Overview of Audio Forensics Robert C. Maher Montana State University Electrical & Computer Engineering Department Bozeman, MT 59717-3780 USA [email protected] Abstract. Audio forensics applies the tools and techniques of audio engineering and digital signal processing to study audio data as part of a legal proceeding or an official investigation of some kind. This chapter summarizes the principal audio forensic tasks, including authentica- tion, enhancement, and interpretation. The chapter explains the relevant procedural and histori- cal background, presents several examples of audio forensic applications, and reviews several important areas for future research and development. 1 Introduction The field of audio forensics involves the scientific interpretation of audio recordings that are obtained from a formal civil investigation or a criminal legal proceeding. Audio forensic evidence is often obtained deliberately from an acoustical recording system such as a cockpit voice recorder, an automated call center recording, or a sur- veillance tape acquired in the course of a criminal investigation by a law enforcement agency. In other cases the evidence may be collected inadvertently, such as a sound- track extracted from an electronic news gathering rig. In any case, the audio evidence must be evaluated to determine its authenticity, the likelihood that its contents can be enhanced and interpreted, and its relevance to the goals of the investigation [25]. 1.1 Types of Audio Forensic Investigations Authenticity An investigation in which audio material is presented for forensic examination may have several needs and goals. One of the common requirements is to determine the authenticity of the recording. The audio forensic examiner seeks to verify that the re- cording was produced under controlled circumstances, was maintained in a docu- mented chain-of-custody, and was not inadvertently or deliberately altered prior to examination. Enhancement Forensic audio examinations often involve recordings that were made surreptitiously or under circumstances that did not permit ideal microphone placement or optimized signal-to-noise ratio. Therefore, the quality of the audio may be compromised by H.T. Sencar et al. (Eds.): Intel. Multimedia Analysis for Security Appli., SCI 282, pp. 127–144. springerlink.com © Springer-Verlag Berlin Heidelberg 2010 128 R.C. Maher additive noise, distortion, poor equalization, or excessive reverberation. Among the most frequent enhancement tasks involve noise reduction of recorded speech to im- prove intelligibility so that an accurate written transcript can be prepared. Interpretation Following authentication and enhancement, the audio material for forensic examina- tion ultimately must be evaluated and interpreted to discover its relevance and importance to the investigation. In the case of a speech recording, this often includes preparation of a transcript, identification of the talkers, interpretation of any back- ground sounds that might uniquely identify the circumstances of the conversation, and so forth. Other types of recordings, such as audio evidence obtained from accident or crime scenes, require specialized analysis to document all tell-tale sounds and timing relationships within the recording. 2 History and Examples of Audio Forensics Investigations Forensic audio examination traces its roots to the 1950s, with the advent of live re- cording systems for use outside of the recording studio. In the United States, the Fed- eral Bureau of Investigation (FBI) has developed expertise since the early 1960s in audio forensics for the purposes of speech intelligibility enhancement and authentica- tion of recordings [18]. 2.1 Audio Forensics and the Law The seminal legal case in the United States that dealt directly with recorded conver- sations is the 1958 ruling in United States v. McKeever (169 F.Supp. 426, 430, S.D.N.Y. 1958). The judge in the McKeever case was asked, for the first time, to determine the legal admissibility of a tape recorded conversation involving the de- fendant. The judge ultimately allowed in court the use of a written transcript of the recorded conversation [25]. The McKeever ruling is particularly important because the judge cited seven specific requirements necessary for a recording to be accepted in court, and these requirements are now assumed by most state and federal courts in the United States. Table 1. Seven Tenets of Audio Authenticity (the McKeever case). (1) That the recording device was capable of taking the conversation now offered in evidence. (2) That the operator of the device was competent to operate the device. (3) That the recording is authentic and correct. (4) That changes, additions or deletions have not been made in the recording. (5) That the recording has been preserved in a manner that is shown to the court. (6) That the speakers are identified. (7) That the conversation elicited was made voluntarily and in good faith, without any kind of in- ducement. Overview of Audio Forensics 129 In summary, the seven tenets require that audio forensic material for use in court must be obtained and preserved in a documented manner, be unaltered, and contain recognizable talkers and other sound sources—or have witnesses to the recording who can verify its veracity. 2.2 The Watergate Tapes The Watergate scandal of the mid-1970s had many ramifications for the legal system. The revelation in 1973 by White House aid Andrew Butterfield that U.S. President Richard M. Nixon had installed an audio recording system in the White House and in the Executive Office Building resulted in an order by Judge John J. Sirica of the U.S. District Court for the District of Columbia that the recordings be turned over to the court for transcription. During the ensuing investigation in 1974 it was discovered that the recording of a White House conversation between President Nixon and Chief of Staff H.R. Haldeman recorded in the Executive Office Building in 1972 contained an unexplained 18 ½ minute segment where a buzzing sound completely obscured the speech presumably contained on the tape. The question for the court was whether the gap was caused by a malfunction at the time of the recording, or by some subsequent accidental or deliberate action that destroyed that portion of the recorded conversation. Chief Judge Sirica appointed a group of technical experts to comprise a special Advisory Panel on White House Tapes to devise and implement a complete physical analysis of the tape itself, the magnetic signals on it, the electrical and acoustical sig- nals generated by playback of the tape, and the properties of the recording equipment used to produce the magnetic signals on the tape. After performing a comprehensive set of tests, the Panel concluded that the 18 ½ minute gap was caused by multiple overlapping passes on the tape by the magnetic erase head of a specific model of tape recorder that differed from the device that produced the original recording. The Panel's tests clearly showed the characteristic magnetic patterns on the tape caused by the recording and the erase heads of the available recording devices [1]. The work by the Advisory Panel on White House Tapes was highly influential in the field of audio forensics. The Panel's methodology is now widely accepted as the model for judging the authenticity of audio recordings. The five steps are summarized in Table 2. 2.3 Other High Profile Cases Several other highly publicized forensic audio cases have helped shape the techniques and reputation of the field. Acoustic evidence and reconstructions have been used in Table 2. Advisory Panel on White House Tapes procedure. (1) physically observe the entire length of the tape (or other data storage medium) (2) document the total length and mechanical integrity of the storage medium (3) verify that the recording is continuous with no unexplained stop/start sequences or erasures (4) perform critical listening of the entire tape (5) use non-destructive signal processing as needed for intelligibility enhancement 130 R.C. Maher the ongoing investigations surrounding the 1961 assassination of President John F. Kennedy in Dallas [29]. Acoustic evidence has also been discussed in connection with the assassination of presidential candidate Sen. Robert Kennedy in 1968 in Los Angeles. Other important applications of forensic audio include interpretation of con- versations and background sounds from cockpit voice recorder data following a commercial aircraft accident [12], and authenticity assessment and enhancement of recordings purportedly made by terrorists [32]. 3 Qualifications of Audio Examiners and Expert Witnesses Audio forensic examiners are often called upon to render their opinions regarding the authenticity of a recording, the source of the recorded sounds, and the identity of the talkers in the recording. In the United States there are a variety of standards for admit- ting the testimony of topical experts that vary from state to state and between state and federal jurisdictions. The standards for expert testimony often cite the 1923 Frye case (Frye v. United States, 54 App. D.C. 46, 293F.1013, DC Ct App 1923), the Daubert case (Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579 1993), or the more recent Kumho Tire Co. v. Carmichael (526 U.S. 137 1999). In general, the ex- pertise standards require that the examiner use methods and develop findings in a manner that is generally accepted by the professional scientific and engineering com- munities [2, 3, 33]. A forensic audio examiner and expert witness will be the most effective when the examiner can demonstrate a pertinent sequence of formal professional education and training, relevant experience in the audio engineering field, and evidence of ongoing education and professional practice. The list of professional accomplishments should ordinarily include a complete listing of the examiner's prior forensic audio investiga- tions, then number of prior appearances as an audio expert in legal proceedings, a list of formal, peer-reviewed publications authored by the examiner, and evidence of membership in appropriate technical organizations, such as the Audio Engineering Society and the American College of Forensic Examiners.