Phonetic accounts of labialisation 3
3.5 Acoustic correlates of labialisation
One of the very first things that phonetics students learn about the acoustics of vowels is that lip rounding lowers the frequency of formants. Basing his argument on Perturbation Theory (as discussed inChapter 2,Section 2.8, p. 64), Stevens (1998) explained that lip rounding can be modelled as a decrease in the cross-sectional area at the open end of a uniform, unconstricted tube where one end is closed and the other end is open. However, he stressed that the downward shift in formants would apply not just to a uniform tube but to any arbitrary configuration that is open at one end, since there is a minimum in sound pressure and a maximum in volume velocity. Therefore, all articulatory-acoustic models should converge on the notion that any
3.5. Acoustic correlates of labialisation 85
articulation involving a decrease in lip area should result in the lowering of formants and indeed, to the best of our knowledge, they do.
However, vocal tract modelling has shown that the effect of lip rounding on formant frequencies is inextricably linked to the configuration of other articulatory parameters, namely the place and degree of the lingual constriction. In his influentialAcoustic Theory of Speech Production, Fant (1960) showed that vowel formants can be accurately predicted by reducing the complexities of the vocal tract to a three-parameter, four-tube model, which was arguably one of the most important scientific breakthroughs in phonetics in the last century (Harrington, 2010). Fant (1960) presents nomograms to display the acoustic consequences of modifying the size and position of the lingual constriction as well as the degree of lip opening. An example of Fant’s nomograms is presented in Figure 3.3adapted from Fant (1989, p. 80). Figure 3.3 relates formant patterns to lingual constriction location with curves for five different degrees of rounding (from non-rounded curve 1 to very rounded curve 5). For these nomograms, the lingual constriction area is kept constant at 0.65 cm2, which roughly corresponds to the narrow area of constriction in close vowels (Harrington & Cassidy, 1999). We first notice that changes to lip opening area predominantly affect F2 and F3, given the observable differences in F2 and F3 across the five lip area curves. Indeed, we know that the downward shift in formant frequencies caused by lip rounding in vowels particularly impacts F2 and F3 because they are affiliated with the front cavity (Vaissière, 2007). However, the nomograms show that formant frequencies are clearly affected by the varying horizontal location of the tongue constriction.
When the tongue constriction occurs in the pre-palatal region (around 14 cm away from the glottis), lip area particularly affects F3. Conversely, F2 is predominately affected by lip area when the lingual constriction is more posterior (8-12 cm from the glottis).
Vocal tract modelling may give some indication as to why front and back rounded vowels are not produced with the same degrees of rounding, which we take to be synonymous with a horizontal contraction of the interlabial space. According to Vaissière (2011), what unites all ‘focal’ vowels is the merging of two adjacent formants in their acoustic profile. Although not a rounded vowel, as a starting point, we note that forfocal [i], F3 and F4 need to be in
Figure 3.3:Nomograms from Fant (1989, p. 80) for incremental values of lingual constriction location from the glottis to the lips with a constriction fixed at a narrow area of 0.65 cm2. Curves
1-5 correspond to different lip areas from 8.00 cm2(no rounding) to 0.16 cm2(strong rounding).
The points of formant merging are circled for[i],[y]and[u].
3.5. Acoustic correlates of labialisation 87
close proximity (Vaissière, 2011). InFigure 3.3, the minimal distance between F3 and F4 for[i]
(circled) occurs when the lingual constriction is positioned about 14 cm away from the glottis, i.e., in the pre-palatal region, and with the largest possible lip area (curve 1). Forfocal[u], F1 and F2 converge, which according to Vaissière (2007), is achieved with two strong constrictions:
one at the palate and one at the lips. We find the smallest distance between F1 and F2 to occur with the smallest lip area curve presented inFigure 3.3 (curve 5), suggesting that close lip rounding is necessary in order to keep the distance between F1 and F2 maximally low when the lingual constriction is back (around 8 cm from the glottis). This suggestion was corroborated by Stevens (1998) who also noted that in the case of a backed tongue position, the condition of minimum F2 is achieved only if the lips are rounded and a narrow opening is formed. For a focal [y], formant merging occurs between F2 and F3 (Vaissière, 2011). The nomograms presented inFigure 3.3indicate that when the lingual constriction occurs at least 10 cm from the glottis, some degree of clustering of F2 and F3 occurs in conjunction with all five lip area curves. However, the minimal distance between F2 and F3 does not occur in conjunction with the most lip rounding, but with a lip area of 2.0 cm2 (curve 3). In the event of stronger lip rounding (i.e., with a smaller lip area), the distance between F2 and F3 increases. The frequency of F2 at the minimal distance between F3 and F2 for[y]is around 2 000 Hz, which is roughly the same frequency of F2 infocal[i]. The F1 offocal [i]and[y]do not greatly differ either.
Therefore, what distinguishesfocal[i]fromfocal[y]is the frequency of F3 (Vaissière, 2011).
The proposal that the lips are not closely rounded for[y]in order to maintain the proximity of F3 to F2 was also suggested by Wood (1986). Similarly, Catford (1977) also explained that front vowels are usually ‘exolabial’, in order to avoid over-lowering the second formant and hence preserve their front quality. We conclude that while close lip rounding is needed for focal[u]to keep F2 maximally low, focal[y]maintains a minimal distance between F2 and F3 by avoiding close lip rounding. Wood (1986) argued that as the reported differences in lip articulation for[y]and[u]have always shown less lip rounding for[y], this difference can be considered a linguistic universal. The use of distinct labial configurations for front and back vowels may thus have acoustic and perhaps perceptual consequences, particularly in languages where rounding in vowels is contrastive, such as Swedish.
Although we have placed the point of formant merging for[u]to coincide with a back lingual constriction (around 8 cm from the glottis) where F2 is at its lowest in the nomograms inFigure 3.3, in reality, the lingual constriction is probably produced closer to the front of the vocal tract. The nomograms show that regardless of lip area, F2 descends quite rapidly as the lingual constriction moves from the pre-palatal region to the mid-palatal one (between 14-10 cm from the glottis) but then plateaus, suggesting that F2 is relatively insensitive to the location of the lingual constriction in the mid to back palatal region when the constriction is narrow. This is an example of a ‘stable region’ as proposed by Stevens (1989)’s Quantal Theory (as discussed inSection 1.5.1, p. 25), suggesting that quite large variations in the horizontal position of the lingual constriction would result in comparatively stable F1 and F2 frequencies (Harrington & Cassidy, 1999). French vowels are generally considered to be the closest you can get to cardinal vowel productions (e.g., Jones, 1972). For the French production of[u], the constriction is located around 11 cm from the glottis and the corresponding formant pattern is F1 = 250 Hz, F2 = 850 HZ, and F3 = 2 700 Hz (Savariaux, Perrier, & Orliaguet, 1995). A lingual constriction at this location actually occurs during F2’s descent as the constriction moves back, prior to its low plateau. As a result, real F1 and F2 values are not as close together as indicated in the position of F1-F2 merging for[u]circled inFigure 3.3. It may be that for French/u/, rather than the merging of F1 and F2, the lowest possible concentration of energy is required, which is what we find with a lingual constriction 11 cm away from the glottis. Indeed, Ménard, Schwartz, Boë, and Aubin (2007) found that for French[u], F1 and F2 need to be minimally low and thatfocalisationof F1 and F2 occurs at the lowest position. We would like to stress that regardless of the place of the lingual constriction, the lowest possible F1 and F2 values always occur with the greatest degree of lip rounding (i.e., with the smallest lip area). The argument that[u]requires close lip rounding with a small lip opening is therefore still valid, whatever the front-back position of the tongue.