If a child either leaves a sound out or replaces it by a different sound or replaces two sounds by a different sound, is this the most that can be said about the issue? From a linguistic perspective: No.
Phonemes and features
It is sometimes suggested (mistakenly in my view) that the smallest unit relevant to the speech of children whose speech is unclear is the ‘speech sound’ or ‘phoneme’. On this basis, children are said to often replace one phoneme by another, such as saying sea as TEA, replacing the S by a T. But in terms of a 1968 idea from Chomsky and Halle, unknowingly updating the 1669 thinking of William Holder, saying sea as TEA is not so much replacing one phoneme by another as changing a feature.
In the case of sea as TEA, the critical feature involves the continuity or ‘continuance’ of the airstream at the tip of the tongue. For S the airstream is allowed to pass through a narrow gap just behind the upper front teeth – at least for most speakers. For T there is a complete, but momentary, obstruction at the same point. So the replacement of S by a T sound is generally known as ‘stopping’.
It seems to me that a feature-changing analysis is better than a phoneme-changing analysis for three reasons. First children who say sea as TEA, often also say four as POUR, reversing the continuous airstream action both by the lips and by the tongue tip. Second, exactly the same thing has happened in the history of English, although in the opposite direction. So in English we have father. In modern Greek the word is as pateras – more like the Classical Latin pater 2,000 years ago and the ancestral Proto-Indo-European form from 6,000 years ago. Both at the lips and the tongue tip, English now narrows the airstream at both points, instead of blocking it completely, as the ancestral language did. Third, in terms of theory, a changing the air steam analysis avoids the repetition by the phoneme analysis. It is thus more parsimonious.
The airstream and other details
It seems to me that for the purposes of clinical linguistics with children learning English the following features are particularly relevant:
• Whether, most fundamentally, the role of the sound in the syllable is as its ‘nucleus’, typically the vowel, or whether it is a consonant.
• The continuance of the airstream (distinguishing T from S) – whether the airstream is continuous or not, where sounds like T are generally characterised as ‘stops’ because of the totality of the closure, and sounds like S are generally characterised as ‘fricatives’ because of the friction;
• The place of any constriction – whether the airstream is ‘stopped’ or ‘bottle-necked’ at the lips, or with the tip of the tongue, or the back of the tongue (distinguishing T from P and K);
• Whether the airstream is initially stopped and then just partially released as in the cases of the initial sounds in chore and jaw;
• The relative timing of any involvement of the vocal folds (distinguishing ‘voiceless’ or ‘unvoiced’ T from ‘voiced’ D, P from B, S from Z, CH from J);
• In the cases of voiceless stops, whether the delay in the voicing is increased by what is known as ‘aspiration’, as in pea, tea, and key in English, but not where the stop follows S, as in spy, sty, and sky.
• Whether the airstream passes through the nose (distinguishing N from D); Nasals, as these phonemes are known, are quite often copied or ‘reduplicated’, as in skeleton, pronounced by many children as SKELINTON. Something similar seems to have happened in my family name, becoming Nunes from an original YUNIS (or Eunice) as people with that name traveled from Turkey or Kurdistan all the way round North Africa to Spain and Portugal.
• Whether the main effect is to constrict the airway or to resonate, with this resonance – or ‘sonority’ characteristic of L, R, N, M, W, Y);
• In the case of fricatives, whether the acoustic noise falls below a given frequency, as it does in the case of TH (distinguishing TH from S, F, and SH).
• In the cases of S and SH (both with the tongue completely inside the mouth) S, unlike SH, makes the constriction with the tip or apex of the tongue.
• In the case of vowels, where the tongue is in the mouth – up at the top, down at the bottom, or in the middle, towards the front of the mouth or at the back, whether the tongue is tensed and as close as possible to the edge of this space, making the vowel long, whether it moves from one position to another, as in the case of the diphthongs in high and how, whether the lips are rounded as in rue and raw, or not as in hay and high.
The case for features
About these other features, whether they are ‘or’ or ‘off’ or whether there is only one possible value, or whether there are more than two values, and thus how many features there are, there is ongoing debate. But as far as I am concerned, the evidence for distinctive features is so clear and so well demonstrated that it is not worth arguing about. The idea was first proposed over 350 years ago in the context of speech pathology. In the 19th century, it was developed and refined both in the light of data from a large number of languages, starting with the history of German and English by the pathbreaking scholarship of Jacob Grimm, otherwise known as the collector of fairy tales. and then by more data from speech pathology and clinical linguistics where it is critically relevant with issues often arising with respect to all the features exampled here.
In the case of Russian, there are phonemes similar to English T and D, but articulated further forward in the mouth. In Irish English, and the French of Northern and Southern France, there are equivalent phonemes with other differences. And in French, Russian, and Spanish, there are variations with respect to a phoneme similar to, but not the same as, English R. How these differences should be represented is not well-agreed or obvious. But I propose a way of doing this by a new understanding of a function in the system of speech which I think should be called Glue.