VOWELS
It was mentioned earlier that most sounds in speech are produced by passing a stream of air from the lungs through one or more resonators belonging to the phonetic apparatus. There are four principle resonators: (1) the pharyngeal cavity, (2) the oral cavity, (3) the labial cavity, and (4) the nasal cavity. Air flows through these resonators.
The presence or absence of obstructions in the course of the airstream modifies the nature of the sound produced. By classifying the different types of obstructions that are possible, articulatory phonetics distinguishes four different major sound classes: consonants, vowels, glides or semivowels, and liquids.
The distinction between consonants and vowels is quite simple. If the air, once out of the glottis, is allowed to pass freely through the oral cavity, the sound is a vowel. If the air, once out of the glottis, is partially or totally obstructed in one or more places in the oral cavity, the sound is a consonant. It should be noted that the line between vowels and consonants cannot be clearly drawn. A continuum exists between the two extremes. In English, there are also intermediate instances: (a) liquids, and (b) glides.
In order for a phoneme to be a vowel, it should meet certain vowel-hood criteria. These criteria include: (a) the degree of openness of the oral cavity also known as the degree of aperture, (b) the degree of tension or laxity of the vocal tract muscles, and (c) amount of duration or length of articulation.
There is a large degree of freedom in the articulation of open vowels. However, this freedom is not endless. On the one hand, no vowel can be more 'open' than the standard open vowels (fourth degree of aperture). On the other hand, a vowel could not be much more 'close' than the standard close vowels (first degree of aperture). Additionally, the close vowels must have a certain minimum duration in order to be perceived as a vowel rather than a consonant.
The chief characteristic of vowels is the freedom with which the airstream, once out of the glottis, passes through the speech organs. The supra-glottal resonators do not cut off or constrict the air. They only cause resonance, that is to say, the reinforcement of certain frequency ranges.
A vowel's timbre (or quality) depends on the following elements: (a) the number of active resonators, (b) the shape of the oral cavity, and (c) the size of the oral cavity. There are three possible resonators involved in the articulation of a vowel: (a) the oral cavity, (b) the labial cavity, and (c) the nasal cavity. If the soft palate is raised, the air does not enter the nasal cavity, and passes exclusively through the oral cavity; if the soft palate is lowered, however, air can pass through nose and mouth simultaneously. If the lips are pushed forward and rounded, a third, labial resonator is formed; if, on the other hand, the lips are spread sideways or pressed against the teeth, no labial resonator is formed. It is thus the number of resonators at stake in distinguishing nasal vowels (nasal resonator active) from oral vowels (no nasal resonance), and rounded vowels (labial resonator active) from unrounded vowels (no labial resonator/no labial resonance).
Ordinarily, English vowels do not involve the nasal cavity. They can, however, become nasalized in certain contexts (e.g., when they follow nasal consonants). The shape of the oral cavity is determined by the general position of the tongue in the mouth. This divides the vowels into three great classes: (a) front vowels, (b) back vowels, and (c) central vowels. In the articulation of front vowels, the tongue body is in the pre-palatal region. In the production of back vowels, the tongue body is in the post-palatal or velar region. Finally, in the articulation of central vowels, the tongue body is in the medio-palatal region.
The size of the oral cavity is the last factor considered in the articulatory classification of a vowel. The size of the oral cavity depends directly upon the degree of aperture (of the mouth), that is, upon the distance between the palate and the tongue's highest point. Arbitrarily, four degrees of aperture are distinguished, from the most closed (first degree) to the most open (fourth degree). The descriptions of the vocalic articulations (i.e., names given to vowels) should be grouped according to the following principle:
An initial classification is made based on degree of aperture; within each group, the vowels are then divided according to mouth shape, and then as rounded or unrounded.
Vowels differ from consonants in that there is no noticeable obstruction in the vocal tract during their production. Air escapes in a relatively unimpeded way through the mouth and/or nose. If you try saying aaaaa, iiiii, uuuuu, eeeeee, oooooo to yourself you should be able to feel that, although your tongue moves about your mouth, it never actually obstructs the airflow.
You should also be able to feel that the position of the tongue changes for each of those vowels. Vowels are far more difficult to transcribe than consonants and are also an extremely important area of English phonology as they make up the greatest difference between English varieties. In other words, the differences among varieties of English are mostly due to the speakers' articulation of vowels. In addition, vowels are the stress carrying phonemes in stressed syllables.
The relative position of the tongue in the mouth is the most crucial criterion that determines the differences between vowels. The changes of the tongue position can be very slight and difficult to detect. Therefore, phoneticians produced what they called the vowel chart to describe these tongue positions.
As such, the vowel chart attempts to map the positions of the tongue and jaw in articulating vowels. It is worth mentioning that, as it is so difficult to determine the exact position of the lips, tongue and palate, there is no single agreed upon vowel chart.
