Date: Wed, 10 Oct 2007 18:20:05 -0400
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Audio Forensics Experts Reveal (Some) Secrets
By Alexander Gelfand
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12:00 AM
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Catalin Grigoras, a forensic examiner from Bucharest, works in his lab.
Photo: Courtesy of Catalin Grigoras
NEW YORK -- A car door slams. Someone sneezes. Two voices -- one male,
one
female -- emerge from the din of traffic surrounding an urban parking
lot,
their conversation captured by a hidden surveillance microphone.
Suddenly, the sound pops like a needle jumping its groove on an old
vinyl
LP. "Anybody catch that?" Stuart Allen asks, jokingly.
Allen is a forensic-audio examiner, and the pop is only the most obvious
edit he made to a real-life recording he's brought to a workshop on
digital
forensics at the 123rd Convention of the Audio
<http://www.aes.
None of the sharp-eared audio professionals at the Javits Convention
Center
caught another edit on Allen's criminal-investigat
hid that edit behind a speaker's cough, and it was only revealed with
the
help of some sophisticated forensic software.
Allen's sneaky edit is the kind of thing that could be done to cook the
evidence in a criminal trial, and it is exactly what audio forensic
examiners like him are paid to uncover.
Typically, audio forensic examiners are asked to authenticate recordings
presented as evidence in criminal and civil court cases, such as
undercover
surveillance tapes made by the police, recordings presented by feuding
parties in a divorce, or tapes from corporations seeking to prove
employee
wrongdoing or industrial espionage.
Some audio forensic examiners go to extraordinary lengths to validate
recordings.
Catalin Grigoras, a forensic examiner from Bucharest, told the workshop
how
he uses the frequency signatures of local electrical power sources to
pinpoint when and where recordings were made. According to Grigoras,
digital
recorders that are plugged into electrical sockets capture the frequency
signature of the local power supply -- a signature that varies over
time.
Working with electrical companies throughout Europe, Grigoras has
compiled a
database of power signatures spanning several years. He uses a software
package called DCLive Forensics to compare the power signatures
captured on
suspect recordings with the signatures stored in his database. That, in
turn, allows him to determine when (and, to some extent, where) the
recordings were actually made.
The technique can even be applied to recordings made with
battery-powered
recorders, as long as they use electret
<http://en.wikipedia
act like capacitors, electret mikes will register the electrical
signatures
of nearby devices.
In one case, Grigoras claims to have identified the date of a recording
broadcast in Europe, but made in the Middle East, "probably in the
mountains, or in a cave," he says. He didn't mention any names, but it
was
hard not to think of Al Qaeda. Grigoras holds a Ph.D. in electrical
engineering and performs forensic work for the Romanian ministries of
justice and the interior.
Garrett Husveth, a court-approved forensic examiner, argued that audio
forensics experts may soon find themselves on the front lines in
fighting
terrorism.
According to Husveth, child pornographers, drug dealers and terrorist
groups
are starting to use aural steganography
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hiding data in seemingly innocuous carrier files -- to share information
surreptitiously.
Husveth hid a Bruce Springsteen tune after the end-of-file marker on an
MP3
containing James Brown's "I Feel Good." He pointed out that terrorists
could
use similar techniques to distribute secret files through file-sharing
networks or e-mail.
Audio forensics was born during WWII, when acoustic scientists
investigated
the possibility of identifying enemy voices on radio broadcasts. Their
efforts were made possible by the newly invented sound spectrograph, a
tool
for graphing the frequency and amplitude of voice patterns over time.
The police soon began using sound spectrograms to identify voices for
investigative purposes, and spectrographic evidence became widely
admissible
in courts of law.
The advent of digital audio made it far easier to tamper with recorded
evidence. But it also gave investigators a host of new and powerful
tools.
Improvements in forensic-audio software have given the field a big
boost.
Allen, for example, used a software package called EditTracker 2.0 to
dissect his doctored recording. First he played the audio file for the
audience and displayed its spectrogram on a projection screen. Then he
punched a key on his laptop.
Within seconds, EditTracker had scanned the file and flagged a bunch of
"feature discontinuities" -- unexpected bumps in frequency and
amplitude,
miniscule gaps and other unusual events. They're undetectable to the
naked
ear, but could indicate tampering.
The field has benefited from the popular glamorization of high-tech
forensics work in general. Industry insiders call it "the CSI effect."
"It's changed demand and changed expectations,
University of Colorado audio forensics expert who has consulted in cases
ranging from the Oklahoma City bombing to the Columbine shooting.
Still, Allen stressed that the work of the audio forensics examiner is
often
slow and painstaking. An examiner may have to analyze a recording
hundreds
of times in order to determine whether it was fudged -- and, if so,
how.
Nonetheless, it pays well. According to his website, panel moderator Tom
Owen charges a minimum of $4,500 to authenticate an hour's worth of
digital
or analog recording, and $2,500 to testify in court.
Then again, it's not the kind of thing anyone could do at home with a
copy
of GarageBand or Audacity.
"This is very complex work," said Allen. "It doesn't happen with the
push of
a button, like I saw on television the other night."
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