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Volume 7, number 2 (December 2000)


Contents
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Hermann J. Künzel

Effects of voice disguise on speaking

Patterns of voice disguise in forensic cases involving speaker identification or speaker profiling may contain clues to features of the undisguised voice of a speaker. In a longitudinal and synchronous study, 100 subjects were asked to read a text on five occasions during a period of six months, first using their normal voices, and subsequently with two out of three modes of voice disguise, (1) raising fundamental frequency, (2) lowering fundamental frequency, (3) denasalization by firmly pinching their nose. The focus of this investigation is on fundamental frequency (F0). Results show that most subjects were in fact able consistently to change their F0 according to the mode of disguise they had selected. However, there were differences between both sexes with regard to their preference of disguise modes as well as to the individual articulatory ‘strategies’ which they employed to implement them. Results corroborate experience with forensic casework, that is, they show that there is a constant relation between the F0 of a speaker’s natural speech behaviour and the kind of disguise he will use in an incriminating phone call. Speakers with higher-than-average F0 tend to increase their F0 levels. This process may or may not involve register changes from modal voice to falsetto. Speakers with lower-than-average F0 prefer to disguise their voices by lowering F0 even more and often end up with permanently creaky voice. The latter trend can be observed much more clearly in males. Females are generally more reluctant to make drastic changes to their fundamental frequency patterns.
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Paul Foulkes and Anthony Barron

Telephone speaker recognition amongst members of a close social network

This article presents results from a speaker recognition task carried out by a close-knit network of speakers (university friends who have lived in shared accommodation with each other for two years). Ten male speakers recorded a scripted message on to an answer machine via a mobile telephone. Two foil speakers from outside the network were also recorded. Samples of between 8 and 10 seconds were extracted from all twelve recordings, and used as stimuli for an open speaker recognition test performed by the network members. Listeners varied widely in their performance, and one listener failed to recognize his own voice. Some of the voices were easy to identify, but several speakers were consistently misidentified, and one speaker was particularly hard to identify. Both of the foil speakers were sometimes mistaken for network members. Auditory analysis of the voices shows, as expected, that speakers with the most distinctive regional accents and other idiosyncratic features were the most consistently identified. Acoustic analysis of F0 was also undertaken. It was found that the speakers who were most consistently identified were those with relatively high and low mean F0 values, as well as those with the widest and narrowest overall F0 range. Speakers with average pitch values and ranges in the middle of the overall group values proved harder to identify. The findings support the view that average pitch is a robust diagnostic of speaker identity, not only for forensic phoneticians, but also for naïve listeners. They furthermore demonstrate that naïve speaker recognition, even among members of a close-knit social network, is not a task which can be achieved infallibly.
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Harry Hollien and Reva Schwartz

Aural-perceptual speaker identification:

The degree to which speech and/or speech samples are noncontemporary is considered important to the speaker identification process. There are two dimensions to the problem; the first relates to the listener and, especially, to earwitness lineups. Here, the subject or witness is asked to make identifications at various times after having heard (but not having seen, of course) the speaker. It has been found that a person’s memory for a voice decays over time. In the second case, it is the samples of the speaker’s utterances which are temporally displaced. The prevailing opinion here has been that the use of non-contemporary speech samples poses just as difficult a challenge to the speaker identification process as does the decaying memory of a witness. Accordingly, research was carried out to test this possibility (Hollien and Schwartz, in press); it was found that the overall drop in correct identification over latencies from four weeks to six years was only about 15–25 per cent. It was not until the greatest of the time separations was studied (i.e., twenty years) that a substantial drop occurred (to 31 per cent). At this juncture, a number of questions arose; and three of them have been investigated. First, is listener gender important to the process; second, are the identification levels affected by the type of listeners employed and, finally, can external factors serve to differentially degrade listener performance? It was found that the first question could be answered in the negative and the second two in the affirmative. These findings should aid in clarifying some of the relationship between sample latency and identification accuracy.
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Susan Berk-Seligson

Interpreting for the police: issues in pre-trial phases of the judicial process

During the investigative phase of the judicial process, interpreting for those who do not speak the language of the courts is often carried out either by bilingual police officers and other employees of the police department, or by relatives and friends of suspects or detainees. In any of these cases, the norms of professional court interpreting can easily be violated. A review of appellate cases coming from California, Florida and New York reveals that when the police make use of unqualified interpreters during their investigative interviews or interrogations, frequently the Miranda rights of detainees are jeopardized. Issues of hearsay also arise in such cases. The use of ad hoc interpreters in police investigative work is questioned by the author.
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Christophe Champod and Ian W. Evett

Commentary on A. P. A. Broeders (1999) ‘Some observations on the use of probability scales in forensic identification’, Forensic Linguistics, 6(2): 228–41
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Dennis Kurzon

The right to understand the right of silence: a few comments. (A commentary on discussions on the right of silence which appeared in issue 7(1))
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Book Reviews

Book Reviews
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User: WEIMING LIU
Session: 18888

Forensic Linguistics is published by the University of Birmingham Press.

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西北政法大学外国语学院刘蔚铭教授创建与维护

2002-05-062008-01-25