Voice Production Mechanisms
Generation of voice source requires adequate configuration of the airflow from the lungs and vocal fold parameters for oscillation. The sources for voiced sounds
are the airflow pulses generated at the larynx, while those for some consonants (i. e., stops and fricatives) are airflow noises made at a narrow constriction in the vocal tract. The expiratory and inspiratory muscles together regulate relatively constant
pressure during speech. The laryngeal muscles adjust the onset/offset, amplitude, and frequency of vocal fold vibration.
Regulation of Respiration
The respiratory system is divided into two segments: the conduction airways for ventilation between the atmosphere and the lungs, and the respiratory tissue of the lungs for gas exchange.
Ventilation (i. e., expiration and inhalation) is carried out by movements of the thorax, diaphragm, and abdomen. These movements involve actions of respiratory muscles and elastic recoil forces of the system. During quiet breathing, the lungs expand to inhale
air by the actions of inspiratory muscles (diaphragm, external intercostal, etc.), and expel air by the elastic recoil force of the lung tissue, diaphragm, and cavities of the thorax and abdomen. In effort expiration, the expiratory muscles (internal intercostals,
abdominal muscles, etc.) come into action. The inspiratory and expiratory muscles work alternately, making the thorax expand and contract during deep breathing.
During speech production, the respiratory pattern changes to a longer expiratory phase with a shorter inspiratory phase during quiet breathing. Figure2
shows a conventional view of the respiratory pattern during speech. The thorax
is expanded by inspiration prior to initiation of speech, and then compressed by elastic recoil force by the tissues of the respiratory system to the level of the functional residual capacity (FRC).
The lung pressure during speech is kept nearly constant except for the tendency of utterance initial rise and final lowering. In natural speech, stress and emphasis add local pressure increases. The constant lung pressure is due to the actions of the inspiratory
muscles to prevent excessive airflow and maintain the long expiratory phase. As speech continues, the lung volume decreases gradually below the level ofFRC,
and the lung pressure is then maintained by the actions of the expiratory muscles that actively expel air from the lung. It has been argued whether the initiation of speech involves only the elastic recoil forces of the thorax to generate expiratory airflow.
Indeed, a few studies have suggested that not only the thoracic system but also the abdominal system assists the regulation of expiration during speech, as
shown by the dashed line in Fig. 2. Thus, the contemporary view of speech respiration emphasizes that expiration of air during speech is not a passive process but a controlled one with co-activation
of the inspiratory and expiratory muscles.
Figure 2: Respiratory pattern during speech. Top two curves show the changes in
the volume and pressure in the lungs. The bottom two curves show schematic activity
patterns of the inspiratory and expiratory muscles. The dashed line for the expiratory
muscles indicates their predicted activity for expiration
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