Schwa’s acoustic properties

The issue of the spectral quality of schwa is closely related to that of the status of the schwa segment. It determines whether schwa should be considered as a true vowel and whether this vowel is distinct from all the other vowels of the French language. Phonological accounts of the process and representations underlying the alternation between schwa and non-schwa variants differ according to whether schwa is considered a true phoneme, with a distinct identity or not (see for instance Walker, 1993).

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The exact acoustic properties of schwa are a rather controversial issue in the literature. On the one hand, some authors assume that schwa has its own spectral quality, distinct from that of [œ] and [ø]. Rousselot (1913, cited in Malécot & Chollet, 1977) attributes to schwa an aperture between the aperture of [œ] and [ø] (see also Pleasants, 1956). In a large-scale acoustic study comparing schwa with [œ] and [ø], Fougeron, Gendrot and Bürki (2007) find that optional schwas differ from [œ] and [ø] in terms of aperture and labiality.

On the other hand, other authors attribute to schwa the spectral characteristics of an existing vowel. Malécot and Chollet (1977) conducted a computer-based identification study based on a large sample of tokens extracted from conversations. Schwa appears close to [ø]. Their data also show that schwa has less identity than any other French vowel, as shown by the tendency of the computer to confuse this vowel with other vowels.

It has also been proposed that schwa is spectrally variable, being sometimes realized as [œ] and sometimes as [ø]. According to Ayres-Bennett & Carruthers (2001), schwa is much more variable than other vowels. It is also more anterior than the neutral position assumed by the IPA alphabet and realized with a certain degree of labiality. Its spectral quality is that of [œ] or [ø]. In a similar vein, Malécot (1977) suggests that schwa’s spectral quality varies according to the segmental context and speakers’ usage. Despite this variability, it remains however closer to [ø] than to [œ] according to this author.

Finally, a few authors do not attribute to schwa precise articulatory characteristics as they do not consider schwa a real vowel. For Pernot (1929), schwa is a neutral vowel, and thus does not have a well defined articulation. Martinet (1945), on the basis of a survey documenting speakers’ representations, concludes that schwa is not a phoneme but a way to ease the articulation of adjacent consonants. As a consequence, its spectral quality does not really matter.

Several reasons may account for this lack of consensus. Firstly, it is not rare to find that a proposal relies on the author’s intuition only. Secondly, where acoustic studies have been conducted, these usually involved few speakers. The lack of appropriate statistical tests could also be mentioned. Furthermore, most (if not all) studies did not control for the differences in segmental context between the different vowels. Finally, and as underlined by Jenkins (1971), the different proposals may also reflect regional differences.

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In a study based on the PFC corpus (Durand, Laks & Lyche, 2002, 2005), Bürki, Racine, Andreassen, Fougeron and Frauenfelder (2008) compared the spectral characteristics of schwa, [œ] and [ø] in the production of 36 speakers from three different regions: Brunoy (Parisian area), Nyon (Canton of Vaud in Switzerland) and the city of Québec. They provided a strict control of the segmental context. Their results first show that schwa’s spectral quality indeed differs according to the speakers’ regional background. In Brunoy, schwa is less open than in the Nyon or Québec areas. Furthermore, in the three regions, schwa is closer to [œ] in terms of labiality but closer to [ø] in terms of aperture.

Interestingly, the differences observed in the three regions regarding the spectral quality of schwa reflect differences in the entire vowel system. Schwa is less open in Brunoy than in the other two geographical areas (as shown by a lower F1 value), but as can be seen on Figure 4, the whole vocalic system of Brunoy is less open than in the other two regions.

Similarly, the whole vocalic system of Brunoy shows a higher value for F3 than the vocalic systems of the other two regions. Thus, differences in the spectral characteristics of schwa between the three systems seem to reflect differences in the entire vowel systems rather than differences in the vowel schwa only.

Figure 4. Acoustic space F1/F2 of Brunoy (dark grey) and Nyon (light grey) in the upper panel and Brunoy (dark grey) and Québec (light grey) in the lower

panel in Bürki et al. (2008).

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In addition, Bürki et al.’s data show that while schwa always differs from both [œ] and [ø] in terms of at least one formant, it is never different from these other vowels in terms of all three formants. Thus, there is considerable overlap between the three vowels. It is however difficult to draw precise conclusions about the individuality of schwa’s spectral characteristics on this basis, as we do not know whether this overlap is greater than the one between other adjacent vowels. It therefore appears necessary to compare the overlap between schwa and its two neighbors with the overlap between other adjacent vowels in the acoustic space. Only if the overlap is greater for schwa will we be able to conclude that schwa’s spectral characteristics have indeed less individuality. Last but not least, Bürki et al.

show that when segmental context and regional background are controlled for, schwa is not more variable than its two neighbors.

Bürki, Fougeron, Gendrot and Frauenfelder (submitted) examined the durational and spectral variation of schwa in a large number of occurrences extracted from a corpus of radio-broadcasted speech. They show that, like other segments, schwa undergoes temporal and spectral reduction. Furthermore, their data suggest that the spectral variation of schwa mirrors that of other vowels: shorter schwas are more coarticulated and thus take the spectral characteristics of the surrounding consonants (see Gendrot & Adda-Decker, 2010 and Mooshammer & Geng, 2008, for data on the spectral reduction of other vowels). The fact that schwas reduce spectrally in connected speech could also partly explain the discrepant results found in the literature concerning the spectral quality of the schwa vowel.

Another point of interest regarding schwa’s acoustic properties concerns its duration. Several studies report that schwa is a rather short vowel compared to the other oral vowels. In Fougeron et al. (2007) schwa has a mean duration of 50 ms, while [œ] and [ø] are on average 15 ms longer (see also Bartkova & Sorin, 1987; Adda-Decker, Boula de Mareüil, Adda &

Lamel, 2005). It is usually assumed that this shorter duration is due to prosodic constraints, since schwa, unlike all other vowels, is never accented. However, comparing schwa’s duration with that of unaccented [œ] and [ø], Malécot & Chollet (1977) find that schwa is 67 ms long on average while [ø] has a mean duration of 80 ms and [œ] a mean duration of 121 ms.

To summarize, despite the large number of empirical studies on the spectral properties of French schwa, no consensual picture emerges. Whereas schwa does not appear to be more variable than its two neighbors [œ] and [ø], nor completely assimilated to any of them (see

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Bürki et al., 2008), whether it has less individuality than the other vowels remains to be shown. Concerning its duration, whereas all studies agree that schwa is a short vowel overall, the exact reasons for this shorter duration remain to be examined empirically.

In Chapter II, we will provide additional data on the duration of schwas in connected speech.

We will examine the variables influencing schwa duration and show that schwa’s shorter duration cannot be completely accounted for by prosodic constraints.