in the throat of adult men. Within the larynx is the pair of vocal folds. When the folds are closed to allow speech or singing (“adducted”), air flow from the lungs can cause self-excited oscillation. But during breathing, the folds are opened (or “abducted”). Photographs of vocal folds in these two configurations are shown in Fig. 4.
The vocal folds are stretched horizontally, from back to front, across the larynx. They are composed of a layered structure involving distinct tissues. Beneath an outer layer of flat cells called squamous epithelium there is a gel-like layer (the lamina propria). The vocalis and thyroarytenoid muscles make up the deepest portion. These three layers are indicated in Fig. 5 in an animation of the vibration of the vocal folds during singing or speaking in the most common voice register, called the “modal voice” or “chest voice”.
With these anatomical details, we can give a brief account of what is believed to happen to account for the “singer’s formant” (Sundberg [2]). The singer lowers the larynx to lengthen the epilaryngeal tube, between the vocal folds and the junction with the pharynx. At the same time, they may constrict the diameter of this tube so that there is a significant jump in cross-sectional area at that junction. Such a jump would lead to enhanced acoustical reflection and thus create a partial resonance in the tube. The resulting change to the frequencies and mode shapes of the tract resonances gives rise to the clustering of resonances responsible for the formant.
[1] Henry Gray: “Gray’s Anatomy”, Longman (1918). Available online here
[2] Johan Sundberg: “The singer’s formant revisited”, Voice 4, 106-119 (1995). The same text is also available here.