I'm working with the CMU pronunciation dictionary and I can't comfortably say I can understand what difference in sound they're trying to indicate by splitting AA and AO into different phonemes.

Wikipedia gives "balm" and "bot" as examples of the AA sound and "bought" as an example of the "AO" sound, but I don't detect a difference in vowel sound when I say the three words.

I also went through the CMU dictionary to look for examples of AA and AO words and still can't figure it out.

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    Many American English speakers don't distinguish these sounds: this is called the "cot-caught merger". See nohat's answer to the question "Why are many TV personalities beginning to pronounce “daughter” as “dotter”?" – herisson Sep 9 '18 at 3:17
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    The CMU dictionary is presumably only meant to describe American English pronunciation. In British English "balm" and "bot" are as different as "chalk" and "cheese" - they couldn't possibly be described as two examples of the same sound. – alephzero Sep 9 '18 at 9:46
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  • @Nardog They are related questions but they are not the same question. I've already posted an answer to this question, and it would not serve as an answer to that other question. – Nathan Wailes Sep 10 '18 at 0:45
  • @alephzero Not only that, "balm" and "bot" are different even in the parts of America without the cot-caught merger. I have no idea what Wikipedia is on about. – Spencer Dec 8 '18 at 10:22

Ok after continuing to go through the CMU dictionary I found some examples that made sense to me:

"AO" is the sound in "orange" and "port" where you bring your tongue and mouth in tighter than when you make the "AA" sound.

  • For many speakers, calm and father have different vowels. – tchrist Sep 9 '18 at 14:25
  • @tchrist Really? Although phonetically they may differ somewhat, every dictionary I consulted, British or American, had /kɑːm/ as the most common (if not the only) variant. – Nardog Sep 10 '18 at 9:22
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    @Nardog In some Midwestern American dialects, including the one I’m speaking to you in, the word calm starts just like Call me a taxi does. It’s like the vowel in all, and the L matters. You can call that L-colored if you want, but it’s really much closer to the CLOTH or THOUGHT vowel. – tchrist Sep 10 '18 at 12:08

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