41. The non-transitory computer-readable storage medium of claim 21 , wherein one or more of the vowel space metrics are calculated based on a first vowel characteristic, formant F 1 , and a second vowel characteristic, formant F 2 ; wherein the vowel space metrics comprise an area of a vowel triangle defined by a mean F 1 and F 2 values of the AA, IY, and OW vowel sounds.
41. The non-transitory computer-readable storage medium of claim 21 , wherein one or more of the vowel space metrics are calculated based on a first vowel characteristic, formant F 1 , and a second vowel characteristic, formant F 2 ; wherein the vowel space metrics comprise an area of a vowel triangle defined by a mean F 1 and F 2 values of the AA, IY, and OW vowel sounds. 42. The non-transitory computer-readable storage medium of claim 41 , wherein the area is calculated according to: 
 area=√{square root over ( s ( s−D IY , AA )( s−D AA , OW )( s−D OW , IY ))}, where s=0.5*(D IY , AA +D AA , OW +D OW , IY ), V represents mean F 1 and F 2 values for vowel V, and D x,y represents a Euclidean distance between two values in an F 1 -F 2 plane: 
 D x,y =√{square root over (( F 1 x −F 1 y ) 2 +( F 2 x −F 2 y ) 2 )}{square root over (( F 1 x −F 1 y ) 2 +( F 2 x −F 2 y ) 2 )}.