Motor equivalence and labialisation

3.6 Motor equivalence and labialisation

The previous section indicated, perhaps somewhat implicitly, that multiple articulatory param-eters may contribute to the frequency of a given formant. For example, we found that when the lingual constriction area is narrow, F2 lowering may be accomplished by positioning the lingual constriction further back (i.e., from a pre- to a mid-palatal position) or by increasing the degree of lip rounding. Increasedlip protrusionwould have a similar acoustic effect to the backing of the tongue because it would extend the cavity in front of the lingual constriction.

It may therefore be possible to produce the same acoustic output using multiple articulatory configurations, which is sometimes known under the termmotor equivalence(Perkell et al., 1993). As a result,trading relationsmay occur, whereby articulatory movements covary in order to keep a perceptually important acoustic cue constant (Brunner et al., 2011). The production of the vowel[u]is one such example. Perkell et al. (1993) found evidence to suggest that lip rounding and tongue-body raising aremotor equivalentstrategies for F2 lowering in Amer-ican English/u/. They observed atrading relationbetween the degree of the labial and the lingual constriction: if one of these constrictions is too large (a property that tends to increase F2), the other constriction is adjusted accordingly (Perrier & Fuchs, 2015). Another possible trading relationmay exist between lip rounding and the place of the lingual constriction for [u]. Savariaux et al. (1995) assessed how speakers of French behave when lip rounding for[u]

is mechanically perturbed using a lip tube, which fixed the lip opening area at 4.9 cm². They found that 7 of their 11 speakers moved their tongue backwards to compensate for perturbation, which would have a lowering effect on F2.

Motor equivalencemay result in the development of different production strategies for the same sound across speakers. One such case involving the lips may be the production of English /u/. A diachronic process of fronting of the /u/vowel has been well-reported in Englishes worldwide, particularly in UK dialects (Fabricius, 2007; Ferragne & Pellegrino, 2010; Harrington, Kleber, & Reubold, 2008; Harrington et al., 2011). Acoustically speaking,/u/-fronting manifests itself as the raising of the second formant and is generally considered to be the result of the

lingual constriction being articulated at the front of the mouth. However, given the fact that F2 raising may also be the result of less lip rounding, Harrington et al. (2011) suggested that the F2 raising associated with/u/-fronting could be a result of either tongue-body fronting, lip unrounding, or a combination of both. In a study comparing the articulation and acoustics of /u/in a variety of UK dialects, Lawson et al. (2019) found two distinct production strategies, which result in similar F2 values. In English and Irish speakers, the tongue was fronted and the lips were protruded. Conversely, Scottish tongue-body positions were located further back in the vocal tract but were accompanied by lesslip protrusion. Although it would be tempting to consider these different production strategies atrading relation, as Lawson et al. (2019) pointed out, a difference in F1 between the two articulations does not make this possible. However, no correlation was observed between acoustic and articulatory frontness, suggesting that both the tongue and lips play a part in F2 lowering. This finding also highlights that tongue-body frontness should not be inferred from the acoustic signal alone.

3.7 Chapter conclusion

We have shown in this chapter that the phonetic implementation of lip rounding is not as simple as what the binary phonological feature [±round] would suggest. Phonetic evidence has led us to call into question the appropriateness of the termroundingand its application to both consonants and vowels, as segments typically considered rounded, such as front rounded vowels, may not actually be produced with rounded lips. The somewhat restrictive label may have equally led to the labial gesture being overlooked in certain cases, such as Japanese/u/

(Nogita & Yamane, 2018; Nogita et al., 2013). We thus propose to uselabialisationas a more phonetically neutral label, applicable to both consonants and vowels. We definelabialisationas a secondary labial articulation, which results in a reduction of the overall lip area achieved via horizontal labialisationorvertical labialisation. Labialisationmay also result in an increase in the length of the vocal tract when accompanied bylip protrusion.

Acoustic modelling has shown that modifications to the labial articulation have acoustic consequences. However, the lips combine with other articulatory configurations impacting

3.7. Chapter conclusion 91

formant frequencies in different ways. For example, F2 is particularly affected by the size of the lip area when a narrow lingual constriction occurs in the mid to back palatal region.

Conversely, lip area exerts greater changes to F3 when the narrow lingual constriction is positioned further front in the pre-palatal region. In order to produce a maximum acoustic and perceptual contrast between front and back labialised vowels, such as[y]and[u], their respective lip configurations may differ. Fant’s nomograms would predict that for [u], the lowest possible F1 and F2 frequencies are attained with the smallest possible lip area, whereas for[y], as F2 needs to be as close as possible to F3, a larger lip area than the one for[u] is required.Horizontal labialisation, which is associated with back vowels, may therefore result in a smaller lip opening area than thevertical labialisationtypical of front vowels. It seems then that the lips work in harmony with the tongue to form the necessary vocal tract configurations required for any given acoustic cue. Trading relations may occur across the different articulatory configurations which reciprocally contribute to a given acoustic cue. Reliance on one will result in less of another, and vice versa. Indeed,motor equivalencetheory suggests that multiple vocal tract configurations may result in the same acoustic output. As a result, speakers may stray away from their habitual articulatory strategy for a given sound when an articulator is perturbed in some way in order to generate the expected acoustic/perceptual output. Motor equivalencephenomena such as these have provided possible answers to the long-debated question of whether speech production goals are articulatory or acoustic in nature. Motor equivalenceargues in favour of acoustics. However, as Perrier and Fuchs (2015) pointed out, perturbation studies such as Savariaux et al. (1995) have also shown that if compensation is not possible, speakers will still prefer their usual vocal tract shape, suggesting that speech goals may have both articulatory and acoustic components.