Vowel spacing in four-vowel systems

Languages with fewer than average vowel quality contrasts provide a testbed for proposed universals of vowel systems. This paper examines the positioning in two- and three-formant acoustic spaces of the vowels of several languages with four contrastive vowel qualities. All have vowels like /i/ and /a/, with /i/ seeming the most similar across the languages. Most have a back rounded vowel with a higher F1 than that of /i/, resulting in the transcription /o/ rather than /u/. The position of the fourth vowel is highly variable. In San Carlos Apache it is a front mid vowel, in Shipibo a high central-back vowel, and in Paiwan a mid central vowel. The overall maximal separation of vowels in the acoustic space is therefore not the major organizing principle of these vowel systems. Rather, their construction is anchored by the polarity of /i/ vs /a/ (dispersed spectral energy versus compact mid-frequency energy). A high back vowel with close rounding is the next most distinct vowel in terms of spectral energy distribution. But this is avoided, perhaps because vowels of this type have attenuated amplitude, and a lower vowel is preferred. The fourth vowel is free to take a variety of characteristics.