There are thirteen phonetic symbols for vowels but, in English spelling, there are only five letters to represent these sounds. This is why the English spelling system for vowels is often confusing and difficult to learn. Consequently, to accurately represent each of the vowels, it is necessary to use a number of additional symbols that are not contained in the standard alphabet.
Unlike the consonants, which may be produced with or without voicing, the vowels are always produced with voicing (vibration of the vocal cords). The particular vowel that is produced is determined by the degree of opening in the mouth and the position and shape of the tongue and lips.
For example, if you make the vowel /i/ as in the word “see” and then the vowel /u/ in “Sue”, you will notice that your lips are rounded for the /u/ , but not for the /i/. Now, lets add the vowel /ʌ/ as in the word “but”, and then make the sequence of sounds /i-ʌ-u/. Feel your tongue move from the front of your mouth to the back. This is because the tongue is raised or bunched in the front of the mouth for /i/, and then moves to the middle for /ʌ/, and then to the back for /u/. Thus, the positioning of the tongue in the mouth (front, middle, back) is one of the primary ways of classifying the vowel sounds.
This first group of vowel sounds, /i, ɪ, e, ɛ, æ/, is called front vowels because they are produced with the highest part of the tongue arched towards the front of the mouth.
The next group of vowels, /ʌ, ə/ and /ɝ, ɚ/, is called central vowels because they are produced with the tongue arched in the center of the mouth, i.e., midway between the front and back. These vowels are further distinguished according to whether they are stressed or unstressed. To illustrate, say the words “but” and “about”, and you will observe that the vowel in “but” and the first vowel in ‘about” are nearly the same. The main difference is that the vowel in “but” is stressed, and the vowel in “about” is unstressed, i.e., it occurs in an unstressed syllable. This difference is represented phonetically by using the symbol /ʌ/ for the stressed form of the vowel and /ə/ for the unstressed form.
Similarly, the symbols /ɝ/ and /ɚ/ are used to distinguish between the stressed and unstressed form of the /r/-vowel. The symbol /ɝ/ is used for the stressed vowel in words like “bird” and “nurse”, and the symbol /ɚ/ is used for the unstressed form in words like “mother” and “later”. Like the /r/-consonant, the /r/-coloring of the /r/-vowel is produced by a slight curling of the tongue tip.
The next group of vowels, /u, ʊ, o, a/, is called back vowels because they are produced with the back part of the tongue arched toward the back of the mouth. The three vowels /u, ʊ, o/ also differ from all of the other vowels because they are produced with a higher degree of lip rounding. Listen to the back vowels in the sample words and try saying them to familiarize yourself with the sounds and symbols.
The sounds in the last group, /ai, au, oi, iɚ, eɚ, oɚ, aɚ/, are combinations of vowels that function as single vowels. They are produced by starting with one vowel and then gliding or moving to another vowel. They are characterized by a constantly changing quality, and the symbols used to represent them are only suggestive of their starting and ending point. There are many different possible diphthongs besides those included here, but these are some of the most common ones and will serve to acquaint you with this type of sound.