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Forensic Acoustics
one other than presidential biographers and historians af- of acoustical events, and educate the attorneys, defendants,
ter he left office. However, it quickly turned into a political judges, and juries about the meaning, significance, and limi-
and legal bombshell of historic proportions in 1973 when tations of the recorded evidence (Audio Engineering Soci-
White House aide Alexander Butterfield revealed during his ety, 1996; Maher, 2009). Figure 2 depicts an interpretive an-
congressional testimony that there were secret audiotape notation that has been added to a time waveform to assist
recordings of conversations between the President and his with an investigation.
to make the original recording (Advisory Panel on White 1958): “Current advances in the technology of electronics
House Tapes, 1974).
and sound recordings make inevitable their increased use
advisors! After all, this was in the midst of the Watergate investigation and the widespread public concern about the
of this scientific development. Safeguards against fraud or their characteristics, total length, and mechanical integrity;
Forensic Acoustics and the US Courts
other abuse are provided by judicial insistence that a proper (2) verify that the recording is complete and continuous and
veracity of various White House officials who had testified Audio forensics traces its origins to the development of por-
before Congress and in federal court. President Nixon even- table recording equipment, and examples of the use of audio
tually agreed to release edited transcripts of the various con- recordings in US courts date to the mid-1950s. Although the
versations recorded by the secret taping system and later the courts generally began to accept the unique importance of
tapes themselves.
audio recordings, especially in cases involving speech ob-
foundation for such proof be laid.”
does not exhibit any erasures, splices, or stop/start sequenc-
tained via clandestine surveillance or wiretaps, there were In 1974, interest turned to a particular recording of a con-
versation now offered in evidence; (2) that the operator of
significant considerations regarding the Fourth Amend- versation between President Nixon and his Chief of Staff H.
the device was competent to operate the device; (3) that the
ment’s protections against unreasonable searches and sei- R. Haldeman recorded in 1972 in the EOB. The investigators
For a few decades after Watergate, the discipline of acousti- recording is authentic and correct; (4) that changes, addi-
zures and concern about the legal admissibility of a record- were suspicious that the recorded conversation included re-
cal forensics revolved around two common requests: estab- tions or deletions have not been made in the recording; (5)
ing as being a bona fide representation of the sonic events marks about the Watergate cover-up, but when the record-
lish the authenticity of a tape recording and identify the talk- that the recording has been preserved in a manner that is
actually present during the recording process.
ing was examined, the investigators discovered that 181⁄2
ers whose utterances are audible in the recording. The work shown to the court; (6) that the speakers are identified; and
minutes of the recording were obliterated by an unexplained
generally focused on analog audio because up until recently, (7) that the conversation elicited was made voluntarily and
The McKeever Case
the most frequently encountered recording medium in fo- in good faith, without any kind of inducement.”
gap consisting of audible buzz sounds but no discernable Among the key cases in the US federal court system regard-
speech. Investigators suspected that someone had deliber- ing the admissibility of audio forensic evidence is United
ately erased or recorded over that section of the tape to de- States v. McKeever (1958). In the McKeever case, two de-
stroy the originally recorded conversation, perhaps with the fendants were indicted for extortion in an antiracketeering
intention of eliminating incriminating remarks. prosecution involving the International Longshoremen’s
rensic cases was analog tape.
Association. After his indictment, the defendant McKeev- John J. Sirica, Chief Judge of the US District Court for the
Watergate tape, the physical medium itself could provide investigations, and similar capability has been instituted
er had arranged to make a surreptitious tape recording of District of Columbia, determined that the potentially altered
useful forensic information about edits, splices, stop/start/ in other public and private forensic acoustics labs around
a conversation he had with an individual who later was a tape required expert analysis beyond the routine capability
erase sequences, and other alterations affecting authentic- the world (Koenig, 1990). As described in Modern Audio
witness in the trial. During the trial, McKeever’s defense of the court (McKnight and Weiss, 1976). He requested that
ity. Our contemporary digital recorders can provide much Forensics: Digital “Good” and Digital “Bad,” tenet 3, au-
team sought to challenge under cross-examination the wit- the Watergate Special Prosecutor and the counsel for the
greater quality, stability, and storage capacity, but the ability thenticity determination, has become more difficult in our
ness’ testimony by playing a portion of the clandestine tape president jointly nominate a group of six outside technical
to read, alter, and resave the contents of a digital bitstream era of read/write digital recording and computer processing
recording to refresh the witness’ recollection. The court al- experts (including several ASA members) to form a special
and even to manipulate the file’s date and other file system compared with the historic reliance on physical analog mag-
lowed the tape to be played but only via headphones so that Advisory Panel on White House Tapes “ study relevant
data without leaving a physical trace in the flash memory netic tapes.
the witness could hear it but not the jury. When the defense aspects of the tape and the sounds recorded on it” (Advisory
or computer disk file has caused serious concern about our ability to authenticate digital audio forensic recordings. If a
then sought to have the recording played in court so that Panel on White House Tapes, 1974).
Forensic Acoustics and Watergate
the jury also could hear it, the prosecution objected to the
forensic examiner cannot detect any evidence of tampering, A significant turning point in the practice of forensic acous-
it is still conceivable that the digital data could have been tics in the United States occurred 15 years after McKeever
manipulated in some undetectable manner.
during the Watergate scandal. In 1971, late in his first term
The Advisory Panel analyzed the physical tape itself and the use of the tape because its admissibility as evidence had not
electrical signals observed on playback and, ultimately of been established. Specifically, the court had to address the
in office, President Richard Nixon directed the Secret Ser- One proposed approach to address digital audio authenticity
greatest importance, performed magnetic development us- fact that the recording was obtained secretly out of court, the
vice to install audiotaping systems in the Oval Office and is based on detection of a tell-tale hum in the recording due
ing ferrofluid to reveal latent magnetic domain patterns on participants were not sworn, the witnesses disputed whether
the Cabinet Room of the White House, in the president’s to interference from alternating current (AC) power leak-
the tape and the magnetization signatures of the recording or not they recognized even their own voices, and the legal
private office in the Executive Office Building (EOB) next ing into the audio recording (Grigoras, 2005; Cooper, 2008).
and erase heads installed in the tape recorders known to be chain of custody had not been demonstrated to ensure ad-
to the White House, and at Camp David, the president’s re- The electrical network frequency (ENF) of the AC power
present in the White House. The magnetic development of missibility of the audio evidence.
treat in rural Maryland. The existence of these recording grid actually varies slightly from its nominal 60-Hz (US) or
the tape led the Advisory Panel to the conclusion that the
systems was known only to a select group of individuals 50-Hz (Europe) frequency by a time-varying deviation that
The court examined these questions and considered a num- 181⁄2-minute gap consisted of several overlapping start-stop
and to the Secret Service (Nixon Presidential Library and depends on the instantaneous balance between power gen-
ber of prior federal court rulings, then established what the erasures performed with a specific tape recorder available
Museum, 2015). President Nixon presumably assumed that eration and consumption on the grid, which fluctuates from
forensic acoustics community now refers to as the Seven in the White House but not the same device that was used
the recording system's existence would be of interest to no
Tenets of Audio Authenticity (United States v. McKeever,
26 | Acoustics Today | Spring 2020, Special Issue
Summer 2015 | Acoustics Today | 25
24 | Acoustics Today | Summer 2015 Reprinted from volume 11, issue 3
to obtain and preserve evidence possessing genuine proba- The procedures employed by the Advisory Panel became the
tive value. Courts should deal with this class of evidence in standard for audio forensic investigators: (1) examine the
a manner that will make available to litigants the benefits physical tape, reels, and structural housing, documenting
es; (3) listen carefully and critically to the entire tape; and [...]“A review of the authorities leads to the conclusion that,
(4) use nondestructive signal processing as needed for intel- before a sound recording is admitted into evidence, a foun-
ligibility enhancement.
dation must be established by showing the following facts:
(1) that the recording device was capable of taking the con-
Modern Audio Forensics: Digital
“Good” and Digital “Bad”
Since the early 1960s, US Federal Bureau of Investigation
Analog magnetic tape had a variety of drawbacks including (FBI) laboratories have developed techniques and proce-
comparatively poor signal quality, stability, and storage ca- dures for assessing the authenticity and audible contents of
pacity. However, as found with the 181⁄2-minute gap in the forensic audio recordings obtained from law enforcement
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