Phonation Details

Voice is the sound produced as air is pushed through the vocal folds; in other words, voice is the product of phonation. Recall that speech is voice that has been modified via articulation and resonance. Phonation is the result of vocal fold vibrations that alternatively trap and release air. Although it's not entirely clear how the vocal folds vibrate, the most widely accepted explanatory framework is the Myoelastic Aerodynamic Theory, which posits that vibrations are the result of interactions between airflow and the vocal folds. The subglottal pressure that pushes air past the vocal folds drives vocal fold opening, and elastic recoil and the Bernoulli effect close the folds. The cover-body theory of vocal fold vibration ties the morphological structure of the vocal folds to their ability to vibrate, and explains the mucosal wave phenomenon. This theory also explains how we can produce a variety of voice sounds. From innermost to outermost: The thyroarytenoid muscle and its vocalis component make up the body of the fold, which is relatively rigid. The next two layers create the transition layer, or the bridge between the deep and superficial layers. Label them as the deep and intermediate lamina propria; the deep layer comprises the collagen fibers of the vocal ligament, and the intermediate layer comprises elastic fibers. Two layers that form the "cover" of the mucosal fold: the superficial layer of lamina propria (aka Reinke's space), which comprises a gel-like matrix, and, finally, the squamous epithelium. The vibrations of the vocal folds create mucosal waves, in which the looser outer layers slide over the rigid body; this phenomenon begins on the inferior margins and ripples along the surface of the fold. To imagine this, think of a flag waving in the wind. A phonatory cycle as a round of vocal fold opening and closing. Vocal fold vibrations trap and release air to make sounds during a phonatory cycle. Two phases of a cycle: Prephonation is the time when the vocal folds go from their open, resting state to adducted, which is necessary for phonation. The attack phase begins with adduction and continues through the first vibratory cycles. Coordination of these two phases determines the type of attack that we hear. In a simultaneous, balanced attack, the release of air is coordinated with full adduction of the vocal folds. If air is released before the vocal folds are completely adducted, we get a "breathy attack" at the start of the sound; this can sound whispery. On the other hand, if the vocal folds are forcefully adducted before air is released, the subglottal pressure release will be explosive, producing a "throat-clearing" sound at the start of vocalization. Start with the vocal folds at rest; they are open to allow free air flow during breathing. Then, in the prephonation phase, we prepare to create sounds by actively closing our vocal folds; this requires muscular action. The lateral cricoarytenoids and interarytenoids (the transverse and oblique arytenoid muscles) contract to produce medial compression, and the thyroarytenoids with their vocalis portions contract to create tension on the vocal folds. When the vocal folds are completely adducted, subglottal pressure begins to build. At a certain point, subglottal pressure is high enough to start opening the folds; the bottom layers of the folds start to peel apart first. Eventually, the vocal folds are blown completely open, and air flows through, creating voice sounds. Notice that pressure is still higher below the folds than above them. This pressure differential facilitates the Bernoulli effect: the reduced air-pressure between the folds creates a suction-like force. To see the Bernoulli effect in action, hold two pieces of paper in front of your face, as if you're creating a paper walls on either side of your mouth; the paper represent the vocal folds. Then, blow between the pieces of paper, and watch how your breath blowing between them creates a suction that pulls them toward each other. Thus, the folds begin to close. The mucosal wave begins in the bottom layers of the fold, so they adduct first; then, as the wave ripples through them, the upper layers adduct. Again, no muscular action is required for vocal fold closure. The vocal folds are completely closed, so subglottal pressure begins to rise, starting the cycle over. The frequency of vocal fold vibrations, aka voice pitch, is determined by the resting length of the vocal folds. Recall that longitudinal tension is controlled by the cricothyroid, thyroarytenoid, and vocalis muscles. More longitudinal tension produces a higher pitch; past a certain point, we recruit our suprahyoid muscles to assist in pulling the larynx and hyoid bone superiorly and increasing tension. Less longitudinal tension procures a lower pitch; past a certain point, we recruit our infrahyoid muscles to assist in pulling the larynx and hyoid bone inferiorly to produce even lower pitches. The intensity of our voice, aka its loudness, is determined by subglottal pressure, which is mediated by medial compression. We use our lateral cricoarytenoids and interarytenoids to increase medial compression and make our voices louder.