II THE SOUND SPECTROGRAPH

The sound spectrograph, an automatic sound wave analyzer, is a basic research instrument used in many laboratories for research studies of sound, music and speech. It has been widely used for the analysis and classification of human speech sounds and in the analysis and treatment of speech and hearing disorders.

The instrument produces a visual representation of a given set of sounds in the parameters of time, frequency and amplitude. The analog spectrograph is composed of four basic parts; (1) a magnetic tape recorder/playback unit, (2) a tape scanning device with a drum which carries the paper to be marked, (3) an electronic variable filter, and (4) an electronic stylus which transfers the analyzed information to the paper. The analog sound spectrograph samples energy levels in a small frequency range from a magnetic tape recording and marks those energy levels on electrically sensitive paper. This instrument then analyses the next small frequency range and samples and marks the energy levels at that point. This process is repeated until the entire desired frequency range is analyzed for that portion of the recording. The finished product is called a spectrogram and is a graphic depiction of the patterns, in the form of bars or formants, of the acoustical events during the time frame analyzed. The machine will produce a spectrogram in approximately eighty seconds. The spectrogram is in the form of an X,Y graph with the X axis the time dimension, approximately 2.4 seconds in length, and the Y axis the frequency range, usually 0 to 4000 or 8000 Hz. The degree of darkness of the markings indicates the approximate relative amplitude of the energy present for a given frequency and time.

Recent developments in sound spectrography have produced computerized digital sound spectrographs ranging from dedicated digital signal analysis workstations to PC-based systems for acquisition, analysis editing, and playback. These sophisticated computer-based systems provide high fidelity signal acquisition, high-speed digital processing circuitry for quick and flexible analysis, and CD-quality playback. The computerize-based systems accomplish all the same tasks of the analog systems, but with the computer-based systems the examiner gains a host of comparison and measurement tools not available with the analog equipment. The computer-based systems are capable of displaying multiple sound spectrogram, adjusting the time alignment and frequency ranges and taking detailed numeric measurements of the displayed sounds. With these advances in technology, the examiner widens the scope of the analysis to create a more detailed picture of the voice or sound being analyzed.

The accuracy and reliability of the sound spectrograph, either analog or digital, has never been in question in any of the courts and never considered an issue in the admissibility of voice identification evidence. This may be due in part to the wide use of the instrument in the field of speech and hearing for non-voice identification analysis of the human voice and, in part to the fact that given the same recording of speech sounds the sound spectrograph will consistently produce the same spectrogram of that speech.

The contest comes in the interpretation of the spectrograms. Proponents of the aural and spectrographic technique of voice identification base their decisions on the theory that all human voices are different due to the physical uniqueness of the vocal track, the distinctive environmental influences in the learning process of speech development, and the unique development of neurological faculties which are responsible for the production of speech. Opponents claim that not enough research has been completed to validate the theory that intraspeaker variability is less than interspeaker variability.

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