The human voice produces sounds in the following manner:
- Air pressure from the lungs creates a steady flow of air through the trachea (windpipe), larynx (voice box) and pharynx (back of the throat).
- The vocal folds in the larynx vibrate, creating fluctuations in air pressure that are known as sound waves.
- Resonances in the vocal tract modify these waves according to the position and shape of the lips, jaw, tongue, soft palate, and other speech organs, creating formant regions and thus different qualities of sonorant (voiced) sound.
- Mouth and nose openings radiate the sound waves into the environment.
The larynx or voice box is a cylindrical framework of cartilage that serves to anchor the vocal folds. When the muscles of the vocal folds contract, the airflow from the lungs is impeded until the vocal folds are forced apart again by the increasing air pressure from the lungs. This process continues in a periodic cycle that is felt as a vibration (buzzing). In singing, the vibration frequency of the vocal folds determines the pitch of the sound produced. Voiced phonemes such as the pure vowels are, by definition, distinguished by the buzzing sound of this periodic oscillation of the vocal cords.
The lips of the mouth can be used in a similar way to create a similar sound, as any toddler or trumpeter can demonstrate. A rubber balloon, inflated but not tied off and stretched tightly across the neck produces a squeak or buzz, depending on the tension across the neck and the level of pressure inside the balloon. Similar actions, with similar results, occur when the vocal cords are contracted or relaxed across the larynx.
The vocal tract
The sound source from the larynx is not sufficiently loud to be heard as speech, nor can the various timbres of different vowel sounds be produced: without the vocal tract, only a buzzing sound would be heard.
Production of vowels
A vowel is any phoneme in which airflow is impeded only or mostly by the voicing action of the vocal cords.
The well-defined fundamental frequency provided by the vocal cords in voiced phonemes is only a convenience, however, not a necessity, since a strictly unvoiced whisper is still quite intelligible. Our interest is therefore most focused on further modulations of and additions to the fundamental tone by other parts of the vocal apparatus, determined by the variable dimensions of oral, pharyngeal, and even nasal cavities.
Formants are the resonant frequencies of the vocal tract that emphasize particular voice harmonics near in frequency to the resonance, or turbulent non-periodic energy (i.e. noise) near the formant frequency in the case of whispered speech. The formants tell a listener what vowel is being spoken.
vowel a e i o u
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