Note: This reading notes is wrote and edited on the basis of Springer Handbook of Speech Processing.
Abstract
Speech sound is a wave of air that originates from complex actions of the human body, supported by three functional units: generation of
air pressure,regulation of vibration, and control of resonators. The lung air pressure for speech results from functions of the respiratory system during a prolonged phase of expiration after a short inhalation. Vibrations of air for voiced sounds are introduced
by the vocal folds in the larynx; they are controlled by a set of laryngeal muscles and airflow from the lungs. The oscillation of the vocal folds converts the expiratory air into intermittent airflow pulses that result in a buzzing sound. The narrow constrictions
of the airway along the tract above the larynx also generate transient source sounds; their pressure gives rise to an airstream with turbulence or burst sounds. The resonators are formed in the upper respiratory tract by the pharyngeal, oral, and nasal cavities.
These cavities act as resonance chambers to transform the laryngeal buzz or turbulence sounds into the sounds with special linguistic functions. The main articulators are the tongue, lower jaw, lips, and velum. They generate patterned movements to alter the
resonance characteristics of the supra-laryngeal airway. In this chapter, contemporary views on phonatory and articulatory mechanisms are summarized
to illustrate the physiological processes of speech production, with brief notes on their observation techniques.
Overview of Speech Apparatus
The speech production apparatus is a part of the motor system for respiration and alimentation. The form of the system can be characterized,
when compared with those of other primates, by several unique features, such as small red lips, flat face, compact teeth, short oral cavity with a round tongue, and long pharynx with a low larynx position. The functions of the system are also uniquely advanced
by the developed brain with the language areas, direct neural connections from the cortex to motor nuclei, and dense neural supply to each muscle. Independent control over phonation and articulation is a human-specific ability. These morphological and neural
changes along human evolution reorganized the original functions of each component into an integrated motor system for speech communication.
The speech apparatus is divided into the organs of phonation (voice production) and articulation (settings of the speech organs). The phonatory
organs (lungs and larynx) create voice source sounds by setting the driving air pressure in the lungs and parameters for vocal fold vibration at the larynx. The two organs together adjust the pitch, loudness, and quality of the voice, and further generate
prosodic patterns of speech. The articulatory organs give resonances or modulations to the voice source and generate additional sounds for some consonants. They consist of the lower jaw, tongue, lips, and the velum. The larynx
also takes a part in the articulation of voiced/voiceless distinctions. The tongue and lower lip attach to the lower jaw, while the velum is loosely combined with other articulators. The constrictor muscles of the pharynx and larynx also participate in articulation
as well as in voice quality control. The phonatory and articulatory systems influence each other mutually, while changing the vocal tract shape for producing vowels and consonants. Figure
1 shows a schematic drawing of the speech production system.
Figure 1: Sketch of a speech production system. Physiological processes of speech
production
are realized by combined sequential actions of the speech organs for phonation and articulation.
These activities result in sound propagation phenomena at the three levels: subglottal cavities,
cavities of the vocal tract, and nasal and paranasal cavities
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