Anatomy of the
Voice Box
The larynx consists of a cartilaginous framework with a mucosal lining on the inside. There are several muscles on the inside and one paired muscle on the outside of the larynx. These muscles work together in order to properly position the individual components of the larynx during speech, breathing, and swallowing.
Anatomy of the vocal folds
The vocal folds are a laminated structure consisting of mucosa and muscle. The overlying layer is called epithelium, which covers the underlying lamina propria. The lamina propria contains a top layer, called superficial lamina propria (SLP) or Reinke’s space, and a deeper layer, which includes the vocal ligament and the vocalis muscle. The SLP is an amorphous, gel-like substance, which makes the vocal folds pliable and allows them to vibrate during phonation.
Voice
During phonation or the production of speech, the vocal folds are brought together to allow them to vibrate. The laryngeal muscles control the exact position of the vocal folds and they control how airtight the glottic valve is. These muscles can also control the stretch of the vocal folds, which in turn determines their pliability. Due to the flexible layer of the vocal fold, a mucosal wave travels over the vocal folds while the glottis space opens and closes.
Voice production requires the following systems to work hand in hand:
- The lungs need to generate sufficient aerodynamic pressure to push air up through the larynx.
- The vocal folds need to be functional in order to convert this aerodynamic energy into sound.
- The areas above the vocal folds (throat) serve as a resonator and convert the sound, generated by the vocal folds, into a human voice.
Voice production is a very complex system, and it can easily malfunction by the smallest structural change along the vocal folds or by poor use of the laryngeal muscles. This typically results in hoarseness or early vocal fatigue.
Voice pitch
Voice pitch is determined by the number of vocal cord vibrations when speaking. This is also called the fundamental frequency. For men, this is typically between 120 to 150 times per second (also called Hertz), and for women around 200-250 times per second. To be perceived as a female voice, the pitch should at least be around 190 Hz.
Transgender voice and fundamental frequency
The vocal cord of men is approximately 15mm long and of females 11 mm long. Longer male vocal cords vibrate slower than shorter female vocal cords, and therefore, have a lower frequency. Further, male vocal folds are 15-20% thicker than female vocal cords (6mm vs 5mm). Male-to-female transgender voice surgery focuses on making the vocal cords shorter so they can vibrate faster, resulting in a higher fundamental frequency.
Most female-to-male transgender patients receive testosterone which often successfully lowers their voice, and surgery is usually not needed. However, for patients who still do not meet their personal voice goals, a simple procedure can be conducted to help with lowering the voice.
Vocal tract resonator
A common misperception is that a vocal cord shortening surgery will automatically produce a higher and more feminine voice. Just like a string instrument, the resonance frequency of the human voice (different from the fundamental frequency which is the vocal cord frequency) will also depend on the volume of the resonating cavity which includes the throat and oral cavity (called the vocal tract). The larger the vocal tract, the lower the voice sounds. The volume of a natural male vocal tract is 60%- 80% larger than a natural female vocal tract which is why feminization vocal strategies must include learning to squeeze and narrow the vocal tract to achieve the desired female-sounding voice.
Illustration shows a thin slice through a normal vocal cord with all its layers. The purple layer is called Reinke space (or SLP) and is crucial for a smooth-sounding voice.
The vocal tract resonator is shown in light blue. Vocal cords create a sound that is then modified via the vocal tract resonator into a higher or lower voice. The volume of the vocal tract resonator is 50-80% smaller in females compared to men.
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