...The second step in the voice identification process is the spectrographic analysis of the recorded samples. The sound spectrograph is an automatic sound wave analyzer with a high quality, fully functional tape recorder. The speech samples to be analyzed are recorded on the sound spectrograph. The recording is then analyzed in two and one half second segments. The product is a spectrogram, a graphic display of the recorded signal on the basis of time and frequency with a general indication of amplitude.
The spectrograms of the unknown speaker are then visually compared to the spectrograms of the suspects. Only those speech sounds which are the same are compared.3 The comparisons of the spectrograms are based on the displayed patterns representing the psychoacoustical features of the captured speech. The examiner studies the bandwidths, mean frequencies, and trajectory of vowel formants; vertical striations, distribution of formant energy and nasal resonances; stops, plosives and fricatives; interformant features, the relation of all features present as affected during articulatory changes and any peculiar acoustic patterning.4 The examiner looks not only for similarities but also for differences. The differences are closely examined to determine if they are due to pronunciation differences or if they are indicative of different speakers.
When the analysis is complete the examiner integrates his findings from both the aural and spectrographic analyses into one of five standard conclusions; a positive identification, a probable identification, a positive elimination, a probable elimination, or no decision. In order to arrive at a positive identification the examiner must find a minimum of twenty speech sounds which possess sufficient aural and spectrographic similarities. There can be no differences either aural or spectrographic for which there can be no accounting.
