Forensic Phonetics
Forensic Phonetics:
Issues in speaker identification evidence Andrew Butcher
Centre for Human Communication Research Flinders Medical Research Institute Flinders University, Adelaide, Australia
Abstract The field of forensic phonetics has developed over the last 20 years or so and embraces a number of areas involving analysis of the recorded human voice. The area in which expert opinion is most frequently sought is that of speaker identification – the question of whether two or more recordings of speech (from suspect and perpetrator) are from the same speaker. Automated analysis (in which Australia is a world leader) is only possible where recording conditions are identical. In the most frequently encountered real-world forensic situation, comparison is required between a police interview recording and recordings made via telephone intercepts or listening devices. This necessitates a complex procedure, involving auditory and acoustic comparison of both linguistic and non-linguistic features of the speech samples in order to build up a profile of the speaker. The most commonly used measures are average fundamental frequency and the first and second formant frequencies of vowels. Much work is still needed to develop appropriate statistical procedures for the evaluation of phonetic evidence. This means estimating the probability of finding the observed differences between samples from the same speaker and the probability of finding those same differences between samples from two different speakers. Thus there needs to be an acceptance that the outcome will not be an absolute identification or exclusion of the suspect. By itself, your voice is not a complete giveaway. 1. The field of forensic phonetics The use of phonetics as a forensic tool has developed over the past 20 years or so (Hollien 1990; Baldwin & French 1991), but with the rapid expansion in the number of cases depending on the evidence of covert audio and video recordings in recent years, forensic
Issues in speaker identification evidence Andrew Butcher
Centre for Human Communication Research Flinders Medical Research Institute Flinders University, Adelaide, Australia
Abstract The field of forensic phonetics has developed over the last 20 years or so and embraces a number of areas involving analysis of the recorded human voice. The area in which expert opinion is most frequently sought is that of speaker identification – the question of whether two or more recordings of speech (from suspect and perpetrator) are from the same speaker. Automated analysis (in which Australia is a world leader) is only possible where recording conditions are identical. In the most frequently encountered real-world forensic situation, comparison is required between a police interview recording and recordings made via telephone intercepts or listening devices. This necessitates a complex procedure, involving auditory and acoustic comparison of both linguistic and non-linguistic features of the speech samples in order to build up a profile of the speaker. The most commonly used measures are average fundamental frequency and the first and second formant frequencies of vowels. Much work is still needed to develop appropriate statistical procedures for the evaluation of phonetic evidence. This means estimating the probability of finding the observed differences between samples from the same speaker and the probability of finding those same differences between samples from two different speakers. Thus there needs to be an acceptance that the outcome will not be an absolute identification or exclusion of the suspect. By itself, your voice is not a complete giveaway. 1. The field of forensic phonetics The use of phonetics as a forensic tool has developed over the past 20 years or so (Hollien 1990; Baldwin & French 1991), but with the rapid expansion in the number of cases depending on the evidence of covert audio and video recordings in recent years, forensic
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