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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

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).

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