28. The non-transitory computer-readable storage medium of claim 27 , wherein the vowel space metrics comprise a vowel space range, wherein the vowel space range, F 1 Range, is calculated according to 
 F 1Range=Max F1 ( AA )−Min F1 ( IY ), where Max F1 (AA) is the maximum value of the first vowel formant measurements for all AA vowel sounds produced by a speaker, and Min F1 (IY) is the minimum value of the first vowel formant measurements for all IY vowel sounds produced by a speaker.
28. The non-transitory computer-readable storage medium of claim 27 , wherein the vowel space metrics comprise a vowel space range, wherein the vowel space range, F 1 Range, is calculated according to 
 F 1Range=Max F1 ( AA )−Min F1 ( IY ), where Max F1 (AA) is the maximum value of the first vowel formant measurements for all AA vowel sounds produced by a speaker, and Min F1 (IY) is the minimum value of the first vowel formant measurements for all IY vowel sounds produced by a speaker. 29. The non-transitory computer-readable storage medium of claim 28 , wherein the vowel space metrics comprise a second vowel space range, wherein the second vowel space range, F 2 Range, is calculated according to 
 F 2Range=Max F2 ( IY )−Min F2 ( OW ), where Max F2 (IY) is the maximum value of the second vowel formant measurements for all IY vowel sounds produced by a speaker, and Min F2 (OW) is the minimum value of the second vowel formant measurements for all OW vowel sounds produced by the speaker, wherein the non-native speech proficiency score is determined based on the F 1 Range and F 2 Range vowel space metrics.