II. Functions of phonemes.
The opposition of phonemes serves to distinguish the meaning of the whole phrase as well. For example, He was heard badly and He was hurt badly.
Thus we may say that the phoneme can fulfill the distinctive function. Secondly, it is material, real and objective. It is realized in speech of all English-speaking people in the form of speech sounds, that is the allophones belonging to the same phoneme are not identical in their articulatory content through there remains some phonetic similarity between them. The allophones which do not undergo any distinguishable changes in the chain of speech (dark, done, dot, etc.) are called principal. There are quite predictable changes in the articulation of allophones that occur under the influence of the neighbouring sounds (God thanks, riddle, etc) in different phonetic situations. Such phonemes are called subsidiary. The examples below will show the articulatory modifications of the phoneme [d] in various phonetic contexts:
[d] is slightly palatalized before front vowels, deal, did;
[d] is pronounced without any plosion before another stop, bedtime, bad dog, good pat;
[d] is pronounced with the nasal plosion before nasal sonorants [n] and [m], garden, admire, could not, etc.;
The plosion is lateral before the lateral sonorant [l], middle, rapidly, good luck, etc.;
[d] followed by [r] becomes post-alveolar, dry, drive, etc.;
when [d] is followed by the labial [w] it becomes labialized, dweller, etc.;
[d] followed by the interdental [T] and [D] it becomes dental, bad things,
read the text, etc.;
in the initial position [d] is partially devoiced, doctor, deep, etc.;
in the intervocalic position or when followed by a sonorant [d] is fully
voiced, leader, drip;
in the word-final position it is voiceless, road, caused, etc.
Thus, allophones of the same phoneme never occur in similar phonetic contexts, they are quite predictable according to the phonetic environment, but they cannot differentiate meanings.
III. The place of allophones in the sound system.
The
phones are realized in speech, though they do not correspond exactly
to the allophone predicted by this or that phonetic environment. They
are modified by phonostylistic, dialectal and individual factors. In
fact, no speech sounds are absolutely alike.
phonostylistic
variation
phoneme → allophone → dialectal → phone
variation
individual
variation
Allophones of the same phoneme function as the same linguistic unit. The phoneme is an abstract linguistic unit; it is an abstraction from actual speech sounds, that is allophonic modifications. The functionally relevant bundle of articulatory features is called the invariant of the phoneme. For example,
[d] – occlusive, forelingual and lenis characteristics of this phoneme are generalized in the mind of the speaker into what is called the invariant of this phoneme. The phoneme is objective real, it exists in the material form of speech sounds, its allophones. All the allophones of the same phoneme have some articulatory features in common, that is all of them possess the same invariant. Simultaneously each allophone possesses quite particular phonetic features which may not be traced in the articulation of other allophones of the same phoneme. Any change in the invariant of the phoneme affects the meaning. If an allophone of some phoneme is replaced by an allophone of a different phoneme the mistake called phonological, because the meaning of the word is inevitably affected. It happens when one or more relevant features of the phoneme are not realized.
For example, when the vowel [i:] in the word weak becomes slightly more open, more advanced or is no longer diphthongized the word weak may be perceived as quite different word wick. It is perfectly clear that this type of mistakes is not admitted in teaching pronunciation to any type of language learner.
If an allophone of the phoneme is replaced by another allophone of the same phoneme the mistake is called phonetic. It happens when the invariant of the phoneme is not modified and consequently the meaning of the word is not affected.
For example, when the vowel [i:] is fully long in such a word as sheep the quality of it remaining the same, the meaning of the word does not change.
Language learners are advised not to let phonetic mistakes into their pronunciation. If they do make them the degree of their foreign accent will certainly be an obstacle to the listener’s perception.