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 phonetics now plays a crucial role in an increasing number of criminal trials. A forensic phonetician may be asked to prepare reports in a number of areas, of which the following four are the most frequently encountered: 1.1 Speaker identification. This is by far the most commonly required task and the subject of the remainder of this paper. 1.2 Disputed utterances. In view of the usually very poor quality of covert police recordings (especially those made via a listening device), there is often ample scope for a defendant to
challenge the prosecution’s version of what was actually said in the course of a recorded conversation. Forensic phoneticians may be asked to prepare a report on the quality of the recording and the intelligibility of the speech. They may also be asked to prepare an ‘objective’ transcript of the recording. 1.3 Tape authentication. Occasionally a defendant (or a civil litigant) may have cause to question whether an audio recording has been tampered with in some way. Usually the claim is that certain sections have been excised or perhaps transposed. It is not generally within the competence of a phonetician to give an opinion as to the physical condition of a tape, but there may be evidence within the acoustic signal (‘pops’ or abrupt changes in either the signal itself or the background noise) which would be indicative of electronic editing. However, currently available software makes ‘seamless’ editing comparatively easy, and a phonetician may be needed to give an opinion on the only remaining evidence of any tampering – linguistic evidence in the form of unnatural changes in rhythm, tempo or intonation. 1.4 Voice line-ups. The practice of confronting witnesses of a crime with a tape recorded ‘voice line-up’, where the voice of a suspect is included amongst a series of ‘foils’, may be used to obtain evidence of identification in cases where, in the course of committing a crime, an unseen or masked perpetrator...