Доброго дня, Галино!
Зверніться до наступних джерел:
1. David S Taylor. Vowels, Consonants and Syllables in English: An English Teaching Perspective // http://www.fortunecity.com/victorian/vangogh/555/Spell/cons-vowel.htm
2. Peter Roach. English Phonetics and Phonology: A Practical Course // http://books.google.com.ua/books?id=dDVR_zh5wkMC&pg=PA99&lpg=PA99&dq=Phonological problems of English vowels&source=bl&ots=xliP1TiZkj&sig=Q8Tvc-ULApZSMg5JK_vrlpi2vuI&hl=ru&sa=X&ei=oi14T5rdIsLXtAaVm4jMBA&ved=0CDQQ6AEwAg#v=onepage&q=Phonological problems of English vowels&f=false
3. From sound to meaning: hearing, speech and language // http://openlearn.open.ac.uk/mod/oucontent/view.php?id=398684§ion=2.3
4. Красса С. Теоретическая фонетика английского языка // http://www.durov.com/study/1119378694-206.html