Examination of Firearms Review: 2007 to 2010 - Interpol

1. Introduction
This review focuses on Forensic Audio Analysis and the advances in technology during
the past three years. The principal topics covered are divided into five sections:
Authentication, Speaker Identification, Audio Enhancement, Related Fields of Audio
Evidence, and Working Groups and Organizations.
The selection of specific sections to be reviewed was based on a broad literature review
of approximately 100 references which was further refined. For most of the references
the authors reviewed the complete articles; however, the abstracts were sufficient for a
small number of reviews.
Over the past few years there have been several areas of substantial research and
publication. One area of interest is the use of Electronic Network Frequency (ENF)
analysis for authentication of audio recordings. Work has been on-going for several
years in Europe, while in the United States, research has been started in an effort to
implement this technique. Additionally there have been various projects dedicated to
testing automated speaker identification systems and developing better methods for
speaker identification as well as research projects relating to speaker identification,
each of which help strengthen the current state of the science. There also appears to
be an increased focus on forensic audio procedures as more laboratories have
requirements for forensic laboratory accreditation and practitioner certification. In the
United States there is extensive discussion in the forensic science community in
response to the 2009 release of the U. S. National Academy of Sciences released
report entitled “Strengthening Forensic Science in the United States: A Path Forward.”
2. Authentication (Geoff Smith)
The authentication method garnering the most attention is the use of Electric Network
Frequency (ENF) analysis that was first proposed by Grigoras in 2003 (1). It has been
called the “'biggest breakthrough since Watergate [1972 U.S. political scandal]” (2) in
the field of audio authentication. In this method, the frequency changes of the mains
hum (50 Hz or 60 Hz depending on location) in a digital recording are compared to a
historical database of the mains frequency changes. Depending on the conditions, it is
possible to provide details about the recording including the date and time the ENF
corresponds to, the geographical area the ENF corresponds to, if the recording is
original or copied, and the continuity of the recording.
For current methodologies see Grigoras (3) and Cooper (4). These methodologies
include ENF database collection, extraction of the ENF data from a recording, and an
automated procedure for matching the extracted data to the historical database. ENF
analysis has been tested in both continental Europe by Brixen (5), Michalek (6), and
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