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Audio Forensics; An Accurate, Arguable and Authentic Approach to Understanding Audio Evidence
by Edward J. Primeau
Bell Labs were the first to discover that spoken word patterns and sounds could be identified and characteristics examined to identify the individual who made them. This has been a very important advancement in forensic science because the potential to assist law enforcement is well worth the effort it takes to defend the proponents and practitioners. Audio Forensics has been referred by some to be a junk science. After over 25 years examining, editing and clarifying audio recordings, I can attest and scientifically prove that voice identification and audio authentication is an exacting science that has huge benefit to the courts, law enforcement agencies and businesses.
In the following article, I will describe what works and does not work for two of the main activates of Audio Forensic Experts: audio authentication and voice identification. I will also review and break down the steps and processes I employ and why I believe it is a valuable tool in litigation.
I have been retained for dozens of court cases as well as by corporations which involved audio evidence in one form or another to help them understand various aspects of the audio evidence. Some required that I find the truth about the source of the threatening voice like a bomb threat called into 911 or sexual harassing voice mail left on a victims phone.
Others were defendants trying to validate or disqualify a pre recorded audio confession. Evidentiary audio recordings all have one thing in common; they needed an experienced Audio Forensic Expert to review and either qualify (validate) or disqualify them. My job as an Audio Forensic Expert falls under one of two most sought curriculum; either to determine the recordings authenticity or to identify a speaker or qualify or disqualify an accused individual.
Voice Identification Overview
I have been practicing voice identification for over 25 years. Much of my skills and principles have been learned from employment as an audio engineer at Ambience Recordings. Other skills have been learned through reading and successful cases completed. I believe people's voices just like fingerprints and other forensic sciences can be identified through visual inspection of sound waves and spectrum analysis as well as through critical listening skills.
I have conducted voice identification for sexual harassment, workers compensation and employment harassment as well as various threatening voice mail messages like bomb threats and other threatening voice mail messages.
In our country today we are guilt until proven innocent, the opposite of our United States Constitution. It is my job to determine the truth about voice recordings using visual, electronic and auditory inspection of the both the evidence recording and an exemplar (voice sample taken for the purpose of comparison).
A typical case I would review might involve a telephoned bomb threat or harassment call that was recorded on audiotape or digital voice mail. After the police arrested a suspect, I would be retained by either the state (court) or defense to determine the truth about that audio recording.
The first step is to examine the original evidence and learn as much about the recording as possible. How was it created? Who created it? What machinery was involved?
Then with the help of the court or defense lawyer, I create an exemplar of the accused voice to compare characteristics visually, electronically and auditory.
In almost every legal case I have been engaged in has allowed my report and or testimony into evidentiary status to aid with due process. I believe my success rate is high due to the fact that I employ three testing platforms as outlined above.
Steady advances in computer technology have had a huge impact on Audio Forensic voice identification. Having experience as an acoustic engineer having listened to literally hundreds of hours of spoken word recordings in addition to sophisticated electronic software programs have contributed to my success with voice identification.
Voice Identification Example #1
One case I examined involved a bomb threat. Bomb threats make up a fairly large segment of voice identification activity. The call in question was made from a pay phone outside of a convenience store to a 911 operator. This is scientifically evident when police traced the call.
The caller identified herself by name as an employee of Xyz Company. When the police arrived at Xyz Company, they find the accused employee who identified herself to 911 operators and arrest her. The employee denied making the call.
She is immediately arrested and charged with making a bomb threat call, guilty until proven innocent. I was retained by the defense to prove the truth beyond a certain degree of scientific certainty that our client did not make the bomb threat call.Voice Identification Procedure
When comparing spoken word samples for the purpose of identification, I base my processes on historical information I have learned from the scientific community, state police crime labs, other forensic experts and designers and developers of electronic (especially computer) equipment and testing software programs. My process requires the examination of every aspect of the words spoken visually, electronically and auditory not simply pathologically. The words themselves, the way the words flow together, the pauses between the words, the way the words are formed by the mouth and larynx can be measured using three sciences. A major key to successful voice identification is to also apply critical listening skills (auditory examination) when examining the recordings. Noise floor and electronic measurement of speech and other audible sounds in the recording must be considered and measured. Forensic procedure requires careful examine of all audio evidence characteristics and follow procedures as outlined by the scientific community.
These scientific procedures begin with the analysis of the quality of the audio recording. It is important to establish that the quality of the recording in question is acceptable and workable. Sometimes, it may be necessary for an audio forensic expert to apply some light equalization or other non destructive audio processing to reduce or remove background noise that may interfere with the forensic examination. The expert must also make sure the audio recording is long enough to conduct forensic testing. In my opinion, the audio evidence or sample must be longer than thirty seconds.
Voice Examination Example #2
I was retained to examine a telephone answering machine recording of a single word. I took the case in spite of the one word recording because defense counsel convinced me to and needed help because his client was being discriminated against by the court.
His client had been violated of his parole and put back in jail because the recipient of the call (whose answering machine made the recording) testified under oath it was the accused. The court listened and concluded that it was the accused putting him back in jail shortly after he was released. My job was to analyze the one word recording and determine what I could.
I discovered it was not the accused using unconventional procedures (which I explained under oath to the judge) trying to convince the court this was unacceptable in the scientific community and could not be determined either way. Our attempt was unsuccessful.
Voice identification requires the forensic examiner to discover similarities in all three areas of investigation as well as differences.
Here are the processes I use when conducting voice identification:
1. Visual examination of the original recording, analogue or digital. This includes examination of the physical characteristics of the tape itself (if analogue) or analogue or digital recorder. It is important to examine the cassette tape (standard, mini or micro) or other analogue or digital source to determine if there are visual signs of tampering or alteration.
2. Once the physical evidence has been examined, the next step is to load the recording in question into a forensic computer. Visual examination of the sound wave, sonogram and spectrograph reveal speech characteristics and patterns of verbal delivery as well as electronic characteristics. At this point, the recording has been digitized so forensic software can analyze and conduct various tests.
3. If possible, for authentication or voice identification, an exemplar or comparison recording should be made on the original recorded to compare the original recording characteristics. This same forensic examination process that is applied to the evidence is also applied to the exemplar to determine that the characteristics are the same and the recording is from the same audio recorder.
4. When conducting voice identification, it is important to create an exemplar of the accused for audio comparison using as exact conditions and equipment as possible to the measurements taken from the evidence as outlined above. The speech must be the same as the speech on the evidence in order for the testing to be accurate. As an audio forensic expert, I often have to coach the accused into the same energetic voice tone and inflection as the evidence recording. However, it is still possible to compare speech if the exemplar is not as exact as the evidence. This may cause inaccuracies but still reveal comparisons worth arguing.
5. Critical listening skills are used to examine the speech pattern, pronunciation, voice tone and inflection, accent, dialect and specific speech characteristics (like a lisp or significant s delivery). There is a rhythm of how an individual speaks and regardless if trying to disguise their speech (in an attempt to fool the forensic examiner), the rhythm and speech patterns as described above still show through. The expert must pay careful attention to the rhythm of spoken word formations. I listen to single words as well as phrases and sentences. I like to compare original evidence sections of spoken word recordings as well as individual words. This is best accomplished by editing exemplars and original recordings back to back. It is extremely helpful to then make these sub files of words and sentences within the section back to back with exemplars. I repeat the assembly over and over to accommodate critical listening skills with the auditory identification process. That way, your ear can experience the sounds, vowel formations and consonants without interruption.
There are many of character traits that can be experienced in a spoken word recording. It is important for the Audio Forensic Expert to become familiar with the evidence speech patterns and visual and electronic characteristics. These characteristics are evident in a persons voice even if they attempt to disguise their voice and are compared to the exemplar.
Using many of the same tools as described above, audio authentication can aid the legal system in determining the validity of audio evidence that is being considered as evidence in litigation.
When authenticating an audio recording, it is important that the Audio Forensic Expert pay careful attention to tone consistency of the audio recorded signal (speech) as well as the recordings noise floor.
The consistent audio recorded signal is important because audio recordings that are not authentic are most always edited or fabricated assemblies of two or more audio recordings for the purpose to deceive the person (s) listening to the recording. Using the tools described above, the Audio Forensic Expert can measure the tone consistency to determine authenticity.
Those same tools can also measure the noise floor looking for inconsistencies in the room tone or background noise of the recording. These breaks or changes in either audio recorded signal or background noise are signs that the audio recording being considered may be counterfeit or fake.
Critical Listening Skills
I have been working with professional speakers and other spoken word recordings since 1980 and have developed my critical listening skills to a degree that when I first hear evidence and exemplar recordings back to back, my opinion begins the conclusion process.
In my early days as an audio engineer, I learned to edit ¼ reel to reel tape with razor blades to make a recording sound like it was recorded start to finish without a single mistake. Some of my edits were pretty tricky. I got so good I could split words in two and even three edits to fix a problem or shorten a script. After awhile, I became very familiar with speech characteristics and patterns as well as vocal tone and pronunciation.
The best way to become skilled with voice identification is to listen to dozens if not hundreds of hours of forensic evidence to become familiar with the various speech pathological characteristics and develop critical listening skills.
There can sometimes be differences in speech patterns that can help identify clues. Look for several similarities as well as differences, nasal resonance differences, voice tone with regard to inflection both similarities and differences.
Voice Identification Conclusions
When conducting the examination, the Audio Forensic Expert must look for similarities as well as differences in all three testing platforms to help arrive at a conclusion (beyond a reasonable degree of professional certainty).
After the investigation and testing procedures are complete, the forensic experts report must arrive at one of the following conclusions. They are: positive identification, probable identification, positive elimination, possible elimination or inconclusive.
The key to successful voice identification is to have a methodology and standard procedure developed that you strictly follow every time you conduct an identification and comparison.
Audio Authentication Conclusion Every tone change in either the audio recorded signal or background noise must be documented and analyzed as a whole before considering the recording genuine or authentic. All forensic concerns must be documented and listed in the forensic report to prove the Audio Forensics findings.
The Audio Forensic Report
It is my belief that the Audio Forensic Report should include:
1. The introduction; what you were asked to do and how you came about the evidence to examine.
2. The testing processes you employed to examine the audio evidence
3. Your conclusion of the tests including your opinion as to the relevant facts and concerns
4. Your curriculum vita (resume) to establish credibility as an Audio Forensic Expert
5. An article you have had published about the testing you have just completed as it relates to the case you are engaged in.