Many law enforcement laboratories are equipped with at least one sound spectrograph, although there are several types to choose from.  This machine plots the frequency of a complex sound according to time and intensity.  Its function is based on the idea that the human voice is produced by a combination of physiological structures and harmonics. 

The vocal column begins in the vocal folds and ends at the lips.  The vocal folds function acoustically as a closed end so that the vocal column becomes a closed-tube resonator.  The tension of the vocal folds determines the vibrational frequency.  When a sound is produced, those harmonics nearest the resonant frequency of the vocal column increase in amplitude.  If the shape of the mouth, throat, or lips changes, the frequencies vary with the change.  

The sound spectrograph converts the sound of a voice into a visual graphic display known as a voiceprint.  The analog spectrograph has four parts: a magnetic tape recorder unit, a tape scanning device, a filter, and an electronic stylus that writes the information onto electrically sensitive paper. 

A high-quality tape is fastened to the scanning drum, which holds a 2.5 segment of tape time.  The process takes about eighty to ninety seconds to complete.  As the drum revolves, an electronic filter is activated that allows only a certain band of frequencies to get through to the recorder.  These frequencies are translated into electrical energy that gets recorded by the stylus.  As the process continues, the filter moves into increasingly higher frequencies and the stylus records the intensity levels of each defined range.  The final print shows a pattern of closely spaced lines that represent 2.5 seconds worth of all of the distinguishable frequencies of that person's voice as it was taped.

The horizontal axis on a voiceprint represents the parameter of time, registering how high or low a voice is.  The vertical axis is the frequency.  The degree of darkness within each region on the graph illustrates the degree of intensity, or the voice's volume. 

Two kinds of prints can be made: bar prints, which are utilized for identification, and contour prints, which help to file the prints in a computer.

Recent developments include digital spectrographs that can be used with a computer for enhanced comparison and measurement, but some specialists still prefer the older analog model.

Comparisons are made between voice samples and when sufficient similarity exists between one pattern and another, the voices are believed to have a high probability of originating from the same person.  For forensic purposes, the voiceprint interpreter needs a recording of the suspect's voice (e.g., from an interview) to compare to the sample made in the context of a crime, such as an obscene phone call or taped conversation.  Other people's voices, unrelated to the crime, are used for elimination factors (points of dissimilarity).

Interpreters use two methods of identification:

Aural: listening to the voice on tape to compare single sounds and series of sounds for similarities and discrepancies; the examiner also listens for breath patterns, inflections, unusual speech habits, and accents.

Visual: reading the voiceprints to compare their appearances.

First, the examiner evaluates the recording of the unknown suspect, to make sure it has sufficient quality and clarity for analysis.  Then the examiner turns to the voices of the known person to ensure that the recording has similar clarity.  The best test cases have the suspect repeat what was said on the "unknown voice" tape, or at least include as many of the same words as possible. 

The aural and visual methods are combined to come up with one of five conclusions: 

• positive identification
• probable identification
• positive elimination
• probable elimination
• no decision.

The highest standard requires the identification of twenty speech sounds that possess similarities.  "Positive elimination" derives from twenty or more differences, and the rest fall on a spectrum in between.

Some critics of this technology claim that it has never been adequately developed to prove that voiceprints are as individual as fingerprints.  However, those who work closely with it on a regular basis insist that the spectrograph is highly accurate.

Tom Owen, who runs Owl Investigations, Inc.,  has thirty-five years of experience in the recording arts and is a certified Voice Identification Examiner.  He teaches at the New York Institute of Forensic Audio and offers specialized courses on audio and video analysis.  He also consults with law enforcement agencies around the world on specific cases, and for more than twenty years has served as an expert witness in both criminal and civil proceedings.  His agency has a fully equipped processing laboratory, which includes five different types of spectrograph machines for voice identification and speech enhancement.  

With Michael McDermott, Owen has written an extensive article on the history, methods, and forensic applications of voice technology, and he takes on fifty to sixty cases each year.  "It's not uncommon," he says, "that at a murder scene or shooting, you have a tape made from a 911 call where the victim might have been calling for help, or else the person might have been on the phone talking to a relative.  Someone shoots the person, the victim dies and the shooter doesn't realize that the machine was recording.  I would get that tape and see if the intruder said anything before he shot the person.  Sometimes we get results.  Then there are civil incidents, like someone calling to threaten you.  If you don't pay this money, he's going to damage your car or kill your pet.  You also have divorce proceedings where recordings get made, and you have people who keep calling to say something and then hanging up.  We can analyze those calls."

In fact, in one case, a murderer himself called the police to offer the location of the body.  He said that he was an acquaintance and used another man's name.  That man was eliminated and the murderer identified through voiceprint analysis.

Owen uses the full range of spectrograph analysis, but he admits that the technology could still be better.  "You can't accurately print all the 256 shades on the gray scale," he says.  "The printers have gotten better, but only the most expensive ones really get the full range of resolution, and it's often not worth the cost of such a machine."

Recently he completed a study on twenty-five female voices of varying races and ages, doing a one-to-one analysis to determine the degree of error.  The results were striking:  "When you're comparing a known and an unknown voice using a verbatim exemplar [the samples contain the same verbal communication], there are no errors.  That's ninety-nine percent of what we do today.  We don't try to pick a voice out of a pack."

Because voiceprints are generally used in cases where the accuracy rate is so high, Owen is confident that they make a real contribution to the legal process.  However, the history of admissibility of voiceprints has mirrored what has happened in court with other technologies in their early stages.  Courts are conservative and the sound spectrograph has had to prove itself.