Viseme classifications of Dutch consonants and vowels

Videotaped lists of meaningless Dutch syllables were presented in quiet to four subject groups, differing with respect to their knowledge of and experience with lipreading (lipreading expertise). Syllables consisted of all Dutch consonants within three vowel contexts, and of all Dutch vowels within four consonant contexts. Three speakers pronounced all syllable lists. The aim of the research was (1) to establish viseme classifications of Dutch vowels and consonants; (2) to interpret the visual-perceptual dimensions underlying this classification and relate them to acoustic-phonetic parameters; (3) to establish the effect of lipreading expertise on the classification of visually similar phonemes (visemes). In general, viseme classification proved very constant with different subject groups: Lipreading expertise is not related to viseme recognition. Important visual features in consonant lipreading are lip articulation, degree of oral cavity opening, and place of articulation, leading to the following viseme classification: /p,b,m/, /f,v,υ/, /s,z,■/, and /t,d,n,j,l,k,x,r,■,h/. In the acoustic domain, these features may be related to spectral differences. Vowel features in lipreading are lip rounding, degree of lip opening, and vowel duration, yielding the following visemes: /i,■,e,ε,εi,a,■/, /u,y,œ,■/, /o/,o/, and /au,œy/. In the acoustic domain, lip rounding may roughly be related to the second formant, lip opening to the first formant.