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Expert Evidence in Forensic Phonetics
By Allen Hirson,
Governor of the Expert Witness Institute (UK),
Senior Lecturer in Phonetics at City University London
As a forensic speech scientist based in London most of my forensic case work is on English
and the expert evidence that I provide is adduced within the local Common Law system.
However, the material on which I work is anything but parochial. Modern multilingual
society and the globalisation of organised crime is reflected in a wide range of languages
on the evidential recordings submitted for forensic analysis; over the years I have worked
on 27 such languages. Such recordings are submitted for forensic analysis because they
contain evidence relating to criminal cases including smuggling, fraud, terrorism, murder,
rape, and accident investigations - on telephone recordings, covert audio recordings, video
recordings, voicemails, indeed the list seems endless. Very occasionally, I work on civil
cases.
So long as the legal proceedings are in the UK, I am on familiar ground as regards the
judicial procedure.
I am instructed by either the Prosecution (e.g. the police) or the
Defence (solicitors acting on behalf of one or more defendants), but in either case my
responsibility is to the court rather than any interested party. This guiding principle of
independence is zealously defended and by studiously disregarding any partisan pressures,
the impartial evidence serves its purpose of assisting the jury to decide which way the
scales of justice should tip.
But how, one may wonder, is the speech scientist to deal with the cacophony of languages
with which s/he may be confronted? English presents challenge enough! But exercising
the caution in the professional code of conduct of the International Association for Forensic
Phonetics and Acoustics (the IAFPA), I am able to collaborate with language specialists in
the relevant language who also have familiarity with instrumental phonetic techniques and
knowledge of the variation (sociophonetics) of the language concerned. Through such a
Adresse correspondance :
92 rue Anatole France
92300 Levallois-Perret (France)
S/C Compagnie des Experts
près la Cour d’Appel de Versailles
5, rue Carnot - 78000 Versailles
www.experts-institute.eu
Tel : +33(0)1 41 49 07 60
Fax : +33 (0)1 41 49 02 89
E-mail : [email protected]
collaboration, the forensic scientist and language specialist combine their skills to work on
the recorded speech evidence.
A close collaboration with linguists
Cases on which I have worked that involve languages other than English have included a
case in which a child swallowed a handful of caustic soda. Clues as to who may have been
responsible were sought in the background speech captured on a telephone call to the
emergency services.
This was in European Portuguese.
In another case - an alleged
historic rape - a covertly recorded meeting (in Punjabi) revealed many relevant details
regarding the allegation. And in a case involving the collision of two oil tankers near The
Philippines, radio communications in Tagalog were analysed to establish the cause of the
accident. In each case, cIose collaboration with a relevant language specialist helped to
resolve the question in hand (see below).
When the evidence is contested, I expect to be called to give ‘live’ evidence at court
together with the language specialist; opposing experts are called by the other side. At
court, I would first give ‘evidence in chief’ addressing questions from counsel from my own
‘side’, followed by probing ‘cross-examination’ by the opposing barrister assisted by their
own experts. In this way, the evidence is properly tested, since any inconsistencies or
inaccuracies will be picked up by equivalent experts in the same field. Ultimately, the jury
of twelve men and women decide the outcome of the trial, and so the expert must direct
their evidence towards the jury.
Naturally, the evidence must be pitched at the right level for the jury to understand, and I
often imagine that I am addressing a new and unknown group of students at the University.
Owing to its key position in the legal process, the jury is generally regarded with
considerable respect, but nevertheless certain types of evidence are not admissible in the
UK courts if a High Court has ruled that it may confuse the jury. The relevance in the
context of speech evidence is that Bayesian statistical reasoning is in this category, and
this puts some constraints on admitting automatic speaker recognition (ASR) evidence in
this country.
The contents of the proofs and the identity of the speaker
The questions that are asked in relation to evidential speech recordings depend upon the
case in question. The two main questions are about (a) speech content when the speech
is indistinct, and (b) the identity of the speaker. On occasion other issues may arise such
as forensic lip-reading, the integrity of a recording (whether or not it may have been
tampered with), questions regarding background noises such as birdsong - that might
reveal, for example, where the recording was made, or even questions about what
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speakers or listeners may recall of the recorded event. At the core of the field is Phonetics
(speech science), but there are overlaps with Audio-engineering, Signal Processing, Pattern
Recognition, Bioacoustics, and Perceptual Psychology.
It is speaker identity that currently excites most interest, not only because our ability to
recognise individuals from their voices is intrinsically fascinating, but also because of
intense interest in automatic speaker recognition (ASR) technology. Despite its promise,
this technology has many shortfalls, and court rulings in several countries (and the current
author) consider it to be unreliable in the evidential domain. Interestingly, ASR claims to
identify speakers independently of language, comparing samples and generating likelihood
ratios with reference to a ‘background population’. ASR models the speech as the product
of a human vocal tract rather than as a linguistic system and by reducing the comparison
of speech samples to a problem of pattern recognition, Linguistics and Phonetics are
deemed to be largely irrelevant. By contrast, practicing experts in this field analyse the
phonetic details of pronunciation in relation to the relevant language, underpinning
phonetic judgements with acoustic measurements. Similarities and differences are then
evaluated in terms of population norms, speaking situation and ‘channel effects’ (e.g.
whether or not the speech was transmitted by telephone).
The expert then gives expert evidence regarding the likelihood that two recordings do or
do not originate from the same speaker, based upon their experience and expert
knowledge, with the disclaimer that such evidence is not comparable to DNA or fingerprint
evidence. Juries are able to understand such evidence, which is scientifically well-founded.
The language specificity of forensic speech analysis might be graphically illustrated by an
example from an utterance encountered recently during transcription of a covert audio
recording prior to speaker identification.
Based upon a phonetic and spectrographic
assessment, the utterance in question was transcribed as follows:
Yeah listen, yeah, he’s gone [gaɹəʔi] mate.
The phonetically transcribed [gaɹəʔi] did not immediately map to any known word/s, and
based on the hypothesis that I might have missed an indistinct preposition (‘to’)
immediately prior to the word (presumed singular), the possibility of a proper noun arose.
But, where, I wondered was Garity? Alternative spellings were explored and the word
‘garrity’ was eventually tracked down in Dalzell & Victor’s (2005) New Partridge Dictionary
of Slang and Unconventional English. Not a place at all, it turned out, but a slang word for
‘wild’ – and well known, I later discovered by the Police Officer in the case. The word is
derived from the name Eddie Garrity, (also known as Ed Banger), a singer in a short-lived
punk band from Manchester (1976-1978) called ‘Ed Banger and the Nosebleeds’.
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Ed
Banger was renowned for going wild on stage, and his name was absorbed into the English
language to mean just that. Naturally, this sleuthing did not solve the case, but it did help
to resolve the forensic transcription, before isolating the speaker of interest, phonetically
profiling and comparison with a speech sample from the prime suspect.
Transcripts of evidential recordings need to be meticulously accurate, in order to capture
details of importance and also to ensure that speakers are not confused. If the extracted
speech sample is contaminated with extraneous speakers, the results could be
compromised. As in all forensic science, the devil is in the detail. Slang and colloquial
language use, even argots (akin to French ‘Verlan’) are often decrypted in forensic phonetic
casework. Deciphering what has been said and by whom frequently requires expert
analysis that ultimately can provide valuable – and in instances such as ‘garrity’, surprising
evidence.
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