According to the source, sākin mīm followed by bāʾ (ب) is a case for ikhfāʾ, commonly termed labial concealment (ikhfāʾ shafawī). The mīm is realised with nasal resonance before the bāʾ instead of as a fully separated, released consonant.
Both letters use the lips, so this is best learned through listening and correction. Concealment does not mean omitting mīm entirely, nor doubling the bāʾ in advance.
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Second rule: mīm before bāʾ
According to the source, sākin mīm followed by bāʾ (ب) is a case for ikhfāʾ, commonly termed labial concealment (ikhfāʾ shafawī). The mīm is realised with nasal resonance before the bāʾ instead of as a fully separated, released consonant.
Both letters use the lips, so this is best learned through listening and correction. Concealment does not mean omitting mīm entirely, nor doubling the bāʾ in advance.