Perturbation

Vocally healthy speakers can phonate quite steadily: they can utter a certain vowel with nearly constant pitch, loudness and quality. However, subjects with diseases of laryngeal nerves, tissues and muscles often cannot. This unsteadiness of acoustic parameters of speech perturbation of fundamental frequency and amplitude may offer important clues to the absence, presence and perhaps the nature of the disorder.

Links
Utrecht Lexicon of Linguistics