Speech is produced by joint activation of three subsystems—respiratory, phonatory, and articulatory (see Figure 1). The respiratory system produces an outflow of air under relatively constant pressure. The modulation of this airflow to produce the sounds we know of as speech is characterized by the source-filter theory, according to which the phonatory system provides the sound source for speech and the articulatory system filters this source.

The phonatory system acts on the outward airflow of the respiratory system to produce pho-nation, or voicing. The sound source is the vibration of the vocal cords (or folds), which are two opposed horizontal muscular slivers running from front to back within the cartilaginous larynx. They are drawn apart during quiet breathing. Prior to the first cycle of vocal fold ...