Human Vocal Anatomy and Voice Production Explained

The Anatomy of Human Phonation

The process of human voice production, known as phonation, involves a complex set of anatomical structures. These can be broadly divided into three distinct zones:

• Subglottic Cavity
• Larynx (Glottic Cavity)
• Supraglottic Cavity

Subglottic Cavity: The Powerhouse of Voice Production

The subglottic cavity comprises the diaphragm, lungs, bronchi, and trachea. This region is crucial for generating the airflow necessary for phonation.

The diaphragm is a muscular septum, shaped like a dome, that separates the chest and abdominal cavities. Its movement drives the respiratory cycle:

• Inspiration: When the diaphragm contracts and the ribs rise, the volume of the chest cavity increases. This draws outside air through the nose or mouth, pharynx, and bronchi into the lungs.
• Expiration: Conversely, when the diaphragm relaxes, it returns to its dome shape, causing air to be expelled from the lungs.

It is primarily during expiration that phonation occurs. Although rare, it is also possible to produce sounds during inspiration, such as in children's cries or sobbing.

The Larynx: Voice Box Anatomy and Function

The larynx, often called the voice box, is a tubular structure composed of a system of muscles and cartilages. It forms the glottic cavity.

Laryngeal Cartilages

Key cartilages of the larynx include:

• Cricoid: Located at the base of the larynx, it is ring-shaped, thinner at the front and thicker at the back.
• Thyroid: Situated above the cricoid, it is the largest cartilage and forms the “Adam’s apple.”
• Arytenoid: Two pyramid-shaped cartilages, positioned posteriorly and wide, resting on the cricoid. These are crucial for vocal cord movement.

Vocal Cords (Vocal Folds)

The vocal cords are actually a pair of muscular lips (vocal folds) positioned symmetrically on either side of the medial line of the larynx. They contain elastic tissue (the ligament). Above these are another pair of lips called false vocal cords or ventricular bands, which do not participate in phonation. Between the true and false vocal cords are the ventricles of Morgagni.

The constitution of the vocal cords influences the sound produced: thick vocal cords produce a deep (severe) sound, while flat and thin cords produce a high-pitched sound.

The Glottis

The glottis is the triangular-shaped opening bounded by the vocal cords. This space can be closed by the approximation of the vocal cords, which occurs through the movement of the arytenoid cartilages. This closure is essential for phonation.

The Process of Phonation

During normal breathing, the glottis remains open, similar to its state during the articulation of certain voiceless consonants. For phonation to occur, the glottis must close completely along the midline.

• If the space between the arytenoid cartilages remains open, allowing air to pass, it produces a whisper.
• If the closure is complete, the glottis is ready to vibrate, provided the appropriate tension from the cricoarytenoid muscles is applied for the desired vocal register.

It is also possible to allow only a part of the vocal cord to vibrate, thereby shortening the length of the vibrating body and producing a sharper or higher tone. These physiological mechanisms align perfectly with the physical laws governing the natural frequency of vibrating bodies.

The vocal cords vibrate horizontally: they close and open, allowing air to pass. The cords approximate each other, starting from the bottom, until complete closure. Subglottic air pressure (during expiration) then tends to separate the vocal cords again, starting from the bottom, until the opening is complete and air can exit. The air exiting the larynx vibrates accordingly.

The length and thickness of the vocal cords directly affect the vibration frequency: longer and thicker vocal cords result in slower vibrations (lower pitch), while shorter and thinner cords lead to higher frequencies (higher pitch). This explains why women and children naturally speak and sing in a higher register than men. The volume of the resonators also plays a role in this.

Supraglottic Cavities: Resonators of Sound

The supraglottic cavities, including the pharynx, oral cavity, and nostrils, act as resonators for the laryngeal tone. Sometimes, a fourth resonator can be added by the projection and rounding of the lips, further shaping the sound.