A diphthong is two vowel sounds together. That means your mouth has to move: there is a beginning and an ending position. Study this diphthong with illustrations and up close, slow motion speech.
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The ‘ai’ as in ‘buy’ diphthong. The first sound of this diphthong does not occur on its own as a vowel in American English. A, a, ai. It’s similar to the ‘aa’ as in ‘bat’ in that the back of the tongue raises towards the soft palate, but it doesn’t raise as high as it does in the ‘aa’ as in ‘bat’ vowel, a, a, ai. Also, you may find that the corners of the lips pull out a little bit, and the top lip raises a bit, exposing some of those front teeth, a, a. The tip of the tongue remains forward in this sound, lightly touching behind the bottom front teeth. The second sound of the diphthong is the ‘ih’ as in ‘sit’. In this sound, the tip of the tongue is also forward, touching just behind the bottom front teeth. So that does not move in this diphthong. However, the front/mid part of the tongue does raise towards the roof of the mouth, ai, ai. So as it reaches up towards the roof of the mouth, the jaw will close somewhat, ai, ai, buy.
Here we see the two sounds of this diphthong. You can see in the first sound, aa, that the jaw is more dropped. On the second sound, ‘ih’, the tongue is more forward. Here are the two sounds in profile. Again, note that in the first sound, the jaw is dropped more. Here, parts of the mouth are drawn in. you can see that the tongue position in the first sound is drawn slightly back, which makes it flatter. In the second sound, the tongue is much more raised in the front. In both sounds, the tip of the tongue does lightly touch behind the bottom front teeth. The ‘ai’ as in ‘buy’ diphthong. Sample words: shy, write, good-bye. Sample sentence: My sister is bright with a nice sense of style. Now you will see this sentence up close and in slow motion, both straight on and from an angle, so you can really study how the mouth moves when making this sound.
My, lips press together for the M, jaw drops, here’s the ‘ai’ as in ‘buy’ diphthong, you could see the tongue come up towards the end. Sister is bright, lips come together for the B. And here it is again, the ‘ai’ as in ‘buy’ diphthong. Tongue down and then pushes up towards the front. The tip goes up to make the T. Lips come together for the W. With a nice – tongue up for the N and drops for the ‘ai’ as in ‘buy’ diphthong. Sense of, lip comes up to make the V sound, style. Another ‘ai’ as in ‘buy’ diphthong. Jaw drops for the first part, tongue moves up for the second part, and the dark L.
Now from the angle. My, ‘ai’ as in ‘buy’ diphthong. Jaw drops for the first part and then the middle-front part of the tongue moves up for the second part. Sister is bright, another ‘ai’ as in ‘buy’. Jaw drops, forward part of the tongue moves up, and now here the tip goes up to make the T. With a, tongue tip up to make N and drops, ai, nice, sense of, lip up to make the V sound. Style, another ‘ai’ as in ‘buy’ diphthong. Jaw drops, tongue is down, and then the mid-front part of the tongue raises to make the second part of the diphthong. That’s it, and thanks so much for using Rachel’s English.