Pure vowels articulate a single vowel phoneme. Dipthongs articulate combined vowel phonemes. According to the International Phonetic Alphabet (IPA), there are eight dipthong vowels. These are vowels that combine vowel sounds to create a double sound. Dipthong consonants fit the same definition, a doubling of consonant sounds, and are more familiar to most people. Some familiar consonant dipthongs, which provide an orientation for understanding vowel dipthongs, are sh, ch, th, pl, sn, st, as in sheet, church, these, place, snack, and street.
The eight English vowel dipthongs are represented by the vowel sounds in "stay" "try" "toy" "fear" "there" "sure" "mouth" and "nose." The IPA alphabet signifies these eight dipthongs with symbols, some of which I can't represent here. Others are signified as follows. The dipthong (double sound) in stay is signified as eI; try is aI; fear is I schwa; there is e schwa. The links below give complete IPA dipthong information.
The diphthong in English
The diphthongs are vowels in the production of which the tongue moves from one position in the mouth towards another position. In order to indicate this glide (movement) from one position towards another, the phonetic symbols for each diphthong is a combination of the two vowels- one in which the tongue is in position initially and the towards which the tongue moves. The two symbols represent a single sound and not two sounds. Of the eight diphthongs,
(a) in the three tongue glides towards/I/, that is, /ei,ai/
(b) in the two tongue glides towards /u/, that is,/au,au/, and
(c) In the three the tongue glides towards /a/, that is,/ea,ua/.
The diphthongs in which the tongue moves towards the vowels/I, and U/are called closing diphthongs and those in which the tongue moves towards /a/ are called centering diphthongs. Closing diphthongs gliding to/I/ /
The glide starts from a point just below the front, half-close position and moves in the direction of /I/. With the movement of the tongue towards/I/, the lower jaw moves upwards closer to the upper jaw than it was for /e/, the first element of the diphthong. The lips are spread.
For the production of /ai/ the tongue glides from a point near the front open position, towards the RP vowel /I//. Along with the glide the lower jaw moves from an open position to an appreciably closer position. The lips, which are in the neutral position at the beginning gradually change to a loosely spread position as for /I/. The vowel occurs in both accented and unaccented positions.
For the production of /oi/ first the back of the tongue moves towards the position between open and half-open and the lips are open rounded. Then the tongue glides in the direction of the vowel /I/.the lips, which open rounded at first, change the neutral towards the end. The jaw movement is less than for the diphthong/ai/.
Generally, the vowel occurs in accented syllables. Its occurrence syllables are rare. Closing diphthongs gliding to /u///:/au,au/.
/au/as in boat
For the diphthong/au/ the glide is from the central position between half-close and half-open, and moves in the direction of/u/. The movement of the jaw is very slight. The lips are neutral at the beginning of the glide and become rounded towards the end.
/ea/as in rare
For the diphthong/ea/ the glide begins in the front between the half-close and half open, closer to the half-open position, and moves in the direction of /a/. The glide moves in the direction of the opener /a/if/ea/occurs finally as in bear, and in the direction of the less open variety of /a/if it occurs non-finally as in scarce, various. The lips are neutrally open throughout the production of the glide.
The pure vowels of English
The vowels can be plotted on a vowel diagram with reference to the cardinal vowels. We shall first plot the pure vowels. Since all vowels are voiced in English and there is no nasalized vowels we assume that during the production of English vowels the vocal cords are vibrating and the soft palate is raised to shut off the nasal passage. We describe vowels in terms of the parts of the tongue raised and the relative height to which it is raised in the mouth. The front of the tongue raised can be fairly high mouth than for the vowel in the bead or it can be slightly lower in the mouth for the production of the vowel in bead for example, in bid and lower for the vowel in bed and still lower for the vowel in bed. Similarly, the back of the tongue is high in the mouth for the production of the vowel in cool; it is slightly lower in the mouth low in the mouth for the vowel in calm. Another important feature used to describe vowels in the position of the lips during their production. Sometimes the position of the lips is the only distinguishing features-between two vowels. For example, the two vowels .a:/as in calm, and/D as in hot, are both vowels during the production of which the back of the tongue is low in the mouth, the only features that differentiates /a/; from /D/ is the position of the lips. We have referred to long and shorts vowels. When the talk about ‘long` vowels what we refer to is the relative length of long vowels. In identical environments long vowels are longer than short vowels. For example, the vowel /I,/ in feet is longer than the vowel/I/ in fit. These words differ in respect of the vowel alone. The two consonants /f/ and /t/ are common to both. Apart from this, each vowel has different degrees of length depending upon the phonetic environments in which it occurs. For example, generally a vowel is longer when it is followed by a voiced consonant or when it occurs finally in a word than when it is followed by a voiceless consonant, for example, the vowel Jar/ in side and sigh is longer than in sight. The /ae/ in tag is longer than the /ae/ in tact. A vowel in the final position in words is longer than it is before voiced consonants. For example, the vowels/a/ is longer in the word car than in the word card. Thus vowel length is a variant which depends upon the position that it occupies in a word.
Vowel is a syllable that can be pronounced with free passage of air. For example the type of sound we make when a doctor examining our throat asks us to open our mouth and say ah... Consonants as opposed to vowels are syllables that can not be pronoun without some closing or restriction of the organs of speech. A vowel can be an independent syllable, or it can be combined with a consonant to form a syllable. In English the letters a, e, i, o, and u always represent vowels. In addition letters w and y also sometime represent vowels. Diphthongs are two vowels pronounces as a single sound. For example 'ou' as pronounced in the word out is a diphthong.