The reduction of the six Bulgarian vowels (i, E, a, 3, O, u) to a four- or (in some dialects) three-vowel subsystem (i, (3), 3, u) in unstressed syllables is generally accepted. But a number of studies disagree on the exact nature of the reduction process. Claims differ as to whether or not /a/ merges phonetically with /3/, and /O/ with /u/, or whether the assumed neutralized oppositions take a phonetically intermediate quality ( and [o]). Previous acoustic analyses have been based on very few, and almost exclusively male speakers. The present study uses all the men's and womenfs vowels from the Babel contemporary standard Bulgarian (CSB) read speech corpus. Mid-vowel F1 and F2 values were normalized to remove inter-speaker differences and statistical comparisons of stressed and unstressed vowel productions performed. Results confirm the raising of unstressed /a/ and /O/ reported previously, but raising is found to affect all vowels when unstressed except /i/. Unstressed /a/ and /O/ are raised to the quality of stressed /3/ and /u/ respectively (effectively neutralizing the opposition), but remain distinct from unstressed /3/ and /u/, which are raised from their stressed vowel positions. The mechanism underlying the reduction process is discussed.
Bibliographic reference. Andreeva, Bistra / Barry, William / Koreman, Jacques (2013): "The bulgarian stressed and unstressed vowel system. a corpus study", In INTERSPEECH-2013, 345-348.