Speech is produced by the speech organs, where airstream causes the vocal folds to vibrate (this applies to the egressive airstream mechanism). The created sound then moves through the articulatory system, attaining its final form – one of the sounds used in the language of the speaker. This text is an overview of what happens with air on its way out of the vocal tract.
The air from the lungs enters the larynx, a structure that consists of several cartilages: the thyroid, cricoid and arytenoid (Ogden, Introduction 40). The larynx is about 11 cm long and has 2.5 cm in diameter (Clark 30). The angle that is formed by the sides of the thyroid cartilage is 90° in males and 120° in females (30). This physical difference influences the voice quality intrinsically (but, the quality can be culturally influenced as well ).
The epiglottis, a leaf-shaped cartilage that closes the airways during swallowing, thus protecting sensitive tissue, is located above the larynx. The larynx houses vocal folds, “typically about 17 to 22 mm long in males and about 11 to 16 mm long in females” (32). The cartilage structure that surrounds the vocal folds and the vocal folds themselves form the glottis, a “laryngeal valve aperture” (32).
Above the epiglottis is the pharynx, a muscular passage that connects the oral cavity, the larynx and the velum. The pharynx is passively involved in speech (42), because it modifies the size of the space between the oral cavity and the larynx. The velum, a soft tissue, is placed above the pharynx. It directs the airflow in speech: if raised it closes the velopharyngeal port, an opening to the nasal cavity  (46).
The oral cavity is a space in vocal tracts where humans can exert the greatest control of its size and shape (O’Connor, Phonetics 34), which makes it critical for “determining the phonetic qualities of speech sounds” (Clark, Introduction, 47). The oral cavity is a space between the lips (anteriorly ), the palotaglossus muscle (posteriorly), the tongue (inferiorly) and the roof of the mouth (superiorly) (47). The lips, the tongue and the angle of the mandible have an important role in speech sound production, although not of equal importance (for example, it is possible to make a distinctive sound with the mandible fixed) (47). Considering the complex muscular and neural structure of the mobile parts that surround the oral cavity it is no surprise, then, “that the characteristics of vowels depend on the shape of the open passage above the larynx” (Jones, Outline 29). Of course, this refers not only to vowels, but to all speech sounds; what makes vowels interesting, however, is the lack of any closure in the passages, so their quality is conditioned by the shape of the passages, or “inherent properties of the cavities” (Crystal 27).
When the tongue is moved backwards or forwards, the space in the pharyngeal region changes, and with the movement upwards and downwards (usually followed by mandible movement) the space defined by the hard palate and tongue changes in volume and shape (Stevens 22). According to Johnson the volume  of the vocal tract in males is about 170 cm3 and 130 cm3 in females; when the mandible is lowered for about 1 cm (average in speech), the volume increases to 190 cm3 and 150 cm3, respectively (24). Citing Goldstine, Johnson gives 41.1 cm as an average vocal tract length in adult females, 6.3 cm for pharynx length and 7.8 cm for the oral cavity length. In males, the values are 16.9 cm, 8.9 cm and 8.1 cm, respectively (25). This shows that the oral cavity in both sexes is almost of the same length, while differences are reflected in the length of the pharyngeal region (25).
The physiology of the vocal tract links anatomy with phonetics. It describes, in terms of mechanics, properties and dimensions of the environment where speech sounds are created.
 “There are cultural effects too: in English-speaking cultures, it is common for males to enhance their intrinsically lower f0 by lowering their larynx, and for females to enhance their intrinsically higher f0.” (Ogden, Introduction 46)
 The velopharyngeal port is very important in discussing nasal sounds, where the air stream has a complex path that includes several cavities and an intricate physical model.
 Anterior/posterior – in anatomy, the axis from head to the opposite end of body.
 The values refer to the measurements when the vocal tract is in the neutral configuration.