1. Ladefoged Peter 2003.Phonetic Data Analysis: An Introduction to Fieldwork and Instrumental Techniques. Malden: Blackwell.
2. Ladefoged P. 1982.A Course in Phonetics. New York: Harcourt, Brace, Jovanovich.
3. Nathan S. 1987. On second-language acquisition of voiced stops.Journal of Phonetics, (15): 313–322.
4. Chiba Kajiyama 1958.Vowel: Its Nature and Structure. Tokyo, MA: The Phonetic Society of Japan.
5. Corder S.P. 1967. The significance of learners’ errors.International Review of Applied Linguistics, 05.