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Frontal lip shape for French and English vowels
Journal of Phonetics (1992) 20, 3-14
Frontal lip shape for French and English vowels Jean-Pierre Zerling Institut de Phonetique, Universite de Strasbourg II, 22 rue Descartes, 67084 Strasbourg Cedex, France Received lOth July 1990 and in revised form 1st July 1991
This study examines lip shape in French and English vowels. It is based on our own newly gathered data and some data in the published literature for more than 2000 vowel tokens. Only frontal shape of the lip opening is analyzed . Labial configurations for vowels in the two languages are described, and several uncommon characteristics are pointed out, which sometimes do not fit closely with the assumed phonological features. The data suggest that not only shapes , but also strategies may differ greatly from one language to the other; for instance, there are differences in the relationship between lip opening and tongue height.
1. Introduction It is commonly acknowledged that the lips play a very important part in speech
production. In French, labial configurations have been studied from several different viewpoints, including specification of lip parameters for individual phonemes (Abry et al., 1980; Abry , Boe & Schwartz, 1989) and for coarticulatory variability (Bonnot et al. , 1984; Abry & Boe , 1986; Maeda, 1989). Various methods of investigation have been used, such as EMG (Gentil, 1980; Bonnot et al., 1983), acoustic measurements (Majid et al., 1987), and automatic digitizing of video images (Lallouache & Worley, 1988) . Data and descriptions leading to a better understanding of lip behavior is helpful in many fields of research, including phonetics and phonology (Bonnot & Bothorel, 1989), articulatory to acoustic modelling and synthesis (Maeda, 1982; Abry & Boe, 1983), and perception from a visual image of the face (Gentil , 1981; Cathiard, 1989; Tseva, 1990). The present study is part of a more complete set of studies that we have undertaken (Zerling, 1990) and complements our earlier discussion of other aspects of labial parameters, such as their coarticulation across neighboring phonemes (Zerling, 1980), the articulatory strategies used to realize them (Zerling, 1984, 1989a, c), and the extent of their token-to-token variability within a single phoneme (Zerling, 1989b ). These studies show that behind the apparent simplicity of the binary phonological feature [±round] there lies a complex pattern of activity for the lips, yielding continuously variable values for the measured dimensions of labial opening and protrusion as a result of coarticulation with the phonemes in the context and of interspeaker and cross-language differences (Zerling, 1991). It is 0095-4470/92/010003 + 12 $03 .00/0
© 1992 Academic Press Limited
4
J.-P. Zerling
mainly the last point-cross-language differences-that we will discuss here. First, we will summarize the main labial characteristics of French vowels. Then, from the same descriptive framework, we will comment on English vowels using Fromkin's (1964) and Linker's (1982) data. Finally , a comparison between the two languages will lead us to a more general discussion of labial features for vowels. 2. Method This study is about frontal lip shape . Therefore, data mainly come from frontal photographs and films, but lateral X-ray films were also used for further confirmation and discussion. Two sets of data were obtained for the 14 oral and nasal French vowels [i, e, c, a, y, 0, re, u, o, J, £,&,a, 3] . The first consists of 1238 tokens of steady-state vowels extracted from V or VC types uttered by 105 native speakers, all with some knowledge of phonetics. Each token was uttered separately as a monosyllabic word. Data acquisition lasted for several months. Because elicitation initially was only of V types, where [c, re, J] do not occur, these low-mid vowels were produced by only about 60 of the speakers, yielding about 60 tokens for them (as opposed to about 100 tokens for most of the other 10 vowels) . Also, while the rounded nasal /&/ was systematically elicited from each of the speakers , not all speakers had it as a distinct form from the corresponding unrounded /£/, so for [a:] there are only 67 tokens. The second set of data is derived from 40 short sentences read by four speakers, for which both X-ray films and films showing frontal views of the mouth are available (Bothorel et al., 1986). For this particular study, 165 vowels were located and excised from the sentences; as a consequence the segmental contexts are very diverse. In both sets of data, measurements were computed from digitized views of frontal lip opening. Three measures were made: horizontal (A) and vertical (B) separation between the inner surfaces of the lips, and the area of the opening (S). A shape factor K2 (=A/B) was also computed. In this paper, only graphic analyses of the vertical and horizontal separations (B as a function of A) will be shown, although each claim in the analysis has been confirmed by statistical tests such as correlation and Student's t-test. Also, the paper will sometimes refer to analyses of lateral X-ray film images, which were used to confirm vertical separation and to give complementary information about protrusion and tongue position . For English vowels, data for the same parameters were obtained from Fromkin (1964) and Linker (1982). Fromkin's corpus consists of 600 utterances, 12 American English vowels pronounced 10 times by five subjects in the words heed, hid, hay, head, had, hod, hud, herd, hawed, hoed, hood, who. Linker's corpus consists of 72 tokens, the nine English vowels [a, re , e, c , i, 1, u , u, o] pronounced by eight male speakers in [sV] syllables . 3. French vowel results Figure 1 plots summary ellipses of vertical separation (B) as a function of horizontal separation (A) for all tokens from the first data set (isolated steady-state vowels) . Each ellipse contains 90% of the points for one particular phoneme. As may be seen from the figure, French steady-state vowels fill the (A, B) articulatory space in a characteristic way; three groups of nearly superimposed ellipses are clearly evident.
Lip shape in French and English
5
--r---.----,--- - --~- · --·-~ · ------y·-- - - - ,----,---r---
30 _r--,
/ a --.. .
\
/
./---1-
I
~/~--/ji_-,<~:\!: /
6_ //
//·J i 'l\/
~. \
-o·- -·,' 1
!~!;> ce -., ~\\ /-;;:;::r i
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'
/ /.
I'
'
/
- lab
1
1
u ' ,_ ~'/ --~------0 '----'----""--'~-·-~-~-~ - '-- '-·
0
70
Hori zo ntal width A (mm)
Figure 1. Relation between horizontal width (A) and vertical height (B) of lip opening in frontal view for 14 steady-state French vowels pronounced by 105 speakers . E ach ellipse surrounds 90 % of the tokens for a given vowel phoneme .
Considering what is currently accepted about labiality, we chose to attribute a particular name and symbol to each of these three groups (and to the vowels in them) in the following manner : (1) [-lab]= nonlabialized = [i, e , £ , a , £, (ce)] (2) [+lab]= (medium) labialized= [ce , ::l , 6. , (ce)] (3) [++lab]= strongly (or super) labialized= [y , e , u , o , 3]. (Note that in Modern French, /ce/ is often realized as unlabialized . That is , many speakers neutralize it with/£/ .) The higher values of labial opening width (A) for [-lab] vowels justifies labelling them as "spread" whereas others will be called " rounded" because of their lower values of A , B, and S (not shown here) . Figure 2 shows all of the data points with a 90% summary ellipse for each of the three groups . There are several important points to be made about these three groupings in the figure. First , the so-called [+round] vowels clearly fall into two completely distinct groups , [++lab] and [+lab], with not one case of confusion between them . On the other hand, the labialized [+lab] and nonlabialized [-lab] groups do show an area of overlap , an area where the same frontal shape may sometimes be used for one or the other type. Nevertheless , a closer look at these categories shows that pairs of vowels which overlap in this way almost always differ in fact by at least one other articulatory parameter: either the tongue position , or nasality , or lip protrusion. All of these other differences are easily observed on lateral X-ray pictures. The only exception is the opposition between the low-mid vowels [£ , ce]; for this pair there may be some doubt as to whether protrusion is sufficient to make the acoustic difference, the rest of the articulatory shape being ve ry similar. However, not all speakers produced both of these vowels . Figure 3 shows the tokens of these two vowels produced by the 67 speakers who pronounced both of them ; for these speakers the frontal difference is clearly maintained . The second important point concerns the nasal vowels. Before making this point ,
J.-P. Zerling 30
though, we must first note that we excluded from Fig . 2 the nasal vowel [&], the realizations of which happened to cover both [-lab] and [+lab] ellipses. It is a well known fact that the French labial opposition /f.,&/ is disappearing; for many speakers the historically rounded vowel is neutralized to be homophonous with the unrounded vowel. The distribution of the three remaining nasal vowels [f., a, 3] then is interesting: each one fits into a separate category, and together, they fill the whole articulatory space. 30
c
Horizontal width A (mm) Figure 3. Comparison of steady-state realizations of [c) vs [re) for 67 speakers . Ellipses as in Fig. 1.
Lip shape in French and English
7
The last point is perhaps the most important; it may be seen from Fig. 1 that within each of the three categories, no regular correlation between lip opening and tongue height seems to exist: the same degree of lip separation may be found indiscriminantly for any vowel in the group. This is contrary to the idea that for lower vowels the more open jaw posture for the relatively low tongue body position should pull down the lower lip . This lack of articulatory constraint on lip opening for [-lab] vowels may be explained in two different ways. First, the small acoustic consequences of lip area variation for vowels with spread lips (Fant, 1960) may allow considerable lattitude in production. Second, there may be adequate lingual compensation to offset any variation in lip configuration. It has been shown from lateral X-ray pictures for the same corpus by Bothorel et al. (1986) and Zerling (1991), that tongue height variation from one vowel to another is greater when less distinction is made at the lips. All these observations were made for sustained vowels. In order to determine whether they remain valid in connected speech, we applied the same technique to the vowels excised from read sentences . The result is shown on Fig. 4. The three categories are definitely preserved. The main difference from Fig. 3 is the smaller amount of variation for the [-lab] and [+lab] vowels. Interestingly, there is no difference for the [++lab] vowels which already had little variation in Fig. 1. This difference between the [++lab] and the other two groups is due probably to acoustical constraints: for very small lip areas, even minor variation of the area may have major acoustic consequences (Fant, 1960) . Although we do not show it here, our data (Zerling, 1990) generally agree with those of other studies on French (e.g. , Descout, Boe & Abry, 1978; Abry et al. , 1980; Linker, 1982; Abry & Boe, 1983).
30
Horizontal width A (mm) Figure 4. Relation between lip horizontal width (A) and vertical height (B) for 143 French vowel tokens excised from connected speech produced by four speakers. Ellipses contain 90% of the tokens in each of the categories: [-lab] vowels and 3 groups of labialized vowels: /y, u/ , /e, o/ , and /re, :J/ .
J.-P. Zerling
8
4. English vowel results Having described .French vowel characteristics, we will now do the same for English ones, using available published data. Figure 5 shows mean values of A and B calculated from Fromkin's (1964) data plots and compares them with the three categories proposed above for French. Figure 6 shows Linker's 1982 data, which were comparable to ours so they could be used without modification and plotted directly as summary ellipses. A comparison of the two graphs reveals a completely different behavior for the two groups of subjects. On the whole, Fromkin's data rather resemble those for French vowels: the non-labial vowels [i, 1, e, £, re] look very much like our [-lab] tokens, with rather high and almost constant width values. Also we saw previously in Fig. 1 that [+lab] and [++lab] vowels in French define a sloping line that runs at roughly 45° up from the lower left corner in the figure. We note here that Fromkin's measurements for the [+round] English vowels [u, u, o, ac, ;,]lie along the same line: both width and height parameters have definitely lower values than do the nonlabialized vowels. These two series can be called "rounded" and "spread", as in French . Yet, there is a major difference from French in that both categories show a direct relation between lip opening and tongue height. (Recall that no such relationship is seen in French .) Because of their medial position on the graph, the last two vowels [a, A], whose symbols suggest nonlabiality, can be called " neutral", after Jones (1960). Additional data recently published by Bolla (1989) for one set of vowels uttered by one British female speaker suggest a labial behavior similar to that of Fromkin's speakers. In Linker's data, by contrast, although there are still two groups, there is a very
30
Horizontal width A (mm)
Figure 5. Mean position of 12 American English vowels in (A, B) space for 600 tokens pronounced by five speakers (data calculated from plots in Fromkin, 1964). Ellipses show the three French steady-state categories, for comparison.
Lip shape in French and English
9
30
Horizontal width A (mm)
Figure 6. Relation between lip horizontal width (A) and vertical height (B) for 72 tokens of nine American English vowel phonemes pronounced by eight speakers (data from Linker, 1982). The ellipses surround 90% of the tokens for each phoneme.
different distribution in (A , B) space, mainly due to very low B values. This is particularly obvious in Fig. 7, which compares Linker's English data to the French groupings. The fact that A values are rather high and B values rather low eliminates a possible systematic difference in scaling between the two experiments . Nor do we believe that the context can explain the difference, since Linker's vowels were systematically pronounced in an alveolar context ([sV]) syllables), as were most of
30
Horizontal width A (mm)
Figure 7. Comparison between the categories for Linker's (1982) American English vowel tokens (dashed-line ellipses) and the three French categories for tokens from connected speech (solid ellipses). Each ellipse contains 90% of the tokens for the category .
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J.-P. Zerling
Fromkin's vowels ([hVd] syllables). Therefore , we think that these vowel measurements reflect an articulatory behavior common to that particular group of speakers, and distinguishing ·them from Fromkin's speakers. Unfortunately, no precise details are given in the paper. Although we know for certain that Linker's reference to "English" is to American English (personal correspondence , Vickie Fromkin), we cannot help remembering a social behavior rule that we were taught a few years ago while learning British English : "keep a stiff upper lip" . This is precisely what these data suggest. Another (possibly related) difference between Fromkin's and Linker's data is that there is fairly complete overlap among the nonlabialized vowels [i , 1, e, £, re, a] in Fig. 6: the same lip opening may be used for any vowel, whatever the vertical position of the tongue . This type of articulation may be called "spread" , since it is characterized by high values of A . On the other hand, given the low values of B, the name "flat" would be even more suitable than " spread". The so-called "rounded" vowels of the second group in Fig. 6 differ from the spread or flat vowels mainly by their lower A values , which means that the shape is not as spread. At the same time , the vertical opening gets progressively larger in the order [u] < [o] < [u]. In this case, there seems to exist some correlation between lip position and tongue height. More important to us is the fact that although these vowels are labialized, as can be seen from the low area values, it is difficult to think of them as " rounded", since the lip shape has nothing in common with that of all the rounded vowels that were previously described for French and English . Here, the mouth opening is definitely flat rather than round. For that reason, we would prefer calling these vowels "flat'· . (They also may be " compressed", as in Lindau's (1975) use of the word "compressed" for the Swedish central vowel [u].) Thus, in conclusion, we decide to describe this set of vowels respectively as [labial flat] and [nonlabial flat] in order to distinguish them from Fromkin's [round] and [spread] vowels . (In addition, the [flat] vowels may be compressed, although we cannot tell , having no information about muscular activity .) Because no further details are given in the two studies, we cannot fully explain the difference, although it is clear that the two groups of speakers differ greatly in their vocalic labial strategies.
5. Discussion Taking into account these results , as well as data available in Zerling (1990) , we will now propose a classification of vowels according to labial characteristics. We suggest describing vocalic labial characteristics with the following parameters: (1) A binary mode of labialization : nonlabial vs labial, as determined by the phonological specification. (2) A type of labialization, chosen from the following five qualitative categories: -labial rounded (protruded) with or without -labial flat (nonprotruded)} lip compression -nonlabial flat -nonlabial neutral -nonlabial spread.
I. Description of vowels and categories in terms of mode , type, degree , and strategy of labialization for French (after our data) and American English (after Fromkin, 1964 and Linker, 1982)
TABLE
Corpus French Zerling
English Fromkin
Vowels
Mode
Type
[i, e, c , a,£]
[-lab]
nonlabial
spread
[a:,:) , ii, &]
[+lab]
labial
[y , 0, u, o, 5]
[++lab]
labial
[i , I , e, £,
[-round]
nonlabial
protruded =round protruded =round spread
[a, A]
[-round]
nonlabial
neutral
[+round]
labial
protruded =round
[i , I , e , £,
[-round]
nonlabial
flat
[u , o, u]
[+round]
labial
flat
[u, u, o, English Linker
Category
;J , :)]
Degree
Strategy
1 medium to large 1 medium 1 small progressive medium to large progressive medium progressive small to medium
Degree independent of vowel in the category
1 rather small small and slightly progressive
t""'
-15" c.,
~
-§.,
s· Degree related to vertical tongue position
.,;:s~ ("") ~
I:>
;:s ~
~
~.....
t:; ·
Degree related to vertical tongue position
~
....... .......
J.-P. Zerling
12
Labialized vowels are realized either by rounding and protruding the lips, or by firmly bringing them together. In either case, the aim is to reduce the area of the lip opening. The distinction between [labial flat] and [nonlabial flat] is obtained by controlling the horizontal width A, and consequently the area S. (3) A degree of labialization. This is a quantitative measure which specifies the lip opening, i.e. , the vertical lip space B and the area S. This parameter may be controlled either by jaw or lip movements (Macchi , 1988) . (4) A strategy that controls the various degrees of labialization of all the vowels in the category, making them for example, either: -dependent on tongue vertical position -independent of the tongue position . We will use the term "labial category" to designate any group of vowel phonemes or tokens having the same mode and the same type of labialization, with variation within the group governed by the same strategy . Our model assumes that behavior is governed by articulatory and biomechanical constraints as well as by learned patterns of motor control. All these characteristics clearly depend on the language and on its "articulatory basis" (Straka, 1989), to which we now add personal behavior-i.e., an individual speaker's articulatory and coarticulatory habits. Table I summarizes the vowel data described in this study, using these parameters of mode, type, degree, and strategy. Figure 8 gives a theoretical graphical representation of how these categories would behave in plots such as Figs 1-7. Of course, more data from other languages will be necessary to test and to possibly modify or complete this representation, and we have already collected some data for Thai, Cantonese , Finnish, Polish, and Swedish (Zerling , 1991). In the meanwhile, we suggest that each different type of labialization proposed here might stand for a natural labial category, or constitute a "natural labial axis" (Rossi, 1983)-i.e. , a group of vowels involving the progressive and non-contradictory activity of one muscle or of a group of muscles, as opposed to a "non-natural" axis demanding a complete reorganization of muscular activity from one vowel to another.
+labial
-labial
{ may be compressed
Horizontal width A Figure 8. Graphical representation of hypothesized vowel dispersions in the (A , B) frontal labial space for the different theoretical categories.
Lip shape in French and English
13
References Abry, C. & Boe, L. J. (1983) L'encodage labial des voyelles du fran<;ais, Speech Communication, 2, 123-128. Abry, C. & Boe, L. J. (1986) 'Laws' for lips, Speech Communication, 5, 97-104. Abry, C., Boe , L. J. & Descout , R. (1979) Voyelles /abiales et voyelles /abialisees en francais. 9th Int. Cong. Phon. Sc., Copenhagen, p. 177. Abry, C ., Boe, L. J. & Schwartz, J. L. (1989) Plateaus , catastrophes and the structuring of vowel systems, Journal of Phonetics, 17, 47-54. Abry, C., Boe , L. J., Corsi, P., Descout , R., Gentil, M. & Graillot , P. (1980) Labialite et Phonetique. Donnees fondamentales et etudes experimentales sur Ia geometrie et Ia motricite labiales. Institut de Phonetique de Grenoble, 304 p. Bolla , K. (1989) A phonetic conspectus of English. The articulatory and acoustic features of British English speech sounds , Magyar Fonetikai Fiizetek, Hungarian Papers in Phonetics, 20, 402. Bonnot, J. F. & Bothorel , A. (1989) Co-dependance des traits phonetiques, sensibilite au contexte et variabilite parametrique . In Melanges de Phonetique Generate et Experimentale offerts ii Pela Simon. pp. 95-116 . Strasbourg: Institut de Phonetique. Bonnot, J. F. , Chevrie-Muller, C., Greiner, G. , Guidet , C. & Maton , B. (1984) Coarticulation anticipante et coarticulation retentive en fran<;ais: physiologie de quelques indices electromyographiques , 13° J. E. P. (Journees d'Etudes sur Ia Parole) pp. 215-216 . Brussels: GALF. Bonnot, J. F., Chevrie-Muller, C. , Greiner , G ., Maton, B. & Guidet , C. (1983) Etude de l'encodage moteur des traits de nasalite et de labialite ii partir de /'activite EMG des muscles orbiculaires ( 00) et elevateur du voile (LP). 11th Int. Cong . Acous. Toulouse , p . 76. Bothorel, A., Simon, P., Wioland, F. & Zerling, J. P. (1986) Cineradiographie des voyelles et consonnes du fran{:ais. Recueil de documents synchronises pour 4 sujets : vues laterales du conduit vocal, vues frontales de !'orifice labial, donnees acoustiques, Strasbourg: lnstitut de Phonetique. Cathiard, M. A. (1989) La perception visuelle de Ia parole: aper<;u de l'etat des connaissances, Bulletin de l'Institut de Phonetique de Grenoble, 17/18 , 109-193. Descout, R., Boe, L. J. & Abry , C. (1978) Labialite vocalique et labialite consonantique en fran<;ais. Un jeu des levres au feminin, Premiers resultats , 9° J.E. P. (Journees d'Etudes sur Ia Parole) pp. 177-189. Brussels: GALF (and in Labialite et Phonetique , 1980, 111-126). Fant , G. (1960) Acoustic theory of speech production. The Hague: Mouton. Fromkin , V. A. (1964) Lips positions in American English vowels, Language and Speech, 7, 215-225. Gentil, M. (1980) Labialite en fran{:ais: etude phonetique et aspects physiologiques des lev res. These de 3e cycle, Universite de Grenoble III. Gentil, M. (1981) Etude de La perception de La parole: lecture labiale et sosies /abiaux. IBM, France, personal communication. Jones , D. (1960) An outline of English phonetics, 9th edn , Cambridge: Heffer. Lallouache , M. T. & Worley, C. (1988) Saisie, edition et traitement d'images et de signaux articulatoires: /evres et miichoires, Seminaire Production de Ia Parole: Modeles et donnees , 2-3 February 1988, Grenoble . Lindau , M. (1975) Features for vowels. (UCLA Working Papers in Phonetics, 30.) Linker, W. (1982) Articulatory and acoustic correlates of labial activity in vowels: a cross-linguistic study, UCLA Working Papers in Phonetics, 56, 134. Macchi, M. (1988) Labial articulati.on patterns associated with segmental features and syllable structure in English, Phonetica, 45 , 109-121. Maeda , S. (1982) A digital simulation method of the vocal-tract system, Speech Communication, 1, 199-229. Maeda, S. (1989) Articulation compensatoire des voyelles : ana lyse de donnees cineradiographiqucs avec un modele lineaire. In Melanges de Phonetique generate et experimentale offerts ii Pela Simon. pp. 545-562. Strasbourg: Institut de Phonetique. Majid , R., Abry, C. , Boe , L. J . & Perrier, P. (1987) Contribution ii La classification articulatori-acoustique des voyelles: etude des macro-sensibilites ii !'aide d'un modele articu/atoire. 11th Int. Con g. Phon. Sc., Tallin, USSR. Rossi , M. (1983) Niveaux de !'analyse phonetique: nature et structuration des indices et des traits , Speech Communication, 2, 91-106. Straka, G. (1989) Base articulatoire. Essai d'une mise au point. In Melanges de Phonetique generate et experimentale offerts ii Nla Simon. pp. 757-768. Strasbourg: Institut de Phonetique. Tseva, A. (1990) L'arrondissement dans !'identification visuelle des voyelles du fran<;ais. Premiers acquis, Bull. LCP Grenoble, 3, 149-186. Zerling, J.P. (1980) Coarticu/ation /abiale et aire aux levres dans des groupes occlusives-voyelles en fran{:ais . Seminaire International Labialite , GALF, Lannion , February 1980. Zerling , J. P. (1984) Phenomenes de n ~.sa lit e et de nasalisation vocaliques : etude cineradiographique pour deux locuteurs, TIPS: Travaux de l'Institut de Phonetique de Strasbourg , 16 , 241-266.
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Zerling, J.P. (1989a) Les trois degres de labialisation des voyelles isolees en franc;ais. Etude pour 105 locuteurs. In Melanges de Phonetique Generale et Experimentale offerts ii P€/a Simon. pp. 807-831. Strasbourg: Institut de Phonetique. Zerling, 1. P. (1989b) Strategies labiales vocaliques en franrais. Variabilite des parametres frontaux. pp. 116-119. Seminaire Variabilite et specificite des locuteurs , SFA-GCP, Marseille , 20-21 June. Zerling, J.P. (1989c) The three degrees of labialization of the French steady-state vowels. A study for 105 speakers. Eurospeech 89. pp. 445-448. European Conference on Speech Communication and Technology, Paris , 26-28 September. Zerling, 1. P. (1990) Aspects articulatoires de Ia labia/it€ vocalique en franrais . Contribution ii Ia modelisation ii partir de labiophotographies, labiofilms, etfilms radiologiques. These d'Etat, Universite de Strasbourg II. Zerling, J.P. (1991) Labia/it€ vocalique: Etude comparee des types, degres et strategies articulatoires de plusieurs langues, vol. 3, pp. 46-49, 12eme Congres International des Sciences Phonetiques, Aix-en-Provence , 19-24 August.
Frontal lip shape for French and English vowels Jean-Pierre Zerling Institut de Phonetique, Universite de Strasbourg II, 22 rue Descartes, 67084 Strasbourg Cedex, France Received lOth July 1990 and in revised form 1st July 1991
This study examines lip shape in French and English vowels. It is based on our own newly gathered data and some data in the published literature for more than 2000 vowel tokens. Only frontal shape of the lip opening is analyzed . Labial configurations for vowels in the two languages are described, and several uncommon characteristics are pointed out, which sometimes do not fit closely with the assumed phonological features. The data suggest that not only shapes , but also strategies may differ greatly from one language to the other; for instance, there are differences in the relationship between lip opening and tongue height.
1. Introduction It is commonly acknowledged that the lips play a very important part in speech
production. In French, labial configurations have been studied from several different viewpoints, including specification of lip parameters for individual phonemes (Abry et al., 1980; Abry , Boe & Schwartz, 1989) and for coarticulatory variability (Bonnot et al. , 1984; Abry & Boe , 1986; Maeda, 1989). Various methods of investigation have been used, such as EMG (Gentil, 1980; Bonnot et al., 1983), acoustic measurements (Majid et al., 1987), and automatic digitizing of video images (Lallouache & Worley, 1988) . Data and descriptions leading to a better understanding of lip behavior is helpful in many fields of research, including phonetics and phonology (Bonnot & Bothorel, 1989), articulatory to acoustic modelling and synthesis (Maeda, 1982; Abry & Boe, 1983), and perception from a visual image of the face (Gentil , 1981; Cathiard, 1989; Tseva, 1990). The present study is part of a more complete set of studies that we have undertaken (Zerling, 1990) and complements our earlier discussion of other aspects of labial parameters, such as their coarticulation across neighboring phonemes (Zerling, 1980), the articulatory strategies used to realize them (Zerling, 1984, 1989a, c), and the extent of their token-to-token variability within a single phoneme (Zerling, 1989b ). These studies show that behind the apparent simplicity of the binary phonological feature [±round] there lies a complex pattern of activity for the lips, yielding continuously variable values for the measured dimensions of labial opening and protrusion as a result of coarticulation with the phonemes in the context and of interspeaker and cross-language differences (Zerling, 1991). It is 0095-4470/92/010003 + 12 $03 .00/0
© 1992 Academic Press Limited
4
J.-P. Zerling
mainly the last point-cross-language differences-that we will discuss here. First, we will summarize the main labial characteristics of French vowels. Then, from the same descriptive framework, we will comment on English vowels using Fromkin's (1964) and Linker's (1982) data. Finally , a comparison between the two languages will lead us to a more general discussion of labial features for vowels. 2. Method This study is about frontal lip shape . Therefore, data mainly come from frontal photographs and films, but lateral X-ray films were also used for further confirmation and discussion. Two sets of data were obtained for the 14 oral and nasal French vowels [i, e, c, a, y, 0, re, u, o, J, £,&,a, 3] . The first consists of 1238 tokens of steady-state vowels extracted from V or VC types uttered by 105 native speakers, all with some knowledge of phonetics. Each token was uttered separately as a monosyllabic word. Data acquisition lasted for several months. Because elicitation initially was only of V types, where [c, re, J] do not occur, these low-mid vowels were produced by only about 60 of the speakers, yielding about 60 tokens for them (as opposed to about 100 tokens for most of the other 10 vowels) . Also, while the rounded nasal /&/ was systematically elicited from each of the speakers , not all speakers had it as a distinct form from the corresponding unrounded /£/, so for [a:] there are only 67 tokens. The second set of data is derived from 40 short sentences read by four speakers, for which both X-ray films and films showing frontal views of the mouth are available (Bothorel et al., 1986). For this particular study, 165 vowels were located and excised from the sentences; as a consequence the segmental contexts are very diverse. In both sets of data, measurements were computed from digitized views of frontal lip opening. Three measures were made: horizontal (A) and vertical (B) separation between the inner surfaces of the lips, and the area of the opening (S). A shape factor K2 (=A/B) was also computed. In this paper, only graphic analyses of the vertical and horizontal separations (B as a function of A) will be shown, although each claim in the analysis has been confirmed by statistical tests such as correlation and Student's t-test. Also, the paper will sometimes refer to analyses of lateral X-ray film images, which were used to confirm vertical separation and to give complementary information about protrusion and tongue position . For English vowels, data for the same parameters were obtained from Fromkin (1964) and Linker (1982). Fromkin's corpus consists of 600 utterances, 12 American English vowels pronounced 10 times by five subjects in the words heed, hid, hay, head, had, hod, hud, herd, hawed, hoed, hood, who. Linker's corpus consists of 72 tokens, the nine English vowels [a, re , e, c , i, 1, u , u, o] pronounced by eight male speakers in [sV] syllables . 3. French vowel results Figure 1 plots summary ellipses of vertical separation (B) as a function of horizontal separation (A) for all tokens from the first data set (isolated steady-state vowels) . Each ellipse contains 90% of the points for one particular phoneme. As may be seen from the figure, French steady-state vowels fill the (A, B) articulatory space in a characteristic way; three groups of nearly superimposed ellipses are clearly evident.
Lip shape in French and English
5
--r---.----,--- - --~- · --·-~ · ------y·-- - - - ,----,---r---
30 _r--,
/ a --.. .
\
/
./---1-
I
~/~--/ji_-,<~:\!: /
6_ //
//·J i 'l\/
~. \
-o·- -·,' 1
!~!;> ce -., ~\\ /-;;:;::r i
1/(j '(
'
/ /.
I'
'
/
- lab
1
1
u ' ,_ ~'/ --~------0 '----'----""--'~-·-~-~-~ - '-- '-·
0
70
Hori zo ntal width A (mm)
Figure 1. Relation between horizontal width (A) and vertical height (B) of lip opening in frontal view for 14 steady-state French vowels pronounced by 105 speakers . E ach ellipse surrounds 90 % of the tokens for a given vowel phoneme .
Considering what is currently accepted about labiality, we chose to attribute a particular name and symbol to each of these three groups (and to the vowels in them) in the following manner : (1) [-lab]= nonlabialized = [i, e , £ , a , £, (ce)] (2) [+lab]= (medium) labialized= [ce , ::l , 6. , (ce)] (3) [++lab]= strongly (or super) labialized= [y , e , u , o , 3]. (Note that in Modern French, /ce/ is often realized as unlabialized . That is , many speakers neutralize it with/£/ .) The higher values of labial opening width (A) for [-lab] vowels justifies labelling them as "spread" whereas others will be called " rounded" because of their lower values of A , B, and S (not shown here) . Figure 2 shows all of the data points with a 90% summary ellipse for each of the three groups . There are several important points to be made about these three groupings in the figure. First , the so-called [+round] vowels clearly fall into two completely distinct groups , [++lab] and [+lab], with not one case of confusion between them . On the other hand, the labialized [+lab] and nonlabialized [-lab] groups do show an area of overlap , an area where the same frontal shape may sometimes be used for one or the other type. Nevertheless , a closer look at these categories shows that pairs of vowels which overlap in this way almost always differ in fact by at least one other articulatory parameter: either the tongue position , or nasality , or lip protrusion. All of these other differences are easily observed on lateral X-ray pictures. The only exception is the opposition between the low-mid vowels [£ , ce]; for this pair there may be some doubt as to whether protrusion is sufficient to make the acoustic difference, the rest of the articulatory shape being ve ry similar. However, not all speakers produced both of these vowels . Figure 3 shows the tokens of these two vowels produced by the 67 speakers who pronounced both of them ; for these speakers the frontal difference is clearly maintained . The second important point concerns the nasal vowels. Before making this point ,
J.-P. Zerling 30
though, we must first note that we excluded from Fig . 2 the nasal vowel [&], the realizations of which happened to cover both [-lab] and [+lab] ellipses. It is a well known fact that the French labial opposition /f.,&/ is disappearing; for many speakers the historically rounded vowel is neutralized to be homophonous with the unrounded vowel. The distribution of the three remaining nasal vowels [f., a, 3] then is interesting: each one fits into a separate category, and together, they fill the whole articulatory space. 30
c
Horizontal width A (mm) Figure 3. Comparison of steady-state realizations of [c) vs [re) for 67 speakers . Ellipses as in Fig. 1.
Lip shape in French and English
7
The last point is perhaps the most important; it may be seen from Fig. 1 that within each of the three categories, no regular correlation between lip opening and tongue height seems to exist: the same degree of lip separation may be found indiscriminantly for any vowel in the group. This is contrary to the idea that for lower vowels the more open jaw posture for the relatively low tongue body position should pull down the lower lip . This lack of articulatory constraint on lip opening for [-lab] vowels may be explained in two different ways. First, the small acoustic consequences of lip area variation for vowels with spread lips (Fant, 1960) may allow considerable lattitude in production. Second, there may be adequate lingual compensation to offset any variation in lip configuration. It has been shown from lateral X-ray pictures for the same corpus by Bothorel et al. (1986) and Zerling (1991), that tongue height variation from one vowel to another is greater when less distinction is made at the lips. All these observations were made for sustained vowels. In order to determine whether they remain valid in connected speech, we applied the same technique to the vowels excised from read sentences . The result is shown on Fig. 4. The three categories are definitely preserved. The main difference from Fig. 3 is the smaller amount of variation for the [-lab] and [+lab] vowels. Interestingly, there is no difference for the [++lab] vowels which already had little variation in Fig. 1. This difference between the [++lab] and the other two groups is due probably to acoustical constraints: for very small lip areas, even minor variation of the area may have major acoustic consequences (Fant, 1960) . Although we do not show it here, our data (Zerling, 1990) generally agree with those of other studies on French (e.g. , Descout, Boe & Abry, 1978; Abry et al. , 1980; Linker, 1982; Abry & Boe, 1983).
30
Horizontal width A (mm) Figure 4. Relation between lip horizontal width (A) and vertical height (B) for 143 French vowel tokens excised from connected speech produced by four speakers. Ellipses contain 90% of the tokens in each of the categories: [-lab] vowels and 3 groups of labialized vowels: /y, u/ , /e, o/ , and /re, :J/ .
J.-P. Zerling
8
4. English vowel results Having described .French vowel characteristics, we will now do the same for English ones, using available published data. Figure 5 shows mean values of A and B calculated from Fromkin's (1964) data plots and compares them with the three categories proposed above for French. Figure 6 shows Linker's 1982 data, which were comparable to ours so they could be used without modification and plotted directly as summary ellipses. A comparison of the two graphs reveals a completely different behavior for the two groups of subjects. On the whole, Fromkin's data rather resemble those for French vowels: the non-labial vowels [i, 1, e, £, re] look very much like our [-lab] tokens, with rather high and almost constant width values. Also we saw previously in Fig. 1 that [+lab] and [++lab] vowels in French define a sloping line that runs at roughly 45° up from the lower left corner in the figure. We note here that Fromkin's measurements for the [+round] English vowels [u, u, o, ac, ;,]lie along the same line: both width and height parameters have definitely lower values than do the nonlabialized vowels. These two series can be called "rounded" and "spread", as in French . Yet, there is a major difference from French in that both categories show a direct relation between lip opening and tongue height. (Recall that no such relationship is seen in French .) Because of their medial position on the graph, the last two vowels [a, A], whose symbols suggest nonlabiality, can be called " neutral", after Jones (1960). Additional data recently published by Bolla (1989) for one set of vowels uttered by one British female speaker suggest a labial behavior similar to that of Fromkin's speakers. In Linker's data, by contrast, although there are still two groups, there is a very
30
Horizontal width A (mm)
Figure 5. Mean position of 12 American English vowels in (A, B) space for 600 tokens pronounced by five speakers (data calculated from plots in Fromkin, 1964). Ellipses show the three French steady-state categories, for comparison.
Lip shape in French and English
9
30
Horizontal width A (mm)
Figure 6. Relation between lip horizontal width (A) and vertical height (B) for 72 tokens of nine American English vowel phonemes pronounced by eight speakers (data from Linker, 1982). The ellipses surround 90% of the tokens for each phoneme.
different distribution in (A , B) space, mainly due to very low B values. This is particularly obvious in Fig. 7, which compares Linker's English data to the French groupings. The fact that A values are rather high and B values rather low eliminates a possible systematic difference in scaling between the two experiments . Nor do we believe that the context can explain the difference, since Linker's vowels were systematically pronounced in an alveolar context ([sV]) syllables), as were most of
30
Horizontal width A (mm)
Figure 7. Comparison between the categories for Linker's (1982) American English vowel tokens (dashed-line ellipses) and the three French categories for tokens from connected speech (solid ellipses). Each ellipse contains 90% of the tokens for the category .
10
J.-P. Zerling
Fromkin's vowels ([hVd] syllables). Therefore , we think that these vowel measurements reflect an articulatory behavior common to that particular group of speakers, and distinguishing ·them from Fromkin's speakers. Unfortunately, no precise details are given in the paper. Although we know for certain that Linker's reference to "English" is to American English (personal correspondence , Vickie Fromkin), we cannot help remembering a social behavior rule that we were taught a few years ago while learning British English : "keep a stiff upper lip" . This is precisely what these data suggest. Another (possibly related) difference between Fromkin's and Linker's data is that there is fairly complete overlap among the nonlabialized vowels [i , 1, e, £, re, a] in Fig. 6: the same lip opening may be used for any vowel, whatever the vertical position of the tongue . This type of articulation may be called "spread" , since it is characterized by high values of A . On the other hand, given the low values of B, the name "flat" would be even more suitable than " spread". The so-called "rounded" vowels of the second group in Fig. 6 differ from the spread or flat vowels mainly by their lower A values , which means that the shape is not as spread. At the same time , the vertical opening gets progressively larger in the order [u] < [o] < [u]. In this case, there seems to exist some correlation between lip position and tongue height. More important to us is the fact that although these vowels are labialized, as can be seen from the low area values, it is difficult to think of them as " rounded", since the lip shape has nothing in common with that of all the rounded vowels that were previously described for French and English . Here, the mouth opening is definitely flat rather than round. For that reason, we would prefer calling these vowels "flat'· . (They also may be " compressed", as in Lindau's (1975) use of the word "compressed" for the Swedish central vowel [u].) Thus, in conclusion, we decide to describe this set of vowels respectively as [labial flat] and [nonlabial flat] in order to distinguish them from Fromkin's [round] and [spread] vowels . (In addition, the [flat] vowels may be compressed, although we cannot tell , having no information about muscular activity .) Because no further details are given in the two studies, we cannot fully explain the difference, although it is clear that the two groups of speakers differ greatly in their vocalic labial strategies.
5. Discussion Taking into account these results , as well as data available in Zerling (1990) , we will now propose a classification of vowels according to labial characteristics. We suggest describing vocalic labial characteristics with the following parameters: (1) A binary mode of labialization : nonlabial vs labial, as determined by the phonological specification. (2) A type of labialization, chosen from the following five qualitative categories: -labial rounded (protruded) with or without -labial flat (nonprotruded)} lip compression -nonlabial flat -nonlabial neutral -nonlabial spread.
I. Description of vowels and categories in terms of mode , type, degree , and strategy of labialization for French (after our data) and American English (after Fromkin, 1964 and Linker, 1982)
TABLE
Corpus French Zerling
English Fromkin
Vowels
Mode
Type
[i, e, c , a,£]
[-lab]
nonlabial
spread
[a:,:) , ii, &]
[+lab]
labial
[y , 0, u, o, 5]
[++lab]
labial
[i , I , e, £,
[-round]
nonlabial
protruded =round protruded =round spread
[a, A]
[-round]
nonlabial
neutral
[+round]
labial
protruded =round
[i , I , e , £,
[-round]
nonlabial
flat
[u , o, u]
[+round]
labial
flat
[u, u, o, English Linker
Category
;J , :)]
Degree
Strategy
1 medium to large 1 medium 1 small progressive medium to large progressive medium progressive small to medium
Degree independent of vowel in the category
1 rather small small and slightly progressive
t""'
-15" c.,
~
-§.,
s· Degree related to vertical tongue position
.,;:s~ ("") ~
I:>
;:s ~
~
~.....
t:; ·
Degree related to vertical tongue position
~
....... .......
J.-P. Zerling
12
Labialized vowels are realized either by rounding and protruding the lips, or by firmly bringing them together. In either case, the aim is to reduce the area of the lip opening. The distinction between [labial flat] and [nonlabial flat] is obtained by controlling the horizontal width A, and consequently the area S. (3) A degree of labialization. This is a quantitative measure which specifies the lip opening, i.e. , the vertical lip space B and the area S. This parameter may be controlled either by jaw or lip movements (Macchi , 1988) . (4) A strategy that controls the various degrees of labialization of all the vowels in the category, making them for example, either: -dependent on tongue vertical position -independent of the tongue position . We will use the term "labial category" to designate any group of vowel phonemes or tokens having the same mode and the same type of labialization, with variation within the group governed by the same strategy . Our model assumes that behavior is governed by articulatory and biomechanical constraints as well as by learned patterns of motor control. All these characteristics clearly depend on the language and on its "articulatory basis" (Straka, 1989), to which we now add personal behavior-i.e., an individual speaker's articulatory and coarticulatory habits. Table I summarizes the vowel data described in this study, using these parameters of mode, type, degree, and strategy. Figure 8 gives a theoretical graphical representation of how these categories would behave in plots such as Figs 1-7. Of course, more data from other languages will be necessary to test and to possibly modify or complete this representation, and we have already collected some data for Thai, Cantonese , Finnish, Polish, and Swedish (Zerling , 1991). In the meanwhile, we suggest that each different type of labialization proposed here might stand for a natural labial category, or constitute a "natural labial axis" (Rossi, 1983)-i.e. , a group of vowels involving the progressive and non-contradictory activity of one muscle or of a group of muscles, as opposed to a "non-natural" axis demanding a complete reorganization of muscular activity from one vowel to another.
+labial
-labial
{ may be compressed
Horizontal width A Figure 8. Graphical representation of hypothesized vowel dispersions in the (A , B) frontal labial space for the different theoretical categories.
Lip shape in French and English
13
References Abry, C. & Boe, L. J. (1983) L'encodage labial des voyelles du fran<;ais, Speech Communication, 2, 123-128. Abry, C. & Boe, L. J. (1986) 'Laws' for lips, Speech Communication, 5, 97-104. Abry, C., Boe , L. J. & Descout , R. (1979) Voyelles /abiales et voyelles /abialisees en francais. 9th Int. Cong. Phon. Sc., Copenhagen, p. 177. Abry, C ., Boe, L. J. & Schwartz, J. L. (1989) Plateaus , catastrophes and the structuring of vowel systems, Journal of Phonetics, 17, 47-54. Abry, C., Boe , L. J., Corsi, P., Descout , R., Gentil, M. & Graillot , P. (1980) Labialite et Phonetique. Donnees fondamentales et etudes experimentales sur Ia geometrie et Ia motricite labiales. Institut de Phonetique de Grenoble, 304 p. Bolla , K. (1989) A phonetic conspectus of English. The articulatory and acoustic features of British English speech sounds , Magyar Fonetikai Fiizetek, Hungarian Papers in Phonetics, 20, 402. Bonnot, J. F. & Bothorel , A. 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(1989) Articulation compensatoire des voyelles : ana lyse de donnees cineradiographiqucs avec un modele lineaire. In Melanges de Phonetique generate et experimentale offerts ii Pela Simon. pp. 545-562. Strasbourg: Institut de Phonetique. Majid , R., Abry, C. , Boe , L. J . & Perrier, P. (1987) Contribution ii La classification articulatori-acoustique des voyelles: etude des macro-sensibilites ii !'aide d'un modele articu/atoire. 11th Int. Con g. Phon. Sc., Tallin, USSR. Rossi , M. (1983) Niveaux de !'analyse phonetique: nature et structuration des indices et des traits , Speech Communication, 2, 91-106. Straka, G. (1989) Base articulatoire. Essai d'une mise au point. In Melanges de Phonetique generate et experimentale offerts ii Nla Simon. pp. 757-768. Strasbourg: Institut de Phonetique. Tseva, A. (1990) L'arrondissement dans !'identification visuelle des voyelles du fran<;ais. Premiers acquis, Bull. LCP Grenoble, 3, 149-186. Zerling, J.P. (1980) Coarticu/ation /abiale et aire aux levres dans des groupes occlusives-voyelles en fran{:ais . Seminaire International Labialite , GALF, Lannion , February 1980. Zerling , J. P. (1984) Phenomenes de n ~.sa lit e et de nasalisation vocaliques : etude cineradiographique pour deux locuteurs, TIPS: Travaux de l'Institut de Phonetique de Strasbourg , 16 , 241-266.
1-l
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Zerling, J.P. (1989a) Les trois degres de labialisation des voyelles isolees en franc;ais. Etude pour 105 locuteurs. In Melanges de Phonetique Generale et Experimentale offerts ii P€/a Simon. pp. 807-831. Strasbourg: Institut de Phonetique. Zerling, 1. P. (1989b) Strategies labiales vocaliques en franrais. Variabilite des parametres frontaux. pp. 116-119. Seminaire Variabilite et specificite des locuteurs , SFA-GCP, Marseille , 20-21 June. Zerling, J.P. (1989c) The three degrees of labialization of the French steady-state vowels. A study for 105 speakers. Eurospeech 89. pp. 445-448. European Conference on Speech Communication and Technology, Paris , 26-28 September. Zerling, 1. P. (1990) Aspects articulatoires de Ia labia/it€ vocalique en franrais . Contribution ii Ia modelisation ii partir de labiophotographies, labiofilms, etfilms radiologiques. These d'Etat, Universite de Strasbourg II. Zerling, J.P. (1991) Labia/it€ vocalique: Etude comparee des types, degres et strategies articulatoires de plusieurs langues, vol. 3, pp. 46-49, 12eme Congres International des Sciences Phonetiques, Aix-en-Provence , 19-24 August.