Vowel space density as an indicator of speech performance.

The purpose of this study was to develop a method for visualizing and assessing the characteristics of vowel production by measuring the local density of normalized F ₁ and F ₂ formant frequencies. The result is a three-dimensional plot called the vowel space density (VSD) and indicates the regions in the vowel space most heavily used by a talker during speech production. The area of a convex hull enclosing the vowel space at specific threshold density values was proposed as a means of quantifying the VSD.