vocal folds

Two muscular folds running from a single point inside the front of the thyroid cartilage (Adam’s apple) backwards to the front ends of the arytenoid cartilages; also called vocal cords, and sometimes (though not usually in PHONETICS) vocal lips or bands. The vocal folds are very flexible, being shaped by the combined activities of the associated cartilages and muscles. The space between them is known as the GLOTTIS.

The vocal folds have several functions. Their main role in speech is to vibrate in such a manner as to produce VOICE, a process known as PHONATION. When the folds are not vibrating, two main alternative positions are available. They may be tightly closed (‘adducted’), as when the breath is held – a process which produces a GLOTTAL stop upon release. Or they may remain open (‘abducted’), so that the breath flowing through the glottis produces audible FRICTION, as in whispering and the [h] sound. Other ‘phonation types’ are possible, by varying the mode of vibration of the vocal folds in various ways, as in BREATHY and CREAKY voice. Varying the thickness, length and tension of the vocal folds also produces the different REGISTERS in voice production, such as the distinction between ‘falsetto’ and ‘chest’ voice. Lastly, by varying the rate and strength of vibration of the vocal folds, variations in PITCH and LOUDNESS can be introduced into speech.

The question of how precisely the vocal folds operate, from a physiological viewpoint, has been the subject of controversy, and is still not wholly understood. The most widely held theory maintains that the folds are set in vibration aerodynamically, solely by a reaction taking place between their elastic properties and the subglottal air-pressure involved – this is known as the ‘myoelastic’ theory of voice production. An alternative theory, developed in the 1950s, argued that the folds are set in motion as a result of periodic NEURAL stimulation and contraction of the muscles – this was known as the ‘neurochronaxiac’ theory.