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Review of  Forensic Speaker Identification


Reviewer: David Deterding
Book Title: Forensic Speaker Identification
Book Author: Tony G. Alderman
Publisher: Lincom GmbH
Linguistic Field(s): Phonetics
Subject Language(s): English
Book Announcement: 17.549

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Date: Wed, 15 Feb 2006 20:41:43 +0800
From: David Deterding <dhdeter@nie.edu.sg>
Subject: Forensic Speaker Identification

AUTHOR: Alderman, Tony Brian
TITLE: Forensic Speaker Identification
SUBTITLE: A Likelihood Ratio-based Approach Using Vowel Formants
PUBLISHER: Lincom GmbH
YEAR: 2005

David Deterding, NIE/NTU, Singapore

OVERVIEW

The ability to identify speakers based on samples of speech is
becoming increasingly important in forensics, to enable us for example
to determine who a telephoned bomb threat was made by and
whether recorded kidnapping demands were made by a particular
suspect or not. This book considers the effectiveness and reliability of
various methods of identifying and discriminating between speakers
based on measurements of the second and third formants (F2 and
F3) of the five long monophthongs of eleven male Australians
recorded on two separate occasions, particularly by considering the
Likelihood Ratio (the ratio of the probability that the two speakers are
the same divided by the probability that they are not the same).

In the following discussion,
/i/ refers to the vowel in 'heed',
/u/ to the vowel in 'who'd' (in the book, it is shown as a centralised
vowel, a ''barred-u''),
/o/ to the vowel in 'hoard',
/a/ to that in 'hard' (shown as an open central vowel, a ''turned-a''),
and
/3/ to the vowel in 'herd' (a mid central vowel).

SYNOPSIS

Chapter 1 introduces the issues of forensic phonetics and then
provides an overview of the book. In chapter 2, the basic principles of
probabilistic forensic speaker identification are discussed and the use
of formants for representing vowels is considered. Chapter 3 covers
issues concerned with probability theory including the Bayesian
approach to evaluating evidence and also the assessment of
normality. The characteristics of the normal distribution are discussed
in more detail in chapter 4, including statistical measures of deviation
from normality such as skew and kurtosis (the degree to which data is
clustered about the mean). In chapter 5, the vowels of Australian
English are presented with particular reference to the Bernard data
set of measurements of the vowels of 170 male speakers. The
acoustic representation of the five long monophthongs of Australian
English from the Bernard data set is evaluated in chapter 6, and then
the recordings on which this study are based are described in chapter
7. Chapter 8 then presents the results of the current study, comparing
the relative success of F2 and F3 of each of the five long vowels in
separating out the eleven speakers, and then chapter 9 considers the
implications of the study and discusses the way forward.

CRITICAL EVALUATION

This is a short book packed with data. In fact, nearly half of the 143
pages consist of the full tabulated measurements of F2 and F3 of the
eleven speakers and also the results for the effectiveness of each of
the parameters in discriminating between the speakers. While it is
highly commendable that so much detailed information is provided,
and indeed the lengthy tables do allow the reader to get a real feel of
the data (and also to check all the results, should one choose to), it
does sometimes get a bit overwhelming, particularly in chapter 8 when
a comparison of the effectiveness of each of the parameters is
presented first in isolation and then in various combinations.

Sometimes one wishes that more interpretation were provided. For
example, on page 44 we find that four out of five of the vowels have a
positively skewed distribution for F2. But why is this so? And what is it
about /u/ that makes it different from the others? Then we are told that
the F2 of /o/ is bimodal (pp. 45-6) for the Bernard data. Does this
mean there are two different realisations of the vowel in Australia, one
fully back and one less so? Or maybe there is some kind of instability
in the measurement? We learn on page 60 that the F-ratio for the
distribution of F2 for /o/ is low for the eleven speakers in this study,
which indicates that the between-speaker variation is relatively small
but the within-speaker variation is high for the F2 of this vowel. But
why? Is it perhaps related to the bimodality of the F2 of /o/? In chapter
8 (p. 66) we are shown that, when using the Aitken formula for
estimating the Likelihood Ratio, a smoothing factor of 0.05 is best for
the F2 of all the vowels except /u/ and a smoothing factor of 0.4 is
best for the F3 of all the vowels except /u/. But what is it about /u/ that
results in a need for the distribution of its F2 to be smoothed more
than that of the other vowels while its F3 needs to smoothed less? All
these questions seem to be crying out for further interpretation.

We might consider one aspect of the representation of the five vowels
a bit further. On page 43, we are shown a scatter plot of F1 against
F2 for the five vowels, and it appears that the range of F2 for /u/ is
about the same as for /i/. But this is partly an artifact of the scales
used: in percentage terms, a range of 1200 to 1800 Hz (for /u/) is in
fact substantially larger than a range of about 2000 to 2600 Hz
(for /i/). If, instead of linear Hertz scales, the plots were shown on
auditory Bark scales (as is common in acoustic representations
nowadays), the range of F2 for /u/ would be shown as larger than that
for /i/, and this might more accurately reflect the fact that there is
indeed substantial variation in the degree of fronting for /u/ in many
varieties of English, including Australian.

One further issue arises with regard to the data. A substantial quantity
of speech was recorded: 2 recordings on different occasions of 4
repetitions of 24 sentences, so the research is based on an
impressively large set of vowel measurements. However, it is not clear
why for two of the vowels the words were kept consistent, with a fixed
hVd frame, but for each of the other three vowels, another
phonological frame was included: for /i/, 'deed' was used in addition to
three instances of 'heed'; for /a/, 'card' occurs in addition to three
instances of 'hard'; and for /o/, 'board' was recorded in addition to
three instances of 'hoard'. Does this not mean that the influence of the
initial consonant might have increased the variation for /i, a, o/
compared to the other two vowels?

In fact, one might also question whether the general reliance on a
fixed 'hVd' word shape might not substantially underestimate the
degree of variation that exists in the vowels that occur in real speech
data, for the degree of coarticulation from initial and final consonants
may actually be quite significant.

Nevertheless, this book does present some fascinating and
exceptionally valuable results in an important area of research. The
data is carefully presented even if the interpretation might have been
elaborated a little, the foundations for the research are well-grounded
even if there are one or two questions one might ask about the data,
and many people working in this field of trying to establish the
theoretical and practical foundations of forensic speaker identification
will find the thoughtful consideration of so many statistical issues very
useful. This book undoubtedly makes a significant and important
contribution to the growing body of work on forensic phonetics, and
indeed many linguists who are interested in how vowels should be
represented will also find it informative and interesting.
 
ABOUT THE REVIEWER:
ABOUT THE REVIEWER


David Deterding is an Associate Professor at NIE/NTU, Singapore,
where he teaches phonetics, phonology, syntax, and Chinese-English
translation.


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