28. Conclusions
The two-mass model of vocal folds vibration was used
to model the electroglottographic waveform. The established model was used
to check the dependencies found in the experimental measurement of the effects
of word stress and intonation patterns on EGG waveforms. The effect of subglottal
pressure was investigated for the vowels /a/, /e/ and
/u/ and, because the effects of vowel type were negligible, the
relation between the simulated waveform and the vocal folds tension was tested
on /
/ vowels only. The results of the simulation are
in line with the experimental findings: each period of vibration in the EGG
waveform exhibits steeper slopes when the fundamental frequency rises due
to increased subglottal pressure. On the other hand, the increase in fundamental
frequency induced by higher muscular tension does not cause a change in the
steepness of the slopes. Simultaneously, the duty ratio of the EGG grows.
This supports the hypothesis that prominence in non-pitch-accent word stress
and in the pitch accents of sentence intonation in German is controlled by
different phonatory mechanisms, and that these differences are measurable
in the EGG domain.
The steepness relations of the EGG increasing and decreasing
contact slopes correlate well with the skewness of the modelled glottal airflow
signal. Due to the increased tension of the folds, the glottal pulses are
more symmetrical, whereas the EGG slopes remain unchanged. Experimental evidence
for this effect has been found by Slujiter (1995) and Claßen et
al. (1996) among others.
The duty ratio of the EGG, especially the measure of
the relative duration of the no-contact phase, correlates with the Open Quotient.
However, this relation strongly depends on the phase difference in the motion
of the lower and upper margins of the folds and on the closing and opening
angles of the folds. For the closing angles modelled lower than 1º this
dependency remains positive.
The effects of muscular tension were additionally
investigated in terms of the body-cover model of vocal folds motion. It was
shown that the simulation of increased tension leads to the same results
as the two-mass model.
EGG signal parametrization is fully automated and has
been implemented within the state-of -the-art signal processing environment
ESPS/xwaves+. The implementation opens new perspectives in the analysis
of voice quality based on large-scale databases. First attempts have been
described in chapter IV (voice pathology). Further studies in cooperation
with clinical phoniatricians are planned.
