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Voice analysis is the study of speech sounds for purposes other than linguistic content, such as in speech recognition. Such studies include mostly medical analysis of the voice i.e. phoniatrics, but also speaker identification. More controversially, some believe that the truthfulness or emotional state of speakers can be determined using Voice Stress Analysis or Layered Voice Analysis.
Typical voice problems
A medical study of the voice can be, for instance, analysis of the voice of patients who have had a polyp removed from their vocal cords through an operation. In order to objectively evaluate the improvement in voice quality there has to be some measure of voice quality. An experienced voice therapist can quite reliably evaluate the voice, but this requires extensive training and is still always subjective.
Another active research topic in medical voice analysis is vocal loading evaluation. The vocal cords of a person speaking for an extended period of time will suffer from tiring, that is, the process of speaking exerts a load on the vocal cords where the tissue will suffer from tiring. Among professional voice users (i.e. teachers, sales people) this tiring can cause voice failures and sick leaves. To evaluate these problems vocal loading needs to be objectively measured.
Voice problems that require voice analysis most commonly originate from the vocal folds or the laryngeal musculature that controls them, since the folds are subject to collision forces with each vibratory cycle and to drying from the air being forced through the small gap between them, and the laryngeal musclature is intensely active during speech or singing and is subject to tiring. However, dynamic analysis of the vocal folds and their movement is physically difficult. The location of the vocal folds effectively prohibits direct, invasive measurement of movement. Less invasive imaging methods such as x-rays or ultrasounds do not work because the vocal cords are surrounded by cartilage which distort image quality. Movements in the vocal cords are rapid, fundamental frequencies are usually between 80 and 300 Hz, thus preventing usage of ordinary video. Stroboscopic, and high-speed videos provide an option but in order to see the vocal folds, a fiberoptic probe leading to the camera has to be positioned in the throat, which makes speaking difficult. In addition, placing objects in the pharynx usually triggers a gag reflex that stops voicing and closes the larynx. In addition, stroboscopic imaging is only useful when the vocal fold vibratory pattern is closely periodic.
The most important indirect methods are currently inverse filtering of either microphone or oral airflow recordings and electroglottography (EGG). In inverse filtering, the speech sound (the radiated acoustic pressure waveform, as obtained from a microphone) or the oral airflow waveform from a circumferentially vented (CV) mask is recorded outside the mouth and then filtered by a mathematical method to remove the effects of the vocal tract. This method produces an estimate of the waveform of the glottal airflow pulses, which in turn reflect the movements of the vocal folds. The other kind of noninvasive indirect indication of vocal fold motion is the electroglottography, in which electrodes placed on either side of the subject's throat at the level of the vocal folds record the changes in the conductivity of the throat according to how large a portion of the vocal folds are touching each other. It thus yields one-dimensional information of the contact area. Neither inverse filtering nor EGG are sufficient to completely describe the complex 3-dimensional pattern of vocal fold movement, but can provide useful indirect evidence of that movement.