The source specifies inḍimām al-shafātayn for ḍammah: the lips round appropriately. Inadequate rounding can make the sound resemble kasrah or fatḥah. But lip rounding should not turn the short vowel into a prolonged wāw.
Practice: say «بُ، تُ، دُ» briefly. Compare «بَ، بِ، بُ» and make sure all three sound distinctly different.
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Ḍammah: rounding the lips
Correct Vowel Articulation — Fatḥah, Kasrah, Ḍammah, Ishbāʿ and Sukūn
The source specifies inḍimām al-shafātayn for ḍammah: the lips round appropriately. Inadequate rounding can make the sound resemble kasrah or fatḥah. But lip rounding should not turn the short vowel into a prolonged wāw.
Practice: say «بُ، تُ، دُ» briefly. Compare «بَ، بِ، بُ» and make sure all three sound distinctly different.