Forensic science is the use of science in the service of the law.  Sciences used in forensics include any discipline that can aid in the collection, preservation and analysis of evidence such as chemistry (for the identification of explosives), engineering (for examination of structural design) or biology (for DNA identification or matching). A forensic scientist is expert in any technical field and can provide an analysis of the evidence, witness testimony on examination results, technical support and even training in his or her specialized area.

Why is Forensic Science important?

Analysis of forensic evidence is used in the investigation and prosecution of civil and criminal proceedings.  Often, it can help to establish the guilt or innocence of possible suspects.

Forensic evidence is also used to link crimes that are thought to be related to one another.  For example, DNA evidence can link one offender to several different crimes or crime scenes (or exonerate the accused).  Linking crimes helps law enforcement authorities to narrow the range of possible suspects and to establish patterns of for crimes, which are useful in identifying and prosecuting suspects.

Forensic scientists also work on developing new techniques and procedures for the collection and analysis of evidence. In this manner, new technology can be used and refined not only to keep forensic scientist on the cutting edge of science, but to maintain the highest standards of quality and accuracy.

Who provides forensic analysis?

Forensic analysis is usually carried out by experts working individually or in teams. Advanced techniques often require laboratories where the investigative conditions can be carefully controlled and monitored. Private laboratories and government agencies support small and large forensic labs.  One of the largest and best-known is that of the US Federal Bureau of Investigation (FBI). The FBI lab performs all examinations on evidence from crimes being investigated by the their field offices.  Additionally, the lab accepts violent crime evidence from state and local law enforcement agencies that don’t have access to a forensic lab of their own, provided that the evidence has not been previously examined.

This is part of a collection of posts on this subject:

1. Forensic Sciences & Forensics: Science in Law
2. Aural Identification of Familiar Voices
3. Aural Identification of Unfamiliar Voices
4. Spectrographic Comparisons
5. Summary & Conclusion

Today, the FBI investigates all criminal cases in the federal jurisdiction that have not been assigned by Congress to one of the thirty-two other federal law enforcement agencies as well as threats from foreign intelligence or terrorist groups. This includes “applicant matters; civil rights; counterterrorism; foreign counterintelligence; organized crime/drugs; violent crimes and major offenders; and financial crime.” View some of the most famous cases in the FBI archives here.

The FBI also provides investigative support and training to local and international law enforcement agencies. The agency often works closely with other law enforcement agencies in the exchange of information to further an investigation. The information gathered from local law enforcement agencies by the FBI is compiled into a set of statistics describing crime in the US and is known as the Uniform Crime Reports (UCR). This data is used to enable all agencies involved with law enforcement to operate in a fashion that maximizes the management of resources and targets specific areas of crime.

The FBI Headquarters are located in Washington, D.C. The agency is headed by the Director, currently Robert S. Mueller, III, who is in charge of organizing the operations of the agency. The Director is appointed by the President for “a term not to exceed ten years.” The Senate must confirm the appointment.

Outside of Washington D.C., the FBI has fifty-six field offices, nearly 400 resident agencies or satellite offices, four field installations, and approximately 40 Legal Attaches, which are foreign liaison posts. These offices and the Headquarters combined employ more than 27,000 individuals.

From early in the history of the Federal Bureau of Investigation, The US Government recognized the need to centralize forensics and encourage forensic science. The FBI lab was started in 1932 and, in its first year of operation, performed nearly one thousand forensic examinations. Today the lab performs approximately one million examinations a year and has expanded to include extensive training programs, an annual international symposium, and a program for technical assistance to the forensic community.

The FBI laboratory

The FBI lab is actually a collection of related specialized laboratories and facilities including:

CODIS – The Combined DNA Index System is a program that facilitates the electronic sharing of information by outside state and local labs. This system provides forensic labs with software which enables them to access databases of convicted offenders, missing persons and unsolved crimes.  With this system, DNA profiles may be exchanged and compared between labs who are trying to link suspects to crime scenes.

NDIS – The National DNA Index System is part of the CODIS system and allows DNA profiles from convicted offenders to be accessible to forensic labs.

IAFIS – The newest database established by the FBI lab, the Integrated Automated Fingerprint Identification System allows latent fingerprint comparisons to be made between labs. IAFIS is the largest database of its kind.

In addition to technical support to state and local labs, the FBI lab offers procedure manuals that help law enforcement officials properly locate and collect physical evidence from a crime scene.  Details on how to report on evidence and photograph crime scenes, and submission of evidence are made available to investigators.  Guidelines for protecting the safety of investigators are also provided.

To visit the FBI Laboratory click here.

Finally, the FBI lab publishes current research and other information relevant to forensic science in a journal which is available online. To view the current issue, click here.

To view other publications of the FBI, click here.

Artist – A forensic artist works with the descriptions of victims or witnesses to produce detailed sketch of the offender. Traditionally, the artist drew the sketch by hand, but more and more artists are using computer software to create the sketch.

Ballistics Expert – Ballistics experts are interested in the the functioning of firearms and their ammunition. They study the trajectory path and other characteristics of ammunition and can match bullets with particular weapons.

Chemist – Forensic chemists examine a crime scene on a molecular level. They offer a variety of services including fiber analysis, chemical analysis, and particle analysis. These experts most often work on matching or identifying evidence.

Computer Scientist – Professionals working in the forensic field with computers are interested in any evidence having to do with computers. This includes user files, system files, deleted files as well as emails and other contact information stored on a computer.

Cytologist – The cytologist is a relatively new professional in the field of forensics. These scientists examine the tissue left on a bullet in order to determine which part of the body the bullet passed through.

Entomologist – Arthropods and insects are the focus of forensic entomology. Experts in this field apply their knowledge of insects to legal issues in one of three areas: Medicolegal, urban, and stored products pests. Professionals working in the medicolegal area examine arthropods found on human remains to determine rate of decay, time of death and if a body was moved. Professionals working in the urban and stored pests area are interested in the damage insects cause in urban areas and their role in food contamination.

Fingerprint Expert – Dactyloscopy is the practice of using fingerprint analysis for identification purposes. Fingerprint experts employ dactyloscopy techniques to match prints from any of a variety of surfaces at a crime scene with the prints of a victim or a suspect.

Geologist - Forensic geologists analyze soil samples that are found on humans or other pieces of evidence (for example automobiles or shoes) and compares them to other, location-specific samples. Through their analysis they can determine where the individual or piece of evidence has been.

Linguist – The focus of a forensic linguist is the spoken or written word. Their analysis on linguistic evidence can provide information regarding the individual’s intent, education, culture and health. They can often determine if two separate pieces of evidence were left by the same individual.

Odontologist – A forensic odontologist is a dentist who specializes in teeth and bitemark evidence in order to help identify missing persons, victims of mass disaster, victims of homicide, suspects and offenders and to answer other legal questions.

Pathologist – Forensic pathologists are individuals who have specialized in a subfield of pathology (the study of disease and injury through autopsy) which centers on the medicolegal issues involved in the investigation of sudden or unexpected death. After an autopsy and examination of the body, they are able to determine the cause and time of death.

Photographer – The forensic photographer is given the responsibility of taking comprehensive and clear photographic evidence from a crime scene. They reconstruct the crime scene by taking photos from every angle, using several different types of cameras and flash bulbs.

Psychiatrist – A forensic psychiatrist is a medical doctor who deals with mental health issues (diagnosis and treatment) in the legal system. These professionals provide consultations with the courts, attorneys, and other parties involved in the litigation process. They also provide clinical services to victims and offenders.

Psychologist – Forensic psychologists work to apply psychological knowledge to the legal setting. They offer consultation regarding the mental health of victims and offenders to attorneys, court officials and law enforcement officials. They also provide clinical services to victims and offenders.

Sculptor – A sculptor working in a forensic setting creates a three-dimensional likeness of a victim or an offender. This likeness is frequently a clay or computer-generated reconstruction of the individual based on skull or other bone fragments.

Serologist – A forensic serologist examines body fluids, most commonly blood, to provide information for the identification of a victim, suspect or offender. This investigation will often include DNA fingerprinting or the identification of an individual from their DNA.

Toxicologist – Forensic toxicology is a specialized field in chemistry. Professionals in this field are interested in the study of substances that are harmful or poisonous to the human body. They are trained to identify these toxins and they sometimes treat the conditions that result.

• The term “forensic” is derived from the Latin word “forensis,” which comes from the word “forum.” A forum was a public place in Rome where the courts heard trials and orations were delivered, hence its usage today as meaning belonging to or used in a court of law. Forensic science is any science applied to the law or to legal questions.
• The best source of the genetic material DNA is the cell nucleus. Red blood cells do not have a nucleus, thus they are not a good source of DNA
• A single strand of hair can provide a wealth of information to a scientist who knows how to interpret it. However, it is not possible to tell if a hair came from a man or a woman unless the root of the hair is still attached, thus providing DNA, which can provide information about gender.
• Handwriting analysis is based on the premise that every person has a unique style of writing. This style changes over time. For these reasons, a visual comparison of two writing samples can yield information about signature fraud and the approximate date of a writing sample. Infrared light can often demonstrate differences between two types of ink. The markings left by the pen also enable an analyst to differentiate between two samples of writing.
• Alec Jeffreys, and English geneticist, was the first to develop a technique to use DNA for identification purposes. The technique Jeffreys developed enabled the comparison of DNA from the crime scenes where two young girls who were murdered in England in 1983 and 1987 to that of possible suspects. DNA analysis first proved that the main suspect in the case was innocent. Later, the perpetrator, who knew he would be proven guilty with the DNA evidence, confessed to both murders. Blood samples were taken and the DNA from the crime scenes matched the samples.
• Many important pieces of data can be gathered from a human skeleton. Gender can be determined from the bone structure. In particular the pelvic bones differ between men and women. Occupation can frequently be determined because most occupations leave evidence of typical activity on the bones. The most obvious cases occur in persons who were employed as heavy laborers. Disease and sickness also leave evidence on the bones of the afflicted, sometimes even deformities. Intelligence cannot be determined from skeletal remains. Many people believe that skull size is an indicator of intelligence, but this is a myth.
• Victims of fires and explosions are most commonly identified by their dental records. Teeth are very useful in victim identification because they decay much more slowly than bones. Also, the teeth can withstand extreme temperatures that bones cannot. Thus, they are often found even after a victim dies in a fire or explosion.
• Henry Jackson was a burglar in Great Britain. During a robbery in 1902, he placed his hand in wet paint, leaving his fingerprints. His case was the first in which an individual was convicted of a crime based on print evidence.
• The Federal Bureau of Investigation (FBI) Laboratory provides analysis of physical evidence and testimony on findings. The Laboratory also serves as a leader in the development of new technology and technological support. To read more about the FBI Laboratory, click here.
• In 1985, the Bureau of Alcohol, Tobacco and Firearms (ATF) and several other agencies (the Portland Fire Bureau and the Connecticut State Police) teamed up to train new K-9 units in the arson dog program. These dogs are trained to detect accelerants that are often used by arsonists to start fires. The dogs are better at the detection of the accelerants than the electronic detection devices used by human investigators. Their reward? Some are rewarded with food and some are paid in toys such as tennis balls.
• Most professionals agree that if a person knows that they are telling a lie, it can be detected by the polygraph. The polygraph measures the physiological arousal that occurs when people lie. Thus, the theory goes, a person could only “beat” the test if they are telling a lie that they believe to be the truth. Polygraphs are admissible in court if the judge allows the evidence. The US Supreme Court has not yet ruled on the admissibility of polygraph evidence, thus the admissibility varies by state.
• A professional working in the medicolegal area of forensic entomology studies arthropods in order to establish how long a body has been dead or if a body has been moved. Arthropods feed on dead vertebrates and their life cycles are consistent. This makes it possible for an entomologist to determine the length of time a body has been dead. To learn more about the profession of forensic entomology, click here.
• Ballistics experts are able to determine of a person was present when a crime involving a gun was committed by examining the residue samples from the suspect’s skin. The sample from the individual who fired the gun will contain primer residue and gunpowder. The gunpowder and primer contain lead antimony and barium, chemicals which can be detected in the lab. To learn more about ballistics experts click here.
• Locard’s principle is used in crime scene investigations because each individual who is present at a crime scene leaves some evidence of their presence. Each person will also take some evidence from the crime scene. This evidence may not always be detected, because it is often minute, but it does exist.
• Vegetable, mineral, animal, and man-made are the only four types of fibers that exist. The analysis of these fibers compares characteristics of the fibers such as color type, coarseness, diameter, discoloration and cross-sectional shape. The color of the fiber is by far the most important characteristic. It is analyzed using a microspectrophotometer, and is the equivalent of a fingerprint of the fibers. Using this technique, fibers of the exact same color can be matched.
• Eyewitness testimony is actually often unreliable. The reasons for the inaccuracies are many, but include the witness’s relationship with the accused, the amount of time that passes between an offense and the identification of the suspect and the nature of the offense as perceived by the witness. False identification is the explanation for most of the cases in which innocent persons are convicted. Nonetheless, eyewitness accounts are still considered by many laypersons, even those on a jury, to be a convincing piece of evidence. They also believe that inaccuracies can be easily detected.

Bruce E. Koenig
Federal Bureau of Investigation

Summary and Conclusion

Under investigative conditions, individuals can reliably identify voices that are well known to them, but the accuracy rate drops to approximately 78% to 83°/o when unfamiliar voices are compared to known voice samples. The use of expert witnesses does not improve the accuracy rate of aural only voice comparisons. The use of the spectrographic technique continues to decline, even with the establishment of new standards in 1992.

References

Bricker, P. D. and Pruzansky, S. Effects of stimulus content and duration on talker identification,  Journal of the Acoustical Society of America (1966) 40:6:1441-1449.

Clarke, F. R., Becker, R. W., and Nixon, J. C. Characteristics that Determine Speaker Recognition. Technical Report ESD-TR-66-636, Electronic Systems Division, US Air Force, 1966.

Compton, A. J. Effects of filtering and vocal duration upon the identification of speakers, aurally, Journal of the AcousticaI Society of America (1963) 35:11:1748-1752.

Federal Criminal Code and Rules. est, St. Paul, MN, 1991, p. 289.

Hecker, M. H. L. Speaker Recognition: An Interpretive Survey of the Literature. American Speech and Hearing Association, Washington, DC, 1971.

Koenig, B. E. Spectrographic voice identification, Crime Laboratory Digest (1986)13:4:105-118.

Ladefoged, P. Expectation affects identification by listening, Language and Speech (1978) 21:4:373-374.

Pollack, I., Pickett, J. M., and Sumby, W. H. On the identification of speakers by voice, Journal of the Acoustical Society of America (1954) 26:3:403-406.

Schmidt-Nielson, A. and Stern, K. R. Identification of known voices as a function of familiarity and narrow­band coding, Journal of the Acoustical Society of America (1985) 77:2:658-663.

Voice comparison standards, Journal of Forensic Identification (1991) 41:5:373-392.

This is part of a collection of posts on this subject:

1. Forensic Sciences & Forensics: Science in Law
2. Aural Identification of Familiar Voices
3. Aural Identification of Unfamiliar Voices
4. Spectrographic Comparisons
5. Summary & Conclusion

Bruce E. Koenig
Federal Bureau of Investigation

Spectrographic Comparisons

The spectrographic laboratory technique is the most well-known and possibly the most accurate of the laboratory testing procedures presently available for comparing verbatim voice samples under forensic conditions. However, some scientists believe that aural identifications of very familiar voices are more accurate (Hecker 1971). The spectrographic technique has been described in numerous forensic and scientific publications, including an overview article published in the Crime Laboratory Digest (Koenig 1986). Therefore, a detailed explanation will not be rendered here; the following paragraphs provide a brief summary of the examination, a review of the new comprehensive standards passed by the IAI, and its status in government and private laboratories.

When properly conducted, spectrographic voice identification is a relatively accurate but not conclusive examination for comparing a recorded unknown voice sample with a suspect repeating the identical contextual information over the same type of transmission system (e.g., a local telephone line). The examiner uses both the short-term memory process previously detailed and a spectral pattern comparison between identically spoken sounds on spectrograms.

Figures 1A and 1B are sound spectrograms of different male speakers saying “salt and pepper.”

The horizontal axis represents time, divided into 0.1-second intervals by the short vertical bars near the top, and the vertical axis is frequency, ranging linearly from 80 Hz to 4000 Hz, with horizontal lines every 1000 Hz. The speech energy is reflected in the gray scale from black (highest level) to white (lowest level). The frequency range of the voice is analogous to the range of a musical instrument, where the lowest notes are at the lowest frequency and the highest notes at the highest frequency. The mostly horizontal bands of darkness reflect the vocal resonances and are called formants. The closely spaced vertical striations represent fundamental frequency (voice pitch) or the actual vibrations of the vocal cords. The spectrographic technique requires comparison of identical phrases between the voice samples, with a decision made at one of a number of confidence levels. The scientific support of this examination is limited, and the actual error rate under most investigative conditions is unknown. The research to date indicates that the technique has a certain error rate that is independent of examiner-induced errors, with errors of false elimination (the voice samples were actually from the same person, but the examination found that they did not match) appreciably higher than false identification (the voice samples were actually from different persons, but the examination found that the samples matched).

In July 1991, the Voice Identification and Acoustic Analysis Subcommittee of the IAI passed and published its first set of comprehensive spectrographic voice identification standards. These requirements, which became effective January 1, 1992, for all certified IAI members, include examiner qualifications, evidence handling, preparation of exemplars, preparation of copies, preliminary-examination, preparation of spectrograms, spectrographic/aural analysis, work notes, testimony, certification, and miscellaneous subjects. Table 1 lists the minimum qualifications for spectrographic examiners of the IAI and the FBI and updates a similar table published in an earlier issue of the Crime Laboratory Digest (Koenig 1986). Table 2 is another updated and expanded table from the same article concerning minimum criteria for spectrographic comparisons. Tables 1 and 2 and the previously published tables reflect that the upgraded IAI standards are now appreciably closer to the FBI’s criteria. The FBI’s standards require higher educational levels, more words for lower confidence decisions, enhancement procedures when needed, and a higher frequency voice range. The most important legal difference is the FBI’s policy not to provide testimony on spectrographic comparisons due to the inconclusive nature of the examination and the unknown error rate under specific investigative conditions.

Table 1. Minimum Qualifications for Spectrographic Examiners of the AIA and FBI

Table 2. Minimum Criteria for Spectrographic Comparison for the IAI and the FBI

The use of the spectrographic technique since the mid­1980s continues to show a steady decline by both government laboratories and private examiners. As of mid-1993, the New York City Police Department and the FBI were the only government laboratories in this country regularly conducting these examinations. The private sector efforts were limited to less than a dozen part-time examiners. Professional meetings in the field have been sparsely attended, and no major spectrographic research is known to be under way. Problems still persist in the spectrographic voice identification field. Examples of these problems include the following: (1) separate sets of certified examiners making high­confidence decisions for both identification and elimination in the same case;¹ (2) individuals with no experience, training, or education in the voice identification discipline making conclusive decisions under oath in court; and (3) examiners testifying that an unknown voice is not the defendant’s, although admitting their decisions are really inconclusive based upon accepted standards.

Note 1. Los Angeles Board of Civil Service Commisioners. Threat case decided March 25,1992, in which three IAI examiners made an identification at a high-confidence level, while two IM examiners eliminated the suspect.

This is part of a collection of posts on this subject:

1. Forensic Sciences & Forensics: Science in Law
2. Aural Identification of Familiar Voices
3. Aural Identification of Unfamiliar Voices
4. Spectrographic Comparisons
5. Summary & Conclusion

Bruce E. Koenig
Federal Bureau of Investigation

Aural comparisons of unfamiliar voice samples rely on short-term memory. For example, a woman receives a number of different telephone inquiries regarding a classified advertisement. She then receives an obscene telephone call, and she tries to remember if any of the voices match. In a judicial proceeding, a judge and/or a jury may have to decide if a particular crucial comment on an investigative recording was spoken by the defendant, who readily admits to saying the other statements attributed to him on the transcript, or to someone else involved in the conversation. Examiners using the spectrographic technique, described later, play back the separate voice samples concurrently on separate devices or computer files with an electronic patching arrangement to allow rapid aural switching between them or by recording short phrases or sentences from each sample on the same recording (Voice Comparison Standards 1991). The de facto study of unfamiliar voice comparisons (Clarke et al. 1966) determined the following:

1. Sentence length over the range of 5 to 11 syllables is not important variable in identification accuracy.
2. Correct identifications decreased from approximately 90% to 80% when the signal-to-noise ratio (SNR) was reduced from 30 decibels (dB) to 0 dB.
3. Correct identifications decreased from approximately 88% to 78% when the frequency response was reduced from 4,500 hertz (Hz) to 1000 Hz.

Since most investigative recordings have a SNR of 10 dB to 40 dB and a frequency response of 2,500 Hz to 5,000 Hz, the range of expected correct identifications of unfamiliar voices would be 78% to 90%, with most identifications in the 78% to 83% range.

The use of expert testimony for aural identifications of unfamiliar voices provides no assistance to the court and/or to the jury. The notes of the advisory committee on Rule 901 of the Federal Rules of Evidence appropriately reflect this fact as follows: “Since aural voice identification is not a subject of expert testimony, the requisite familiarity may be acquired either before or after the particular speaking which is the subject of the identification…” (Federal Criminal Code and Rules 1991). Additionally, the voice comparison standards of the International Associationfor Identification (IAI) specifically state that it “… does not support or approve the use of… aural only expert decisions…” for voice comparisons (1991).

This is part of a collection of posts on this subject:

1. Forensic Sciences & Forensics: Science in Law
2. Aural Identification of Familiar Voices
3. Aural Identification of Unfamiliar Voices
4. Spectrographic Comparisons
5. Summary & Conclusion

Bruce E. Koenig
Federal Bureau of Investigation

Ongoing law enforcement operations throughout the world are continually capturing the voices of suspects with miniature transmitter/receiver systems, analog and digital on-the-body recorders, telephone intercept devices, and concealed room microphones. Since these recordings are normally utilized for investigative leads and/or legal proceedings, specific speakers must be accurately identified. Voice identifications that occur through self-recognition of one’s voice, eye-witness information, surveillance logs, and the use of a person’s name in the conversation are usually readily accepted. However; voice identifications that involve listening only and/or laboratory tests are often more difficult to evaluate accurately. To provide a better understanding of these voice comparison topics, two types of aural-only comparisons will be discussed, and an update on the spectrographic technique is included.

Aural Identification of Familiar Voices

Recognition of familiar voices is a daily occurrence for most people, as they identify spouses, children, coworkers, friends, and business associates after only a few words spoken over the telephone or by hearing them from an adjacent room. This process involves long-term memory, where recognition occurs through a prior knowledge of speech characteristics, including such attributes as accent, speech rate, pronunciation, pitching, vocabulary, and vocal variance (intraspeaker variability).

Some of the relevant scientific research, and opinions that address the accuracy of identifying familiar voices include the following:

1. Researchers used 7 listeners who were familiar with the 16 chosen speakers through daily contact. The speakers had no pronounced speech defects or accents. Groups of two to eight speech samples of varying lengths were played back to the listeners, which resulted in an identification accuracy of better than 95% for samples lasting from about 1 to 2 seconds. Voice samples were also frequency restricted, but the results reflected only a limited loss of accuracy under conditions normally encountered in law enforcement investigations. In tests involving whispered speech, the duration had to be somewhat greater than three times longer than normal speech samples to obtain equivalent levels of identification (Pollack et al. 1954).
2. Sixteen listeners with no hearing losses, who had known the recorded 10 male coworkers for at least 2 years, were chosen. None of the 10 recorded individuals had either pronounced regional accents or speech abnormalities. When the listeners heard sentences of less than 3 seconds duration from the 10 coworkers, their median accuracy rate of identification was 98% (range of 92% to 100%). When only a disyllable (e.g., mama) was spoken, the median accuracy rate dropped to 88% (range of 73% to 98%) (Bricker and Pruzansky 1966).
3. In a study of coworkers, recordings were made on different telephone lines of four women and seven men, each talking for 30 seconds to 1 minute on a neutral topic such as the weather. An additional recording was prepared of another male; who was relatively unfamiliar to most of the listeners. The recordings were arranged in a random order and played to 10 of the other coworkers, who were asked to identify the speakers. “All the listeners except one correctly identified all the 11 [coworkers]… The one listener who made an error.. confused two speakers who were not well known to him. Three of the 10 listeners knew [the eighth male, who was not a coworker], and correctly identified him. Of the remaining seven listeners, only two said that they could not recognize this speaker. Five listeners wrongly identified this speaker as…” another one of their coworkers. “It is worth noting that four of the five listeners who made the wrong identification were highly skilled, experienced phoneticians…” with doctoral degrees in the field (Ladefoged 1978). This experiment reflects a 100% identification rate for the coworkers’ voices that were well-known to them and an overall average accuracy rate of 96% when the relatively unfamiliar voice was added.
4. Twenty-four individuals were asked to listen to speech samples of 24 coworkers (15 males and 9 females) whom they had known for several years and 4 speakers unknown to the listeners. The speech samples averaged about 30 seconds in length and contained at least 12 utterances of 2 to 4 words each. Listeners rated each coworker on a scale of very familiar to totally unfamiliar prior to the testing. They listened to the samples for as long as they wished and then rated their decisions as follows: (1) guessing, (2) fairly sure, or (3) very sure. Deleting the results of any voice rated totally unfamiliar to the listener, the results showed a 90.4% correct identification rate and 4.3% incorrect identification rate, with 5.3% who said they did not know the speaker. If the 5.3% are deleted, the correct identification rate is 95.4%. “This rate is probably fairly representative of situations where a limited vocabulary is required and can be expected to be even higher in informal conversations where more of the individual speaker’s speech habits are present as cues for identification” (Schmidt-Nielson and Stern 1985).

This research reflects that the identification accuracy rate for familiar voice samples lasting 1 second or longer ranged from 92% to 100% and averaged 95% to 100%. Samples recorded through the telephone or other limited bandwidth systems had little effect on accuracy. The effects of noise and loss of high frequency information were studied in another experiment (Clarke et al. 1966) which found that aural speaker identification was only slightly degraded when progressing from high-quality voice samples to typical investigative recordings. It is obvious from everyday experience and the cited research that identifying familiar voices can be an accurate method for identifying voices recorded in forensic applications, even with the limiting factors of noise and attenuated high frequencies.

This is part of a collection of posts on this subject:

1. Forensic Sciences & Forensics: Science in Law
2. Aural Identification of Familiar Voices
3. Aural Identification of Unfamiliar Voices
4. Spectrographic Comparisons
5. Summary & Conclusion