The studies that have been produced over the years have run the gambit in type, parameter, and result. A quick review of the available published data would leave one with the impression that the spectrographic method of voice identification was only somewhat more accurate than flipping a coin. The diversity of the relatively low number of studies and the range of results has only added to the confusion as to the reliability and validity of this method of identification. When one takes the time and expends the effort to analyze the studies in this field, a very different conclusion becomes evident. When the individual parameters of the studies are taken into account, who was being evaluated, what information was given to the examiner to assess, and what limitations were placed on the examiner's conclusions, a much clearer picture of the accuracy of the spectrographic voice identification method develops. The picture is not one of a marginally accurate technique but rather a picture that clearly shows that a properly trained and experienced examiner, adhering to internationally accepted standards will produce a highly accurate result. The studies also show that as the level of training diminishes and/or the conclusions an examiner may reach are artificially limited, the error rate goes up dramatically.

The training for accurately performing the spectrographic voice identification method has been established as requiring completion of (1) a formal course of study, usually 2 to 4 weeks duration, in the basics of spectrographic analysis, (2) two years of study completing 100 voice comparison cases, usually in a one-to-one relationship with a recognized expert, (3) examination by a board of experts in the field of spectrographic voice identification analysis.

For the most accurate results from the spectrographic voice identification method, a professional examiner (1) will require the original recordings or the best quality rerecordings if the original is not available; (2) will perform a critical aural review of the suspect and known recordings; (3) will produce sound spectrograms of the comparable words and phrases; (4) will produce a comparison recording juxtaposing the known and unknown speech samples; (5) will evaluate the evidence and classify the results into one of five standard categories [ 1 - positive identification, 2 - probable identification, 3. - positive elimination, 4 - probable elimination, and 5 - no decision]. The final decision is reached through a combined process of aural and visual examination.

It is important to remember that the spectrographic method of voice identification is a process that interweaves the visual analysis of the sound spectrograms with the critical aural examination of the sounds being viewed. Taking the results from all of the studies produced shows that if the examiner's ability to analyze both the graphic representations of the voice and the aural cues found in the recordings is limited or restricted, accuracy suffers. Likewise, the amount of training has a direct bearing on the level of accuracy of the results.

In a survey of 18 studies of the accuracy of the spectrographic voice identification method, the results fall into two categories; those with proper training, using standard procedures produce very accurate results, whereas those with inadequate training, using limited analysis methods, produce inaccurate results.

In a study in 1975 authored by Lt. L. Smrkovski of the Voice Identification Unit of the Michigan State police, error rates in voice identification analysis comparisons, based on three levels of training and experience, were evaluated. The following table summarizes the results of that study.

Lt. Smrkovski's results show that proper training is essential. The fact that his results show a higher no decision rate among the professional examiners than the trainee examiners may indicate that the professional is a bit more cautious in his analysis than the trainee.

Mark Greenwald, in his 1979 thesis for his M.A. degree at Michigan State University, studied the performance of three professional examiners (each with eight years experience) and five trainees (each with less than two years experience) using standard spectrographic voice identification methods (visual and aural) and result classifications. Greenwald found that the professional examiners produced no errors when using full frequency bandwidth recordings. When the frequency band width was restricted, the professional examiners still produced no errors, but did increase their percentage of no decision classifications. Greenwald also found that the training level was an important factor and that the trainees in this study had an error rate of 6.1% for false identifications in the restricted frequency bandwidth trials.

In 1986, the Federal Bureau of Investigation published a survey of two thousand voice identification comparisons made by FBI examiners. This survey was based on 2000 forensic comparisons completed over a period of fifteen years, under actual law enforcement conditions, by FBI examiners.

The examiners had a minimum of two years experience, completed over 100 actual cases, completed a basic two week training course and received formal approval by other trained examiners.

The results of the survey are depicted in the chart below.

The FBI results are consistent with the Smrkovski study in that properly trained examiners, utilizing the full range of procedures, produce quite accurate results.

By way of contrast, the 1976 study by Alan Reich used four speech science graduate students with previous experience with speech spectrograms (but untrained in spectrographic voice identification analysis) to examine, using visual comparison only, nine excerpted words. This study produced an accuracy rate in the undisguised trials of 56.67%. When disguise was introduced into this study paradigm the accuracy rate decreased significantly.

Taken as a whole the 18 studies support the conclusion that accurate results will be obtained only through the combined use of the aural and visual components of the spectrographic voice identification method as performed by a properly trained examiner adhering to the established standards. Those studies with poor accuracy results are important in that they demonstrate the weaknesses of improperly performed examinations that do not adhere to the internationally accepted professional standards.

A large part of the debate over the admissibility of spectrographic voice identification analysis in the courts appears due to the fact that the parameters of these studies have not adequately been demonstrated to the courts in the necessary detail which would allow the courts to examine the overall meaning of these studies. Many of these studies look at only one or two aspects of the spectrographic voice identification method. Frequently the results of these restricted scope studies have been misapplied to the entire spectrographic voice identification method resulting in inaccurate information being used as the basis for deciding the admissibility of spectrographic voice identification analysis. It is important to provide an accurate picture of all the studies so the courts will have the foundational information necessary to make an informed decision regarding the admissibility of spectrographic voice identification analysis.

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