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The digital manipulation of vowel formant frequencies for persons with amyotrophic lateral sclerosis

Melanie May Richter, University of Nebraska - Lincoln


Persons with amyotrophic lateral sclerosis (PALS) suffer from a degenerative motor neuron disease that severely affects their speech. Speech therapy helps them compensate or use augmentative and alternative communication (AAC) for communication purposes, however their natural speech may still be unintelligible. Studies have indicated the acoustic changes in PALS' speech that affect the perception of their speech. One primary acoustic cue for vowel perception distorted in PALS is formant frequencies. The present study tested the potential of digitally manipulating (changing) vowel formant frequencies in PALS to an average male's values. The goal was to manipulate the formant frequencies in order to enhance perception and thereby intelligibility. Five male speakers with ALS recorded a wordlist of 70 words. For each word, the researcher identified the vowel location, and the word was imported into the computer program Praat (Boersma & Weenink, 2001). A Praat programming script conducted linear predictive coding (LPC) analysis to separate the source and filter characteristics of the word. Using the identified filter characteristics, the values of the first three formant frequencies for each vowel were removed and replaced by the formant frequencies of that vowel for an average male. Each word was then resynthesized by combining the source and manipulated filter. Additionally, to account for distortion added by the process, an unmanipulated condition was created by using Praat to separate and recombine the source and filter without changing the formant frequencies. Fifty listeners identified the words they heard then indicated how confident they were in their responses. To compare the two manipulation conditions, data were analyzed through repeated measures analyses of variance and follow-up tests. Results indicated the digital manipulation of vowel formant frequencies significantly increased a speaker's vowel intelligibility. Manipulation increased his vowel intelligibility from approximately 40% to approximately 70%. Distinguishing characteristics for him included severely decreased vowel intelligibility, significantly reduced vowel space, and a limited range of the second formant frequency. The challenges and potential of digitally manipulating acoustics are discussed. ^

Subject Area

Health Sciences, Speech Pathology

Recommended Citation

Richter, Melanie May, "The digital manipulation of vowel formant frequencies for persons with amyotrophic lateral sclerosis" (2002). ETD collection for University of Nebraska - Lincoln. AAI3059967.