<P> Vocal resonation is the process by which the basic product of phonation is enhanced in timbre and / or intensity by the air - filled cavities through which it passes on its way to the outside air . Various terms related to the resonation process include amplification, enrichment, enlargement, improvement, intensification, and prolongation; although in strictly scientific usage acoustic authorities would question most of them . The main point to be drawn from these terms by a singer or speaker is that the end result of resonation is, or should be, to make a better sound . There are seven areas that may be listed as possible vocal resonators . In sequence from the lowest within the body to the highest, these areas are the chest, the tracheal tree, the larynx itself, the pharynx, the oral cavity, the nasal cavity, and the sinuses . </P> <P> The twelve - tone musical scale, upon which a large portion of all music (western popular music in particular) is based, may have its roots in the sound of the human voice during the course of evolution, according to a study published by the New Scientist . Analysis of recorded speech samples found peaks in acoustic energy that mirrored the distances between notes in the twelve - tone scale . </P> <P> There are many disorders that affect the human voice; these include speech impediments, and growths and lesions on the vocal folds . Talking improperly for long periods of time causes vocal loading, which is stress inflicted on the speech organs . When vocal injury is done, often an ENT specialist may be able to help, but the best treatment is the prevention of injuries through good vocal production . Voice therapy is generally delivered by a speech - language pathologist . </P> <P> Vocal nodules are caused over time by repeated abuse of the vocal cords which results in soft, swollen spots on each vocal cord . These spots develop into harder, callous - like growths called nodules . The longer the abuse occurs the larger and stiffer the nodules will become . Most polyps are larger than nodules and may be called by other names, such as polypoid degeneration or Reinke's edema . Polyps are caused by a single occurrence and may require surgical removal . Irritation after the removal may then lead to nodules if additional irritation persists . Speech - language therapy teaches the patient how to eliminate the irritations permanently through habit changes and vocal hygiene . Hoarseness or breathiness that lasts for more than two weeks is a common symptom of an underlying voice disorder such as nodes or polyps and should be investigated medically . </P>

Which of the following is not a general category of voice disorders