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Vowels are continuous sounds. The air stream expelled from the lungs acquires a distinct quality, but at no point does it meet any obstruction. Mostly tongue is the crucial factor in creating resonance chambers. It can move from a state of total passivity to the highest point in the mouth close to its roof.
This highly flexible organ is capable of positioning itself to various degrees of height. It is in the nature of vowels that they are voiced. What differentiates them is the way the tongue and the lips shape the channel through which the air passes after leaving the larynx. As we see in the word, ‘least’ tongue is raised, lips are spread and in the word ‘loose’ tongue raised at the back, lips are rounded and in the word ‘last’ tongue is low, lips, lips are somewhat rounded. Vowels that are produced when the tongue is high are called Close Vowels, when the tongue is low are called Open Vowels. When the tongue is pushed forward, they are Front Vowels and when pushed backwards are called Back Vowels. When vowels join with other vowels, they become diphthongs because they glide from one vowel sound to the other as ‘O’ in the word ‘go’ is a diphthong. Peter McCarthy states:
“A pure vowel is one for which the organs of speech remain in a given position for a time. A Diphthong is a vowel sound consisting of a deliberate and intentional glide”
A British Linguist, Daniel Jones proposed a set of reference points known as Cardinal Vowels Chart. There are eight Primary Cardinal Vowels. The cardinal vowels are an abstract yardstick to be used by the linguist. You can see in the graph Cardinal vowels representing tongue positions. The first Cardinal Vowel is usually referred to as C1 and represented by /i/ and so on. Vowels can be described in three variables. Tongue Height (low-high), Tongue Advancement (which part of the tongue is used so we have: Front, Central and Back or also Close, Half Close and Open) and Lip Rounding. English has twelve vowel sounds. In the cardinal chart, you can see seven short and five long vowels. An alternative way of organizing them is whether they are short or long.
Front Vowels: /i:/ - cream, seen (long high front spread vowel) /i/ - bit, silly (short high front spread vowel) /e/ - bet, head (short mid front spread vowel) /æ/ - cat, dad (short low front spread vowel).
Central Vowels: /E:/- burn, firm (long mid central spread vowel) /J/ - about, clever (short mid central spread vowel); this is called schwa. /^/ - cut, nut (Short low front spread vowel).
Back Vowels: /u:/ - boot, glue (long hight back rounded vowel). // - put, soot (short high back rounded vowel). /¿:/ - corn, born (long mid back rounded vowel) // - dog, rotten (short low back rounded vowel) /a:/ - hard, far (long low back spread vowel)
Diphthongs: We can analyse a diphthong or a triphthong as two or three vowels respectively but they are considered a single unit. There are two types of Falling and Rising. In the Falling Diphthong, the first element is more prominent. In the Rising Diphthongs, the second element is more prominent. English Diphthongs are mostly falling ones. Another classification is Centering if the second element is released as the central vowel. The tongue moves to the central position for the second element as in /iJ, J/ and Decentering or Closing if it moves to a less central position as in /a, ai etc./. In short, a vowel that does not change in quality is called a Monophthong and one that changes is a Diphthong. The diphthongs have been marked in the cardinal reference chart.
The centering diphthongs are: /iJ/, /J/, /eJ/ as in dear, poor and care respectively.
The closing diphthongs are: /ei/, /¿i/, /J/, /ai/, /a/ as in page, boy, show, high, cow respectively.