co articulation

7/29/2019 Co Articulation

1/13

Coarticulation and Assimilation

Robert Mannell

Macquarie University, 2008

Active and Passive Articulators

Active articulators are those moveable parts of the vocal tract thatparticipate in the production of speech. They do this by moving towardsor away from passive articulators (other active articulators).

Passive articulators are relatively stationary parts of the vocal tract.

Gesture and Constriction

The goal of active articulator gestures is to produce certain degrees ofstricture (opening / closure) at certain points in the vocal tract.

A speech gesture is the coordinated movement of one or morearticulators to achieve a desired constriction.

Constriction involves the following degrees of stricture:-

Vowel minimal strictureApproximant medium degree of stricture

Fricative open but high degree of stricture

Stop total stricture

Articulatory Phonology

Articulatory Phonology (Browman and Goldstein, 1992) is aphonological theory that regards gestures as the basic units ofphonological contrast. This system of gestures is based on constrictions

involving the lips, tongue tip, tongue body, velum and glottis.

Articulatory Phonology describes 8 tract variables:-

1 LP lip protrusion

2 LA lip aperture

3 TTCL tongue tip constriction location

4 TTCD tongue tip constriction degree

5 TBCL tongue body constriction location

6 TBCD tongue body constriction degree

7 VEL velic aperture8 GLO glottal aperture


2/13

Lip protrusion (LP) accounts for rounding and spreading whilst lipaperture (LA) accounts for differences in degree of constriction (eg.vowel rounding [u] or spreading [i], approximant [w], fricative [v] andstop [b] articulations). Three articulators are involved in LP and LA:upper lips (mostly for protrusion), lower lips (protrusion and stricture),jaw (assists stricture).

The tongue tip can move partly independently of the tongue body.Three articulators are involved in tongue tip constriction location (TTCL)and degree (TTCD). They are the tongue tip, tongue body (tongue bodymovement can assist tongue tip placement), and jaw (its easier toreach the roof of the mouth if the jaw isn't too low).

Tongue body constriction degree (TBCD) and location (TBCL) isimportant for all vowels, and for palatal, velar, uvular and pharyngealconsonant place of articulation (for approximants, fricatives and stops).

Articulators involved in TBCD and TBCL are the tongue body and thejaw (which assists in raising the tongue for some articulations).

Velic aperture (VEL) must not be confused with tongue bodyarticulations (TBCL and TBCD) in which the velum acts as a passivearticulator for a tongue gesture. VEL refers to velum opening andclosing. The velum must be open for nasal stops, must be closed fororal stops and fricatives, and its opening for other sounds is languagedependent.

Glottal aperture (GLO) relates to voicing. GLO is closed for normalvoicing, and partly open (slightly open at the back of the glottis) forbreathy voicing. GLO is wide open for voiceless stops and voicelessfricatives (excluding the glottal stop closure and the glottal /h/fricative). GLO opening and closing is subject to complex languagespecific timing patterns in oral stops. This relates to differences in voiceonset time (VOT). GLO is closed for ejectives and implosives and is alsosubject to important timing constraints in these sounds.

Gestures overlap in an utterance. The relative timing of gestures can beextremely important.

Differing patterns of overlapping gestures distinguish phonemes,syllables and larger speech units (eg. polysyllabic words, phrases).

The Syllable

Two major theories of speech perception regard the syllable as theminimal unit of speech perception. In these theories, speech perceptionis the perception of gestures. These theories are:-

Motor theory of speech perception (Liberman, Mattingly andTurvey, 1967)


3/13

Direct-realist theory of speech perception (eg. Fowler, 1986,Best, 1995)

There is a very large, and growing, body of research that suggests thatthe syllable is the most basic unit of articulatory planning in the brain.

Gestures interact with each other to a greater extent within syllableboundaries than they do across syllable boundaries.

Phonemes in Connected Speech

Phonemes rarely occur in isolation. In English this only occurs in thecitation (isolated word) utterance of the words "I"/"eye"/"aye","owe"/"oh" and "a" (also "air", "ear", "are", "oar"/"or"/"ore", "err" innon-rhotic dialects such as Australian English) plus the interjections"eh" and "oi"/"oy" (note that they are all long vowels or diphthongs).

Phonemes are articulated as part of a syllable and each syllable isnormally part of a longer sequence of speech. The majority of syllablesconsist of more than one phoneme and most syllables are made up ofone vowel and one or more consonants. See the topic "The syllable andthe foot" for more information on syllables.

Articulatory Targets

In simple models of articulatory planning, each phoneme has a single

ideal articulatory target for each contrastive articulator. At the mostabstract level of motor planning each articulator might be thought of ashaving a target position that it must try to achieve for each phoneme.That is the idealised target for a phoneme is invariant. Differences inactual articulations occur as a consequence of physics and timing.

In the following diagram we can see two ideal (and entirely abstract)targets for a particular articulator in two adjacent phonemes. The bluelines represent idealised phoneme boundaries.

Figure 1: Two ideal articulatory targets for a single articulator and two adjacent phonemes.PB1, PB2 and PB3 are the idealised phoneme boundaries for Phoneme 1 and Phoneme 2.


4/13

There are no readily definable articulatory or acoustic boundariesbetween phonemes in continuous speech, except at certain phraseboundaries which are characterised by pauses.

When we talk about boundaries between phonemes we are onlyreferring to an approximate boundary at some point in the transition

between the two phonemes. True acoustic and articulatory boundariesbetween phonemes don't exist. What we are referring to when we talkabout (or plot) phoneme boundaries is a point between two phonemetargets where the two phonemes contribute approximately equally tothe articulatory or acoustic pattern.

The cortical centres that plan articulatory movements during speechproduction can be understood as producing a series of increasinglymore concrete specifications for articulator muscle movement. At themost abstract level of motor planning each articulator might be thought

of as having a target position that it must attempt to achieve for eachphoneme. Such a target would be the ideal target for each phonemeand would depend upon an individual's physiology and linguisticexperience. This model of articulatory planning assumes a fixed idealarticulatory target for each contrastive articulator.

Articulatory Transitions

Phonemes are best described in terms of target positions witharticulatory (and resulting acoustic) transitions between the two

targets. These transitions share the characteristics of the two targets.

A "transition" in speech is caused by the movement of articulatorsbetween phoneme targets. Phonemes can be defined in terms of one ormore idealised articulatory targets. For example, a monophthong vowelis defined as having one target whilst a diphthong vowel is defined ashaving two targets. There are transitions between the targets of twoadjacent phonemes as well as between the multiple targets within asingle phoneme.

Figure 2: An idealised articulatory transition from the target of one phoneme (T1) to thetarget of another phoneme (T2). PB represents the approximate position of the phoneme"boundary".


5/13

The transition between two phonemes shares the articulatory andacoustic characteristics of both phonemes but gradually changes frombeing predominantly like the first phoneme target to predominantly likethe second phoneme target.

Contrastive Articulators

A contrastive articulator is an articulator whose configuration (shape,constriction) or position (place) is important to the accuratetransmission of the intended phoneme to a listener. The listener ideallyshares the same language or dialect and therefore the sameexpectations for the articulatory, acoustic and auditory characteristicsof each phoneme.

Which articulators are contrastive may vary from phoneme tophoneme. An articulator that is contrastive in one language for a

certain class of sounds may not be contrastive for another language.

Velum opening is always contrastive for oral stops (closed) and nasalstops (open), in all languages that have these classes of sound.

Velum opening is contrastive for certain (but not all) vowels in French(distinct nasal and non-nasal vowel phonemes), but is not contrastivefor English vowels (nasality does not change vowel phonemes).

For the vast majority of languages (possibly only one exception), the

position of the tongue body in the front-back and the high-lowdimensions are contrastive for vowels and so the tongue bodyconstriction location (TBCL) is a contrastive for vowels.

In English the configuration of the lips (rounded, neutral, spread) is notcontrastive for vowels (no pair of vowels is contrasted solely on thebasis of lip shape) and so the lips are not contrastive articulators forEnglish vowels (but are for certain pairs of French vowels). This lack ofcontrastiveness does not mean that there are not characteristic lipshapes for English vowels but such lip shapes are redundant as they

can be predicted from tongue position. That is, in English high frontvowels are always lip spread, high back vowels are always rounded andlow vowels tend to have a neutral lip shape.

When an articulator is not contrastive in a phoneme that is surroundedby two other phonemes for which that articulator is contrastive thenthe articulator is free to move slowly (or not at all) within that centralphoneme. A good example in English is nasality, or velum opening. InEnglish velum opening is contrastive for both oral stops (must beclosed) and for nasal stops (must be open) but it is not contrastive forvowels. Vowels are identified as the same phoneme regardless ofwhether the velum is open or closed (but recall that this is not true forlanguages such as French). A vowel between two oral stops has a


6/13

closed velum whilst a vowel between two nasal stops has a significantlyopen velum and the vowel is said to be nasalised.

Articulator Inertia

Inertia is the tendency of a stationary body to resist movement or for amoving body to resist changes in its rate or direction of movement. Themore massive (ie. bigger and heavier) an articulator is the greater itsinertia or resistance to movement. The tongue body is more massivethan the tongue tip and so it moves more slowly than the tongue tip.The jaw is even more massive and so it moves even more slowly.

Figure 3: Two articulators with different degrees of inertia will take a different times to move

the same distance. Articulator 1 (A1) has less inertia than articulator 2 (A2). Therefore,articulator 1 (A1) can move from its first target (T1) to its second target (T2,1) in much less

time than articulator 2 (A2) can move the same distance to its target 2 (T2,2).

The maximum distance that an articulator can move in a given timedepends upon its inertia.

Figure 4: Two articulators with different degrees of inertia can move different maximum

distances in the same time. Articulator 1 (A1) has less inertia than articulator 2 (A2).Therefore, articulator 1 (A1) can move a greater distance from its first target (T1) to its second

target (T2) than can articulator 2 (A2).


7/13

Articulator Agility - Motor units and density ofinnervation

Articulators with low inertia, such as the tongue tip, can bemanoeuvered rapidly. Another variable that affects articulator agility is

the extent to which it is controlled neurally. An articulator with a largerproportion of nerve fibres per unit mass has fewer muscle fibres pernerve fibre. A nerve fibre and its associated muscle fibres is known as amotor unit and each motor unit can potentially be controlledindividually. The larger the number of motor units in an articulator themore controllable its movement is. The tongue tip and the velum havesimilar mass but the tongue tip has smaller and more numerous motorunits. This means that the tongue tip is more manoeuvrable than thevelum in spite of their similar mass.

The high degree of innervation and the larger number of motor units inthe tongue tip doesn't mean that as a whole it moves faster. This islimited by inertia and therefore by its overall mass. The greater degreeof innervation does, however, make it possible to semi-independentlymove sub-structures in the tongue tip (eg. to permit such things astongue tip grooving). These sub-structures are limited in theirindependent movement by their elastic linkage to the rest of the tonguetip, but being sub-structures (rather than the whole structure) of thetongue tip they have lower mass and therefore they have less inertiathan the whole structure.

Target Undershoot

Target undershoot occurs when there is insufficient time for anarticulator to reach its target position. The articulator has too muchinertia to move between two targets in the available time so it doesn'ttravel as far as it would if there was more time. Target undershoot canoccur in both vowels and consonants.

Figure 5: This figure illustrates target undershoot. In this example the ideal target of phoneme

2 (T2) is represented by the articulator position value IT whilst UT represents the undershottarget value. This would be typical of an unaccented short low vowel between two alveolar or

velar stops, for example.


8/13

Overlapping Gestures

A "gesture" is the movement of one articulator, such as the lower lip,the tongue tip or the tongue body, from an articulatory positioncharacteristic of one speech sound to an articulatory positioncharacteristic of the next speech sound.

Articulatory gestures overlap. This means that the movement of eacharticulator occurs at the same time as the movements of all the otherarticulators. These overlapping gestures may not be synchronisedbecause of sequencing requirements, articulator contrastiveness andarticulator inertia.

Figure 6: The gestures of three articulators (A1, A2, A3) overlap in time as they pass through

three idealised phoneme targets (T1, T2, T3).

Articulatory overlap is the basis of coarticulation.

Coarticulation and Syllables

Coarticulation tends to be stronger within syllables rather than acrosssyllable boundaries. This greater coarticulation within syllables isevidence for the cognitive existence of the syllable as a fundamentalunit of articulatory organisation.

Greater degrees of coarticulation between the phonemes in a syllableincrease the perceptual integration of syllables. That is, greater degreesof coarticulation increase the perception that the phonemes in a syllableare connected.

We perceive speech by recognising the (auditorily-transformed)acoustic patterns of syllables. These patterns consist of targets (orrather the effect of coarticulation on the realisation of those targets)and the transitions between targets.


9/13

Coarticulation between Consonants and Vowels

Vowels affect the articulation of adjacent consonants (and adjacent

vowels). Consonants affect the articulation of adjacent vowels (andother adjacent consonants).

Some sounds are more resistant to coarticulation than other sounds.This may be due to differences in phoneme duration, differences in theinertia of contrastive articulators, differences in articulator movementdistances or it may be due to the phoneme inventory effect (seebelow).

Coarticulation is greatest (for a given phoneme duration) when there is

the greatest articulator movement (ie. greatest distance) betweenphonemes.

Many consonants have a high tongue position (eg. [k]) and so they aremore likely to cause target undershoot in low vowels than in highvowels. This is because there is a longer distance to travel from a highconsonant tongue position to a low vowel tongue position than there isto a high vowel tongue position.

Long vowels are more resistant to target undershoot than are shortvowels as there is more time for the articulator to reach its target.

Accented and stressed vowels are more resistant to undershoot thanare unstressed vowels as they are even longer.

In other words (when adjacent to consonants with high tongue bodyarticulations):-

Accented high long monophthongal vowels are the most resistantvowels to target undershoot.

Unstressed low vowels and particularly unaccented short lowvowels are most affected by target undershoot.

Schwa is not realised by a defined target. It is usually very short and istherefore greatly affected by coarticulation with adjacent consonantsbut since it doesn't have a defined target it can't be said to have targetundershoot.

Phoneme Inventory Effects

Coarticulation is resisted (reduced in degree) when it will result inperceptual confusion. The chances of perceptual confusion occurring is

greatest in languages that have a large number of phonemes of aparticular class. The greater the number of members of a class of


10/13

speech sounds the greater the chance that adjacent phonemes will beclose together.

Vowels coarticulate most in languages with a small number of vowels.Some languages have three (or even fewer) phonological vowels. Vowelfronting and height is free to vary considerably in such languages

without a vowel phoneme being confused for an adjacent phoneme.

Consonants coarticulate most in languages with a small number ofplaces of articulation. For example, in English there are only threeplaces of oral stop articulation and only one place of articulation thatuses the tongue body. Vowel articulations are also tongue bodyarticulations. The velar stop consonants /k,g, / in English can vary inplace of articulation (when adjacent to front, central and back vowels)between palatal, velar and uvular without being confused for anotheroral stop phoneme. Australian aboriginal languages, with both a palatal

and a velar oral stop, can't afford to have velar stop phonemes movingtoo far forward as they would be confused with the palatal stopphoneme. Arabic, with both a uvular and a velar oral stop, can't affordto have velar stop phonemes moving too far back as they would beconfused with the uvular stop phoneme.

Coarticulation and Assimilation

Coarticulation is the way in which the movements of differentarticulators affect each other and the ways in which preceding and

following articulations of an individual articulator affect its currentarticulation. Articulatory planning in the brain takes these affects intoaccount when producing a sequence of speech sounds. Some of theseeffects are very slight whilst some are quite strong.

Coarticulation ALWAYS occurs in ALL languages for ALL sequences ofsounds not separated by pauses. Speakers have no choice - they MUSTcoarticulate adjacent sounds. Without appropriate coarticulation (eg. inpoor synthetic speech) the resulting speech sounds unnatural and ishard to understand.

Two opposing forces are at work in speech production and perception.(eg. Boersma, 1998)

There is a tendency to simplify speech patterns to increase easeof articulation as simpler speech is easier to produce.

This is opposed by the competing need ("constraint") to maintainphonological distinctiveness in speech perception.

One of the reasons why we accent certain words (eg. words containingnew information) is so that we can increase their duration, andtherefore can avoid undershoot (ie. more time to reach targets).


11/13

Accented words are the words for which we most need tomaintain maximum perceptual distinctiveness as we can't rely oncontext to perceive them. (constraints maximised)

We tend to not accent words representing given information. Wedon't require maximum perceptual distinctiveness as wordidentification is assisted by context. (constraints reduced)

Function words carry very little of the semantic load and so theyare often reduced as there is no need to maintain their perceptualdistinctiveness. (constraints almost absent)

These patterns of accenting certain words (new information) andreducing the other words (given information) in a sentence, provides apattern of timing that results in the greatest time to reach articulatorytargets in accented words and much greater chance of undershoot forother words (especially function words).

If we always attempted to maximise phonological distinctiveness inspeech perception (without the opposing tendency towards ease ofproduction) then we could perhaps predict the degree to which targetsare achieved or undershot from articulator inertia and the timing ofeach syllable in each accented or unaccented word. However, this isn'talways possible.

We vary in the extent to which we maximise distinctiveness from onephonetic context to another. This variation may be language, dialect,local speech community and individual specific. That is, there are

different constraints on the extent to which we can relax perceptualdistinctiveness and increase ease of articulation. Assimilation is alanguage (or speech community) specific and also a phonetic contextspecific relaxation of these constraints. Assimilation constraints mayallow some allophones in certain contexts but disallow other allophonesin other contexts.

In some cases it may even be possible to increase ease of articulationto a point where a sound assimilates to such a great extent that itbecomes more like another phoneme.

Examples:-

In Australian English, alveolar consonants (especially alveolar oraland nasal stops) readily assimilate to the place of articulation of afollowing consonant.

Sometimes this results in an allophonic change (eg. to dental)and sometimes a phonemic change (eg. to velar or bilabial)

This change in Australian English alveolar stops (particularly thephonemic assimilations) also happens in other, but not all, English

dialects. It isn't permitted in many other languages.


12/13

The extent or degree of such assimilations may vary between discoursecontexts (eg. formal vs. informal)

Even where such a relaxation of constraints is permitted, we often finddifferent degrees of assimilation for different speakers or differentgroups of speakers. (eg. some speakers of Australian English exhibit

greater degrees of contextual nasality than do others).

Careful physiological analysis of alveolar to bilabial or velar phonemicassimilations appears to indicate that some aspect of the alveolararticulation is still present. eg. in alveolar to bilabial assimilation thereis a clear primary bilabial articulation but often a secondary tongue tipgesture can also be detected (so the alveolar feature is still present)

Assimilation is a language (or dialect or speech community) specificrelaxation in constraints that maintain a certain degree of balance

between perceptual distinctiveness and ease of articulation. In otherwords, assimilation could be regarded as a language (or dialect)specific enhancement of the degree of coarticulation.

In the topic "Phonemic (Broad) Transcription of Australian English"there are numerous examples of assimilation where the identity of thephoneme is changed by adopting an articulatory strategy that simplifiesarticulatory effort. Most of these examples involve the conversion of analveolar consonant (oral stop or nasal stop or fricative) into the sameplace of articulation as the following consonant (in these cases alveolar

oral and nasal stops become bilabial or velar and alveolar fricativesbecome palato-alveolar fricatives). There is not a clear case forconsidering this to be driven strongly by coarticulation as the tongue tipis the most rapidly moveable of all articulators. The reverse seems tobe the case. In order to prevent the adjacent sound from being affectedby coarticulation (ie. slower lip or velar movements) the interveningalveolar phoneme is substituted for a homorganic phoneme. This seemsto be an articulatory or phonological choice made in either the higherlevels of articulatory planning or at the even more abstract level ofphonological specification.

On the other hand, in the topic "Phonetic (Narrow) Transcription ofAustralian English" oral and nasal stops are assimilated to the place ofarticulation of the following labiodental, dental or postalveolarconsonants. This can't occur at the level of phonological specification asthere is no change of phoneme involved so this suggests a habitualchoice made at the higher levels of articulatory planning. It seems bestto treat these two types of assimilation (those that change phonemeand those that don't) as being equivalent and therefore not specified ata phonological level but at an articulatory planning level. The fact thatthis is possible in English without confusion is that there are very few

contexts where this would create ambiguity. Such a processes areprohibited in Australian Aboriginal languages which have up to sixplaces of stop articulation. This process tends to be language-specific.


13/13

Further Reading

Clark, J., Yallop, C. & Fletcher, J. (2007), An introduction to phonetics

and phonology, 3rd. edition, Blackwell, Oxford (pages 84-90 andSection 7.17)

Note: Unfortunately, this reference (and many other references oncoarticulation) is expressed in terms of speech acoustics. If you skipover the references to acoustic properties you should still be able tomake sense of much of what is written here.

A useful web site

http://www.ling.lu.se/persons/Sidney/coartdem/

References

Referred to in the notes, but not required reading:-

Best, C. (1995), "A direct realist view of cross-language speechperception", in Strange, W. (ed.) Speech Perception and LinguisticExperience: Issues in cross-language research, Maryland: York Press.

Boersma, P. (1998), Functional Phonology: Formalizing the interactionsbetween articulatory and perceptual drives, PhD dissertation, Universityof Amsterdam.

Browman, C., and Goldstein, L. (1992), Articulatory Phonology: Anoverview, Phonetica, 49, 155-180.

Fowler, C. (1986), "An event approach to the study of speechperception from a direct-realist perspective",Journal of Phonetics,14:3-28.

Liberman, A.M., Mattingly, I.G., & Turvey, M.T. (1967), "Languagecodes and memory codes", in Coding processes in Human Memory, eds.Melton, A.W., & Martin, E., Washington: V.H. Winston

Liberman, A.M. and Mattingly, I.G. (1985), "The motor theory ofspeech perception revised", Cognition, 21:1-36.

co articulation

Documents