Voice analysis was first used in World War II for military intelligence purposes. Its use in forensic investigation dates back to the 1960s and relies on the fact that each person's voice has a unique quality that can be recorded as a voiceprint, rather like a fingerprint, on an instrument called a sound spectrograph. Suspects knowingly or unknowingly leave recordings of the voices on the telephone, voice mail, answering machines, or hidden tape recorders, and these samples can be used as evidence . Forensic voice analysis has been used in a wide range of criminal cases such as murder , rape, drug dealing, bomb threats, and terrorism.
Each person's voice is different because the anatomy of the vocal cords, vocal cavity, and oral and nasal cavities is specific to the individual. Added to that, each person coordinates the muscles of the lips, tongue, soft palate, and jaw differently to produce words. The teeth also have an impact in the way speech is formed. The body's voice-producing apparatus is like an organ pipe producing notes, a tube in which sound waves vibrate, producing sounds which can readily be recorded.
The sound spectrograph records a voiceprint in terms of the frequencies and intensities of the sounds made by an individual while speaking. A good mimic may sound like the person they are imitating, but the voiceprint will be quite different. Of course, a person's voice changes with age, but the voiceprint remains distinctive.
Voiceprint samples may be obtained through covert police operations, such as by investigators wearing hidden microphones or putting surveillance equipment on a suspect's phone. As with fingerprints and shoeprints , samples for comparison can be taken from a suspect, by court order if necessary. The investigator will ask them to speak the same words as those that were recorded on the voice evidence that has been collected. This may be a 911 call from a murderer or a bomb threat call. There is always the possibility that the suspect will try to disguise his or her voice, but the voiceprint expert will probably be able to allow for this.
The investigator has two complementary ways of making an identification through voice analysis. First, he or she will listen to the evidence sample and the sample taken from the suspect, comparing accent, speech habits, breath patterns, and inflections. Then a comparison of the corresponding voiceprints is made. There is no international standard for the minimum number of points of identity needed in this comparison, but ten to twenty speech sounds that correspond are often taken as good proof of identification.
It has been argued that voiceprints may not be as individual as fingerprints. Certainly the technology for analysis is probably not as well developed. However, in one analysis of 2,000 cases by the Federal Bureau of Investigation, the error rate in both false identification and false elimination of suspects was found to be very low.
Voice identification played a key role in the investigation of the crimes of Peter Sutcliffe, the socalled Yorkshire Ripper, who murdered several women in the North of England in the late 1970s. Tapes purporting to be from the Ripper were sent to the police team involved in the case, taunting them for their lack of success in catching him. Voice analysis was at first inconclusive, but it now looks as if the tapes were probably the work of a hoaxer.
Voice analysis has also been applied to the investigation of tapes said to be made by Osama bin Laden, the world's most-wanted terrorist. Since the terror attacks in New York and Washington on September 11, 2001, bin Laden has apparently issued a number of video and audiotapes. Corresponding words on the tapes, like "America," can be compared, but the voiceprints do not match exactly because the same person will never say a word in exactly the same way each time. If there is enough similarity, however, an identification can be made even if it is tentative, especially if there is other evidence. Of course, bin Laden speaks in Arabic, but there is software to handle this and other languages. It may be significant that the most recent utterances by bin Laden have been by audio rather than video tape, raising the possibility that he has been dead for some time and the tape has been made by someone else hoping to raise the morale of al Qaeda. The tape is of poor quality and difficult for analysts to work with. It is unlikely, however, that a mimic could fool a voice analysis expert, even under these conditions. Yet there is the possibility that the tape has been created from previous ones that feature bin Laden's real voice, with new information pasted in to update it. The final possibility is that the tape has been made by one of his sons; parents and children tend to sound similar and may give similar voiceprints. The identification of bin Laden looks as if it will be an ongoing challenge to the forensic voice analysts.
see also Linguistics, forensic stylistics.