A Likelihood-Ratio-Based Study On Forensic Speaker Comparison Using Lateral-Vowel Formant Transitions In Mandarin Chinese

In the study of forensic speaker comparison(FSC),the vowel formant is favorably regarded as a valid parameter owing to its variation features.However,along with the expansion of the discipline,one single parameter can no longer suffice the modern evaluation framework.It is in need of more acoustic-phonetic features to improve the forensic efficacy.This thesis,based on the likelihood-ratio framework,uses the acoustic-phonetic approach to systemically analyze the discrimination of formant transitions between the lateral and vowels in Mandarin Chinese.The main research questions are as follows.(1)Which lateral-vowel formant transitions are identifiable as to forensic speaker comparison?(2)How is the intra-speaker variation affecting the identifiability?(3)How is the validity of the forensic speaker comparison system? In order to answer the research questions,the thesis performs an experiment and then uses PRAAT 6.1to extract data.There,therefore,are 2028 formants measured.Using the multivariate kernel density formula(MVKD),the log likelihood ratios of each compared group are calculated on Matlab 2009 a.At last,a Tippett plot is drawn,and the log-likelihood-ratio cost is estimated to analyze the validity of the FSC system.The major findings of the thesis are threefold.First,formant transitions between the lateral and vowels are applicable to forensic speaker comparison,but the discrimination performance varies among the five syllables.Therein the syllable /là/ is only 25% identifiable,while /lì/ and/lǜ/ are both of 50% discrimination./lè/ and /lù/ are 75% identifiable.So to speak,except for the syllable /là/,the formant transitions of the other syllables are of discriminating potential.Second,the analysis of intra-speaker variation shows that formant transitions are not always stable.The variation rate of the syllable /lè/ is as high as 75%,but /lù/,/lì/ and /là/ are respectively 50%,50% and 25%.The intra-speaker variation of the syllable /lǜ/,0%,is the most desirable one.In general,the formant transitions of /lì/,/lù/ and /lǜ/ are relatively the ideal ones as forensic parameters.Third,according to the Tippett plot,the forensic speaker comparison system overall is proved to be decent as to validity.In addition,the log-likelihoodratio cost denotes that the performance of the syllable /lǜ/ is ideal in that its cost value is only0.72,while the others are greater than 1.Based on the frontier likelihood-ratio framework,the thesis analyzes the formant transitions between the lateral and vowels as parameters and probes into applying the features described above to forensic speaker comparison.This thesis is an exploration of the likelihoodratio framework at home,a referring source for forensic speaker comparison,and a fulfilment of developing parameters for the discipline.