proto-southwestern-tai revised: a new reconstruction - seals 22

10 Marc Brunelle
f0 and open quotient were calculated from the same EGG signal, the open quotient values
corresponding to excluded f0 values were also excluded.
2.4.2 Open quotient (voice quality)
Open quotient corresponds to the proportion of the glottal cycle during which the glottis is
open. The open quotient is correlated with voice quality. A large open quotient
corresponds to a breathier voice quality while a small open quotient corresponds to a
creaky, tight or pressed voice quality (Ní Chasaide and Gobl 1997; Mazaudon and
Michaud 2008 for good overviews).
The open quotient was extracted from the EGG results with a routine based on
Alexis Michaud’s MATLAB scripts (http://voiceresearch.free.fr/egg/). The routine
smoothes the EGG signal, gets its derivative and calculates the open phase of the glottal
cycle from the maximum peaks and valleys of the derivative. Once again, a few suspicious
measurements were removed from the dataset by excluding values that were more than two
standard deviations away from the mean (calculated independently for each speaker).
Since f0 and open quotient were calculated from the same EGG signal, the f0 values
corresponding to excluded open quotient values were also excluded.
2.4.3 Formant frequencies
The first and second formants, which are the acoustic correlates of vowel height and
frontness-backness respectively, were calculated with a Praat script from the acoustic
signal. However, since this signal is noisy and faint for most speakers, the Praat formant
tracker returned numerous suspicious values. These values were excluded in the following
way: Mean F1 and F2 values were calculated for each vowel quality in the speech of each
subject. All values that were more than two standard deviations away for the mean F1 or
F2 for a given vowel quality were then excluded.
2.4.4 Duration
As the speech rate of speakers tends to vary between utterances of the wordlist and even
during the same wordlist, it is necessary to normalize duration values before comparing
them. A common way to normalize duration is to select a fixed part of the frame sentence
as a benchmark for controlling speech rate. Unfortunately, the frame sentence that we used
for this experiment is not suitable for this type of normalization. Speakers know what the
target word is and can lengthen it independently of the rest of the frame sentence. For this
reason, and since duration has never been claimed to play a central role in the Cham
register contrast, duration results will not be reported.
2.4.5 Normalization
In order to facilitate the comparison and visual representation of data from many speakers
and dialects, all acoustic indicators were normalized using a Z-score method. This method
consists in rescaling all the values for a given indicator so that they have an average of 0
and a standard deviation of 1. Z-scores for open quotient and f0 were computed over all of
the measurements made in final syllable vowels. Z-scores for F1 and F2 were computed
similarly, but independently for each vowel.