Our ability to communicate through sound is fundamentally dependent on the vocal cords, which act as the vibrating source for the human voice. These two folds of tissue are responsible for transforming a simple stream of air from the lungs into a complex sound wave. The physical manipulation of these folds directly controls the rate of vibration, known as frequency, which our ears perceive as pitch. Understanding how this manipulation occurs requires looking closely at the anatomy of the voice box and the specific muscles that execute these adjustments.
The Anatomy of Vocal Production
The vocal cords, more accurately called vocal folds, are twin bands of tissue situated within the larynx, or voice box, located at the top of the trachea (windpipe). Each vocal fold is a layered structure, consisting of a muscle core, the thyroarytenoid muscle, covered by a delicate mucous membrane. These folds stretch horizontally across the laryngeal cavity. They attach at the front to the thyroid cartilage (Adam’s apple) and at the back to the paired arytenoid cartilages. The arytenoid cartilages pivot and slide, controlling the position and length of the vocal folds.
How Vocal Cords Generate Sound
Sound generation, or phonation, begins with air pressure built up beneath the closed vocal folds from the lungs. This subglottic pressure forces the folds to separate briefly, creating a narrow opening. As air rushes through this constricted space, its velocity increases, causing a drop in pressure between the folds. This pressure drop is explained by the Bernoulli effect, where faster airflow results in lower lateral pressure, sucking the folds back toward each other. This rapid, repeating cycle of opening and closing creates the characteristic vibration, or mucosal wave, that produces the raw sound wave. The frequency, or the number of these vibratory cycles per second, determines the pitch of the sound we hear.
The Variables That Determine Pitch
The pitch of the voice is a direct function of the vocal fold’s vibratory rate, which is controlled by three physical properties: length, tension, and mass. These variables are manipulated to change the speed of the mucosal wave, much like tuning a stringed instrument. Stretching the vocal folds increases their length and tension simultaneously, causing them to vibrate faster. A faster vibration rate results in a higher frequency and a higher pitch. Conversely, allowing the vocal folds to shorten and relax decreases the tension, which slows the rate of vibration and produces a lower pitch. The third variable, mass, refers to the effective thickness of the vibrating tissue. When the folds are thickened, more mass is involved, leading to a slower rate and a lower pitch, often associated with a chest voice. When the folds are thinned, the effective mass decreases, allowing for faster vibration and a higher pitch, characteristic of head voice.
The Muscles That Control Pitch
The precise manipulation of vocal fold length, tension, and mass is executed by specialized intrinsic laryngeal muscles. The cricothyroid (CT) muscles are the primary agents for raising pitch. When these muscles contract, they tilt the thyroid cartilage forward, which increases the distance between the front and back attachments of the vocal folds. This action lengthens and tenses the folds, causing them to vibrate at a higher frequency. Working in opposition are the thyroarytenoid (TA) muscles, which make up the bulk of the vocal folds. Contraction of the TA muscles shortens the vocal folds, decreasing their tension and causing them to thicken. This shortening and thickening effect lowers the pitch and is essential for achieving the lower ranges of the voice.