Chinese · Pronunciation Guide
doong, low and sagging. An unaspirated d, a rounded vowel closer to "oo" than "oh", and a back nasal ending.
dǒng
As with every dipping tone, the neighbour decides. Before another third tone it rises; before anything else it keeps the dip. Critically, 不 in front of it stays as bù with a falling tone, because the change to bú is triggered only by a following fourth tone — and 懂 is not one.
Listen: 懂
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我不懂你的意思。
Wǒ bù dǒng nǐ de yìsi.
I don't understand what you mean.
Listen: 我不懂你的意思。
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我不懂你的意思。 illustrates exactly that. Bù keeps its falling tone before the dipping dǒng. Wǒ before bù also keeps its full dip, since bù is falling. Then nǐ dips, de is toneless, and yìsi gives a fall plus another toneless syllable. The sentence therefore has three separate third tones and none of them triggers sandhi — worth noticing, because learners assume the rule fires more often than it does.
Nine times out of ten the vowel is too open, coming out as the "ong" of English "long". Round the lips more and close the vowel toward "oo". Aspirating the d gives tǒng, a different word. And the sandhi over-application mentioned above is a genuine issue: seeing 不 next to a third tone, learners often convert it to bú anyway, which is wrong in both directions.