Articulatory dynamics explained
Connected speech involves rapid, coordinated, sequential movements of the articulatory mechanism over extended periods of time. Our ability to move the articulators with precision in connected speech is influenced by two main factors:
- biomechanical performance of the articulatory mechanism
- neuromuscular control mechanism
Naturally, there are limits to the speed at which the articulators can move. The rate is influenced by their mass and size. Any massive object has a property known as inertia. That is to say, a massive object will resist being set in motion. The articulators, therefore, will similarly resist being set in motion and there will consequently be a delay between the nerve impulse that initiates a movement and the actual performance of a particular articulatory maneuver.
We have already considered an example of the effect of delay when we examined the process of nasalization in creating allophones of vowels. Recall that the vowel /ɑ/ in the word mart /mɑt/ is nasalized, i.e. [mɑ̃tʰ]. The reason for this is delay in articulatory movement. Let us explain. We know that the soft palate is lowered during the production of nasal consonants, creating the possibility of air escaping through either the mouth or the nose. However, because the two lips form a complete obstruction in the oral cavity to produce /m/, the air cannot escape through the mouth. Consequently, it passes through the nasal cavity and out of the nose. This describes the activity of the articulators when producing the sound /m/ in isolation. Of course, the word mart has three speech sounds that must be articulated in rapid succession. There must be coordinated effort to yield smooth transitions between each of the three sounds /m/, /ɑ/ and /t/. Owing to natural inertia, which resists setting the articulators in motion, there is a slight delay in raising the soft palate after the /m/ has been articulated. [The palate needs to be raised to produce both the vowel /ɑ/ and the consonant /t/, as these are both oral sounds.] The nasality of the /m/, therefore, overlaps onto the following vowel. This creates some spillage, as a small amount of air continues to escape through the nose after the articulation of the vowel has been initiated. Eventually the system catches up with itself and the soft palate is fully raised by the time the final /t/ is being articulated. Similarly, the diphthong /aʊ/ is nasalized in the word now /naʊ/ owing, again, to an inherent delay in raising the velum after the initial /n/ is articulated, i.e. /naʊ/ → [naʊ̃ː].
Both of the above examples are, once more, of words in isolation. As one might imagine, however, the effects of biomechanical performance are compounded in connected speech, where a whole series of movements must be initiated. Each time an articulator is set in motion there is always the initial inertia to be overcome.
The effect of the neuromuscular control system is the opposite of the articulatory delay that is induced by the biomechanical features of the articulatory mechanism. It appears that, in order to compensate for the intrinsic delays in initiating articulatory movements, the neuromuscular commands that control them may be initiated in advance of them being required. The articulatory characteristics of the target articulation may then appear on an earlier segment of speech.
Consider again the process of nasalization in creating allophones of vowels. We have already seen how the nasalization of vowels that follow nasal consonants may be explained by articulatory inertia (e.g. mart /mɑt/ → [mɑ̃tʰ] owing to a delay in raising the soft palate). Recall, however, that vowels that precede a nasal consonant may also be nasalized. For example, the vowel /æ/ in the name Anne /æn/ may be nasalized, i.e. [æ̃n]. The explanation for this lies in the advanced triggering of neuromuscular commands. We know that all vowels, when spoken in isolation, are produced with the soft palate lowered: they are oral sounds. In contrast, nasals are produced with the soft palate raised. So, in anticipation of the upcoming nasal /n/, the neuromuscular command to lower the soft palate is initiated in advance of it being required in an effort to overcome its resistance to being moved. Consequently, the nasality of /n/ appears on the earlier /æ/ segment, thereby nasalising the vowel. A similar argument may be applied to the word wing /wɪŋ/, which is typically realized as [wɪ̃ŋ]. Again, anticipating the upcoming nasal /ŋ/, and in an effort to compensate for articulatory inertia the neuromuscular command to lower the soft palate is initiated in advance of it being required. Consequently, the characteristic nasality of the consonant overlaps onto the immediately prior /ɪ/ vowel.
Again, both of these examples relate to words spoken in isolation. However, the principle of advanced initiation of articulatory movement applies in a similar same way to connected speech. For example, as the final sound in one word is being completed, the neuromuscular control mechanism may already have initiated a signal in anticipation of the upcoming first sound in the immediately following word. Therefore, the effects of the neuromuscular control mechanism will extend from one word to another in connected speech, in just the same way as the effects of biomechanical performance.