Quote Originally Posted by Max_Killjoy View Post
Don't ask me to use IPA, the vowel system in that thing is a wretched mess missing half the vowel sounds I actually use (evidently they're "between sounds" / diphthongs, so use double-symbols... which seems very arbitrary given some of the sounds that they do give distinct symbols to...
The IPA has one symbol for each sound that a human mouth can produce. The jaw, tongue, and lips have a limited range of motion and can only be combined in a large but finite number of combinations. Some of which can be made simultaneously because they happen in different parts of the mouth and some which can't because they overlap. The "T" sound /t/ is made with the tip of your tongue. The "SH" sound /ʃ/ is made with the sides of your tongue. The "CH" sound /tʃ/ is not an "in-between" sound: it's just what happens when you say /t/ and /ʃ/ at the same time. The "T" and "D" sounds /t/ and /d/ are both made in the same way inside your mouth (the difference is in your throat). Because they both occur in the same spot in your mouth, you can't do them simultaneously. That's why the "-ed" verb ending in English sounds like "T" after unvoiced consonants (because it's hard to switch to a voiced /d/ in the middle of a consonant cluster) and "D" after voiced consonants or vowels, but turns into a separate syllable like "ID" /Id/ after a T or D sound. The "-ed" sounds like "t" in "pushed", "d" in "showed", but it sounds like "id" in "ended".

Diphthongs (and triphthongs in some languages) are multiple vowels pronounced one after the other. A vowel is one sound made by the mouth in one position. The variations in vowels are determined by how open your jaws are, how far forward your tongue is, and how high your tongue is, and how rounded your lips are. If your lips, tongue, or jaw move during the "vowel" then it's not one vowel. Watch your mouth in the mirror while you say "ah" (like when the doctor says "open up and say ah"). Your mouth will be open and your jaw doesn't open wider or close because it's one vowel /a/. Then say "eye". Your mouth will be open the same way as "ah" but then it will close to make the second part of the diphthong, which is why the IPA indicates that diphthong as /aI/. You start by saying /a/ then close your jaw to the position that makes the /I/ sound. It's even more obvious when you say "boy" because you not only close your jaw but your lips go from a rounded to an unrounded position.

Saying that diphthongs are one vowel because you don't have a consonant break between the separate mouth positions is like saying a slide guitarist only plays one note when moving around the fretboard. A person who doesn't understand music might call that one continuous sound a "note" but musicians (and physicists who deal in frequencies) know that it's a glide across multiple notes.