Evaluation of a technique for improving the mapping of multiple speakers’ vowel spaces in the F1 ~ F2 plane

We evaluate a vowel formant normalisation technique that allows direct visual and statistical comparison of vowel triangles for multiple speakers of different sexes, by calculating for each speaker a ‘centre of gravity’ S in the F₁ ~ F₂ plane. S is calculated on the basis of formant frequency measurements taken for the so-called ‘point’ vowel [i], the average F₁ and F₂ for the vowel category with the highest average F₁ (for English, usually the vowel of the TRAP or START lexical sets), and hypothetical minimal F₁ and F₂ values (coordinates we label [u’]) extrapolated from the other two points.

Expression of individual F₁ and F₂ measurements as ratios of the value of S for that formant permits direct mapping of different speakers’ vowel triangles onto one another, resulting in marked improvements in agreement in vowel triangle (a) area and (b) overlap, as compared to similar mappings attempted using linear Hz scales and the z (Bark) scale.