Archive for March, 2012
Over the last few days, I’ve been contacted by news sources wanting to interview me about voice identification. Some 911 recordings had been released a week prior from the Trayvon Martin case out of Florida.
Reporters asked me about voice identification reliability. Voice identification is a reliable science when done properly. Trying to determine who is screaming in the background of a 911 call is not easy but seems to be of great significance since these were the final hours of this young man’s life.
I became interested in this case and decided to listen to and peruse the 911 recordings.
I literally stumbled upon the link of 911 calls from http://motherjones.com/politics/2012/03/what-happened-trayvon-martin-explained#transcript
I listened to the 911 calls and recorded them into my audio forensic computer to critically listen to the events. Two of the 911 calls are very interesting: George Zimmerman’s call and the woman who called 911 when she heard screaming outside her home (which we can hear clearly in her 911 call recording).
This second recording is the same recording where the gun shot is heard that killed Trayvon.
I am not writing this to judge but rather to present the facts as they occurred from a forensic perspective.
As an audio forensic expert I want to make clear that this is not formal voice identification and not meant for legal purposes. It is not a statement of my opinion but rather a presentation of facts as I hear them in two particular 911 recordings. I believe you will hear what I hear.
Listen carefully to the 911 call segment I chose from George Zimmerman. Then listen to the tone of the male voice screaming for help in the background during the second 911 call. Can you see the series of events in your mind’s eye?
In the last 24 hours I have received several calls asking my opinion about the voice screaming for help in the background of a 911 call by a woman who heard gunshots outside her home. I was emailed a link to Mother Jones (http://motherjones.com/politics/2012/03/what-happened-trayvon-martin-explained#transcript) and listened to the recordings which I have posted below from AudioBoo.com (http://audioboo.fm/boos/717346-911-call-screaming-and-gunshots-audible)
911 call with male screaming in background asking for help. Who is the male?
George Zimmerman calling authorities
The male voice in the background can be identified. If I were to conduct a formal voice identification test, here are the steps I would take:
1. Remove the samples of the screaming from the 911 call and isolate them in a digital audio file. Remove background noise and raise the volume of the male screams.
2. Remove portions of George Zimmerman’s voice from the 911 call to serve as an exemplar.
3. Edit both speech samples back to back and compare
4. Ask for a speech exemplar from George Zimmerman screaming for help
5. Conduct a voice identification test.
Do you believe the voice screaming is George Zimmerman, Trayvon Martin or another male?
Based on my 28+ years working with the human voice for identification purposes, I have been asked on several occasions to determine a person’s identity based on a voice sample when an exact exemplar was not available. I call this nontraditional voice identification. I do not make this type of voice identification a day to day practice and I always let our clients know that my findings are preliminary based on experience rather than exact science.
One example is the Elvis Presley voice identification I did. Since Elvis is dead, I could not record an exact exemplar.
Because the family who produced the song was convinced it was Elvis and needed my opinion, I compared the unknown song to known Elvis Presley songs from the same timeframe. My opinion was that the new song is the voice of Elvis. Not everybody agreed with my opinion, however. Since science was not involved, these people had reason to base their opinion on their personal knowledge of Elvis Presley’s style.
The above Elvis story is a good example of nontraditional voice identification. An emergency situation is another example, such as when a threatening call was made with intent to do bodily harm to a person or persons. Emergency voice identification becomes necessary to provide the authorities with evidence so they can order the accused to cooperate and provide the voice identification expert an exact exemplar.
One scenario is when a bomb threat call is made to a place of business. Voice identification becomes necessary to determine if the threat was made by a previous employee. I have worked on cases like these many times. Management often has an idea of who made the threat. Their opinion is based on more information than a voice identification expert would use such as discharge date, personality characteristics and work history. I would conduct an emergency voice identification using known samples of the accused person’s speech, like voice mail recordings when the suspect employee called in sick.
Here are the steps I take for emergency voice identification:
1. Locate a known sample of the accused voice
2. Locate the threatening voice recording
3. Make a list of all speaking characteristics of the known voice. These include stutters, vowel pronunciation, any other speech observations and consistent speaking styles that can be observed.
4. Make a list of all speaking characteristics of the unknown voice. Are there any similarities or differences that can be assessed?
5. Use a spectrum analyzer to note voice spectrum of both samples. Are the samples close enough in spectrum to be a match or different enough to help arrive at a conclusion?
The voice identification expert’s task is to take all information from critical listening, spectrum analysis or electronic measurement, and visual inspection of the wav formation and decide if the voice in question is the same voice or not. The next step is to have the law enforcement officials use the voice identification expert’s report to encourage the suspect to cooperate and record an exact exemplar.
How reliable are audio forensic procedures in the legal system? A student recently asked me that and asked about some of the ins and outs of a career in audio forensics. The student’s specific questions are below and are followed by my answers.
1. How often is speech-based evidence used in court? It’s used very often. Voice identification evidence helps a court to identify people involved in a case and put the puzzle pieces of a case together. Furthermore, voice identification can also sometimes provide substantial enough evidence to cause people to be convicted or released.
Is it a common or rare occurrence? Our business has grown significantly, so I would say that audio forensic procedures in the legal system are common.
2. Is voice identification a lengthy process? It can take up to 4 hours for the average case. This includes 3 types of examination. Audio forensic experts visually examine the sound wave, comparing the evidence and an exemplar (a voice sample of the accused). Next, Audio forensic experts electronically measure the evidence, which is then compared to the exemplar. Finally, and most importantly, audio forensic experts critically listen to compare how the words are spoken and pronounced in the evidence and the exemplar.
3. Are there any factors which can create problems when dealing with a recording? Yes, difference in age between the recording in question and the creation of the exemplar creates problems. People’s voices change with the passing of time. One of the nationally accepted standards for comparing voices is that the voice samples must not be more than six (6) years apart.
4. Is voice identification considered to be reliable within the legal system? YES. However, it’s argued more in some states than others. The forensic expert must carefully examine all scientific evidence and follow procedures. Both law enforcement and defendants seek assistance from audio forensic experts.
When I am asked to investigate alterations of court recordings, I almost always travel to the courthouse to examine the court’s computer recording system. It is not often that I examine audio files recorded in court from a remote location. This is a serious allegation and requires careful scientific investigation and proof if the accuser is to prevail in court.
For example, I had a client request that I examine court recorded digital audio files they believed were altered. Portions were alleged to have been deleted. These court recordings were created with a software program “For The Record.”
FTR has propitiatory CODEC (a form of audio encryption) wrapped around .WAV digital audio files. One day’s worth of court proceedings will yield a lot of digital audio files.
I began this remote forensic investigation by downloading the FTR player after registering as a user on their website http://www.fortherecord.com/.
These digital audio files are automatically created based on the length of the recording and number of microphones active during the recording. My client believed portions of the court testimony as it pertained to her case were missing from the audio file folder that is burned to her CD.
When a serious accusation arises against a court of law, as a forensic expert, I have to make certain I have scientific evidence of alteration, missing files or tampering before I can report to my client confirmation on the alleged alteration.
After conducting forensic testing on the “For The Record Player” I have confirmed that deleting .WAV audio files from the record folder will cause the recording to adjust playback. In other words, portions of the recording would be missing if I delete any of the .WAV files from the folder my client sent me.
Did the court delete the file or were they accidentally left off the CD my client gave me?
I cannot determine from a CD if any files were deleted. I can determine however that deleting files from that record folder will cause portions of the recording to “not be an accurate representation of the facts as they occurred.” The only way to be certain the court deleted or accidentally did not transfer all the files from that trial onto the CD is to examine the original files on the computer that created the files at the courthouse.
No matter who is involved in the case, every allegation of deleting evidence nust be handled with the same scientific procedures.