A Likelihood Ratio−based forensic voice comparison using formant trajectories of Thai diphthongs

This study aims to quantify and compare acoustic parameters of the three Thai diphthongs /ia, īa, ua/ forensically. The likelihood-ratio approach is applied to the parameterized formant trajectories of each instance of each diphthong. The aim of this study is to assess whether such an approach can be used to effectively distinguish, in a probabilistic sense, two or more speech samples as belonging to the same speaker or as opposed to belonging to different speakers. Formant trajectories were fitted using polynomial interpolation. Likelihood ratio values were derived using the multivariate likelihood ratio (MVLR) estimation approach [1] and then calibrated by using the Log-Likelihood Ratio Cost function -Cllr [2]. Speech samples for this study were obtained from 15 male speakers of Standard Thai who performed various tasks aimed at eliciting speech such as giving directions from a map and reading from a script. Such speech data were recorded over two non-contemporaneous sessions separated by at least a week. The results show that the strength of evidence with calibrated Log10LRs > 4 were obtained with the smallest Cllr of 0.01.