Effects of muscle tension dysphonia on tone phonation: acoustic and perceptual studies in Vietnamese female teachers.

Muscle tension dysphonia (MTD) is a hyperfunctional voice disorder commonly seen in professional voice users. To date, published acoustic studies of this disorder have mainly focused on nontonal language speakers, and no publication has documented its impact on lexical tone characteristics. In this study, we examined whether and how this voice disorder affected acoustically and perceptually the characteristics of tones in Vietnamese teachers. Voice data were obtained from 42 Vietnamese female primary school teachers diagnosed with MTD and 30 vocally healthy teachers. Tonal data were analyzed using Computerized Speech Lab (CSL-4300B) and Speech Analyzer. Parameters analyzed included the two most important acoustic cues in Vietnamese tones, that is, tonal fundamental frequency (F(0)) and laryngealization. Tonal F(0) was assessed using a factorial analysis of variance with group and career durations as independent variables. Tonal samples were also perceptually assessed by a panel of native speakers of the same dialect. The results showed that MTD lowered tonal F(0) in high tones and tones with extensive fundamental frequency variation. There was also a significant main effect for career duration; in MTD group, tonal F(0) was lower in teachers with longer career duration. The teachers with MTD showed different patterns of laryngealization compared with the control group. Tone perception was poorer for tones with extensive fundamental frequency variation and without a typical phonation type. The results in this group of teachers supported our hypothesis that MTD impairs lexical tone phonation.