Measuring and comparing vowel qualities in a Dutch spontaneous speech corpus

Recent studies of spoken Standard Dutch support an ongoing change in the phonetic quality of the diphthong /EI/ [1, 2]. However, there is a need for broader analyses and larger data sets. Here, we took Dutch vowel variants of 44 speakers from a spoken Dutch speech corpus, the CGN [3]. The vowels were measured and compared on the basis of 15.000 vowel segments, consisting of productions of /EI/, /Au/, /2y/, /o:/, and /e:/, as well as the anchor vowels /a/, /i/, /u/. It was our aim to analyze changes in vowel quality dependent on the speakers’ sociological backgrounds and ages, and to deal with the variable recording qualities of the corpus. All vowels were taken from spontaneously uttered sentences and were analyzed automatically by means of a principal component analysis (PCA) on the vowels’ bark-filtered spectra, as well as by formant analysis. Recalculating spectral positions in the principal components (pc’s) plane displayed the spectral interaction of the first formants in the pc1-pc2 plane, and explained the better separability of the vowels compared to the F1-F2 plane, as well as the high correlation of the first three formants with pc1 and pc2. The first pc’s turned out to be rather insensitive to sex-differences, but they were sensitive to the signal-to-noise ratio of the speech data. Variable recording qualities manifested themselves in speaker-specific location and size of the vowel spaces. Good signal-to-noise ratios could be transformed to poorer signals by increasing the lowest possible dB values per filter. Having analyzed the influence of noise on our data, we could normalize the data by taking each speaker’s /a-i-u/ positions and the focal point as references for better inter-speaker comparison. The results clearly show different vowel quality patterns dependent on the speakers’ education and age, and indicate a progress of quality changes with as parameters the lowering and the degree of diphthongization of the long vowels and diphthongs.