Towards higher precision in vocal tract length estimation

Abstract

Approaches to vocal tract length (VTL) estimation typically differ in how much credibility is allocated to certain formants as predictors of length, but are alike in allocating equal credibility to all vowel spectra. The latter may be problematic, as asymmetries in the phonotactic frequency of certain vowels and measurement errors for higher formants can influence VTL estimates. Herein, an additional parameter is proposed which privileges vowel spectra that approximate a uniform tube. The proposed metric for this proximity is standard variance (σ) in Φ, where Φ=Fn/(2n-1), n represents the integer label of the formant, and σΦ for a uniform tube is 0. The five estimators detailed in Lammert & Narayanan (2015) were tested on the speech of two adult males. Each of the estimators were run using all vowel spectra as well as only those for which σΦ < 50 Hz. Estimates from all vowel spectra had σVTL well above 1 cm, which is undesirably large, given that the adult vocal tract only ranges from 13 to 20 cm. However, estimates from vowel spectra with σΦ < 50Hz had σVTL less than 1 cm. This higher precision is especially striking since spectra with σΦ < 50 Hz constituted a small fraction of the total.