They are different ways of representing the same
The /aʊ/ symbol finishes with the same /ʊ/ symbol but starts with the /a/ one, meaning the sound starts with the mouth much tighter and further open than with the /ə/ sound of /əʊ/.
TL;DR: To pronounce the /aʊ/ sound correctly, connect the /ɑ/ sound to /ʊ/ with a smooth, gliding movement. Step-by-step pronunciation instructions: Mouth: First, open your mouth very wide for the /ɑ/ sound. Then glide into the /ʊ/, closing it partially.
Unlike with vowels and consonants, when you pronounce a diphthong sound, your mouth will change position and shape as you say the sound, because they combine two vowel sounds. So as you say the /әʊ/ sound your mouth will start on the position of the /əә/ sound and finish on the position of the /ʊ/ or /u:/ sound.
How to Produce /ɒ/? To produce it, drop the jaw just a little and round the lips, unlike the unrounded /ɑ/ sound. Push your lips together and make a short, voiced sound. As you can see in the picture, the mouth is slightly more open than the /æ/ sound and less rounded than when producing the /ɔ/ sound.
1. /aʊ/ as in Town
used to express pain or discomfort when someone gets hurt. Keep in mind that this diphthong is pronounced differently in other forms of English, most notably Canadian English.
Open-mid back rounded vowel Open-mid back rounded vowel In General American the IPA phonetic symbol /ɔː/ corresponds to the vowel sound in words like "thought" and “cloth", and also "north" and “force.” Sometimes the triangular colon /ː/ is replaced by a normal colon, as in /ɔ:/.
The sound /ʊ/ is a back, high, lax, rounded vowel. Spelling: “oo” – took, good. “u” – put, bush.
The sound /ʒ/ is a voiced, alveo-palatal, fricative consonant. Lightly press the middle of your tongue between your alveolar ridge and your soft palate. The sides of your tongue should lightly touch your back upper teeth. Breathe out and allow air to flow past your tongue. Your vocal cords should vibrate.
The biggest difference between these two sounds is that /ɒ/ is a short vowel and /ɔ:/ is a long one. The mouth position is also slightly different, with the mouth in /ɔ:/ being slightly tighter and more rounded.
These are both low, tense vowels. However, /ɔ/ is a back, rounded vowel and /ɑ/ is a central, unrounded vowel. When you pronounce /ɔ/, your lips should form a circle. Note: Many American English speakers do not distinguish between these two vowels.
The sounds /tʃ/ and /ʃ/ are both voiceless, alveo-palatal consonants. However, /tʃ/ is an affricate while /ʃ/ is a fricative. When you pronounce /tʃ/, the air in your mouth should stop (like a /t/) before it is released (like a /ʃ/).
Some common words which practice the pronunciation of /əʊ/ include the following: with "o"+ magic e: alone, bone, close, code, decode, episode, hole, home, hope, joke, lone, note, phone, pole, quote, role, rope, slope, smoke, stole, stone, those, vote, whole.
The two sounds are similar, but the initial "d" in dʒ makes it a sharper sound. You can only make this sound for a brief moment, because of the initial "d." By contrast, the sound ʒ can be continued for a long time, like an "s" or "sh" sound.
These two are pronounced with exactly the same mouth position but /dʒ/ uses the voice, whereas /tʃ/ is just a sudden puff of air similar to a sneeze.
/ʧ/ is pronounced without your tongue moving and with more air released than with /t/. It is similar to the sound of a sneeze, and the air released should be able to move a piece of paper or be felt on your hand five centimetres in front of your mouth.
So thus /ɑ/ would be a better choice in North America.
Across the pond, Received Pronunciation British and other dialects don't merge "father" and "bother" vowels like that, thus you get /ɒ/ to accommodate (by the way, /ɒ/ is the rounded version of /ɑ/.)
Some Ways of Spelling the /ɒ/ Sound
o, as in: log, option, odd, shock, knob, off, God, sod, rod, don, pot, cot, etc.