Voice is produced by vibration of the vocal folds (cords). The vocal folds are a pair of pliable shelves of tissue that stretch across the top of the trachea. They are enclosed within the thyroid cartilage, which is the hard structure that forms the mass in the neck known as the Adam’s apple. The vocal folds, together with the muscles and cartilages that support them, are known as the larynx.
Biologically, the larynx evolved as a valve to protect the airway and lungs. It is positioned where the airway and the esophagus separate. The vocal folds open to allow breathing and close during swallowing to prevent food from entering into the lungs and during voicing/speech.
"Vocal folds" is the modern name for vocal cords. They look like a V-shape and can open (to let air move in and out of the lungs) or close (for airway protection and voice)
They are covered by a soft elastic lining that can vibrate at high speeds
Normal voice production requires the vocal folds to meet completely in the midline
To produce voice, the lungs blow air against vocal folds that are closed, but more loosely than they would be during swallowing
Air pushes through the very small space between them making the covering of the vocal folds, (the mucosa), vibrate
This is the venturi effect:
As air passes through a constriction (or venturi), it speeds up and creates a suction in its wake
This suction draws in the pliable mucosa from each vocal fold, which meets in the midline, only to be pushed aside by more air from the lungs
This cycle creates a repeating undulation which is known as the mucosal wave
The regularity of the mucosal wave is essential to the production of good voice.
The volume of our voice depends on the pressure from the lungs blowing across our vocal folds when they are closed
Stronger lung pressure will increase the volume of the voice
Loudness requires the vocal folds to tense and come together completely
People with vocal fold paralysis or other causes of vocal fold weakness cannot do this and they let the pressure out too quickly
They often complain of a weak voice or an inability to raise the volume of their voice
This refers to the frequency of a sound and can be described as higher or lower
Pitch depends on the length and tension of the vocal folds
This is why men (with longer vocal folds) on average have a lower pitch than women (with shorter vocal folds)
The muscles in the larynx can also change the tension of the vocal folds as we lengthen or shorten them