Masking effects on formant frequency structure variability under selected speaking conditions.

Normal-speaking adults (3 M, 3F) produced 4 vowels (/a/, /i/, /u/ and /o/) in a carrier phrase under free and disruptive speaking conditions (masking and/or bite block). Formant frequency structure variability was not affected by speaking conditions; however, a difference among vowels was obtained. F1 variability for the vowel /a/ was significantly different from the other 3 vowels as was the vowel /i/ for F2. The results were presented as the volumetric relationships which exist between the anterior and posterior F1 and F2 vocal tract cavities. The analysis of F1 and F2 variability suggests that once the place of vowel articulation exceeds a certain physiological boundary in reference to vocal tract cavity size, performance variability accelerates significantly.