Voice analysis refers to the study of speech sounds in other purposes than for the linguistic content, like in speech recognition and similar. Such studies include mostly medical analysis of the voice i.e. phoniatrics, but also speaker identification.
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 his or her 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 evalute these problems vocal loading needs to be objectively measured.
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.