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Forensic tape enhancement: aural identification of recorded voices

Enhancement of tape recorded voices to facilitate transcription & aural identification:
Selected Topics in Forensic Voice Identification

Bruce E. Koenig, October 1993

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.