Voice problems that require voice analysis most commonly originate from the vocal cords since it is the sound source and is thus most actively subject to tiring. However, analysis of the vocal cords is physically difficult. The location of the vocal cords effectively prohibits direct measurement of movement. 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. High-speed videos provide an option but in order to see the vocal cords the camera has to be positioned in the throat which makes speaking difficult.
Most important indirect methods are [inverse filtering]? of sound recordings and electroglottographs? (EGG). In inverse filtering methods, the speech sound 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 pressure pulse which again inversely indicates the movements of the vocal cords. The other kind of inverse indication are the electroglottographs, which operates with electrodes attached to the subjects throat close to the vocal cords. Changes in conductivity of the throat indicate inversely how large a portion of the vocal cords are touching each other. It thus yields one-dimensional information of the contact area. Neither inverse filtering nor EGG are thus sufficient to completely describe the glottal movement.
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