This article discusses the mechanism of speech production in humans. Speech is the acoustic end product of formalized motions of the vocal apparatus. The vocal apparatus consists of the following parts:
- Trachea or windpipe
- Pharynx or throat cavity consists of nasal pharynx (above the velum, rear end of nasal cavity), laryngial pharynx (beneath the epiglottis) and oral pharynx (behind the tongue, between the epiglottis and velum).
- Larynx surmounts the top of the trachea. The cartilaginous frame houses two lips of ligament and muscle called the vocal cords. The slit-like orifice between the cords is called the glottis. Epiglottis shields the opening at the vocal cords and prevents food from going into the trachea.
- Oral cavity is non-uniform in cross-sectional area and approximately 17 cm long in adult male. It is bounded by the lips, tongue, and palate and terminated at the back end by vocal cords. Its cross-sectional area can vary from zero (i.e.” complete closure) to 20 cm2 by movement of the articulators.
- Nasal cavity; Is a non-uniform tube above the palate and extends from the velum to the nostrils. In the adult male the cavity has a length of about 12 cm. It is partitioned over part of its front-to-back extent by the nasal septum. Acoustic coupling between the nasal and vocal tracts is controlled by the size of the opening at the velum.
Speech is resulted when the acoustic information from hearing system and the kinesthetic information from speech musculature is formulated by the central nervous system and sent to the vocal system. Speech utterances begin as air stream or volume velocity waves form the lungs. As air is forced from the lungs it passes through the trachea into the pharynx. These waves travel along the vocal tract and modified by the formalized motions of articulators (lips, tongue, jaw and velum) to be radiated as an acoustic pressure wave from the lips and lips & nostrils.
The performance of articulatory system degrades as a result of impairment of either control mechanism, for example partially or totally deaf persons have difficulty in producing adequate speech.