Unlike the other consonants the two sounds /w/ and /j/ offer no restriction to the flow of air out through the mouth. In this they are quite vowel like, and indeed they are usually classed as semi-vowels. It is worth exploring this from the inside to understand their role both as consonants and as naturally occurring intrusive linking sounds. These two sounds play a key role in smooth connected-up speech.
Say the words we and west. Do this quite slowly, several times until you can notice these four things: 1) Your lips are not static during the sound /w/ but are in movement; 2) The movement begins with your lips forward and rounded, proceeding smoothly into the position of the following vowel; 3) The sound /w/ is voiced; 4) There is no restriction to the flow of air through your mouth during the sound /w/. In fact the movement of /w/ is rather similar to a glide between two vowels, like a diphthong.
With that in mind say this diphthong /ʊǝ/ as in pure and cure. Say the diphthong on its own as a normal glide between /ʊ/ and /ǝ/. And now say the diphthong again but with a stronger lip movement. This stronger movement of the lips changes the sound enough to make the difference between /ʊǝ/ and /wǝ/, and to turn the diphthong into a consonant. And because /w/ is so closely related to a vowel glide it is often referred to as a semi-vowel, and in this case as a bilabial semi-vowel, due to the emphatic movement of the two lips at the beginning. A similar thing happens in the case of /ɪǝ/
Say the words yes and young quite slowly and notice these four things: 1) Your tongue is not static during the sound /j/ but is in movement; 2) The movement begins with your tongue in an /ɪ/ position proceeding smoothly into the position of the following vowel; 3) The sound /j/ is voiced; 4) There is no restriction to the flow of air during the sound /j/. Once again we see that the movement of /j/ is rather like a glide between two vowels, like a diphthong. But in this case a different diphthong.
Say the diphthong /ɪǝ/as in ear and here. Say the diphthong on its own as a normal glide between /ɪ/ and /ǝ/. And now say the diphthong again but with a stronger tongue movement. This emphasis on the stronger tongue movement changes the sound enough to make the difference between /ɪǝ/ and /j/, and to turn the diphthong into a consonant. And again, because /j/ is so closely related to a vowel glide it too is often referred to as a semi-vowel, and in this case as a palatal semi-vowel
So there you have it. The two consonants /w/ and /j/ can morph back and forth between diphthong and consonant, and this will explain many of the pronunciation issues that arise in your class work with these two sounds. And there is something important to say about this process: You have discovered this for yourself through awareness of the muscular movements in your mouth, not just through reading a text on phonology or listening to an explanation. It is that self discovery, and the accompanying proprioception that makes you able to help your students in a different kind of way, as a real learning guide who has been there before and knows what has to be done and how to do it.
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