What this means for learners
MBUM and Fɔn share 25 sounds — roughly 60% of Fɔn's inventory overlaps with MBUM. Shared sounds are ones a speaker already knows from their native language and will generally produce and perceive accurately without explicit training.
The 17 sounds found only in Fɔn represent the greatest pronunciation challenge for MBUM speakers. The adult brain tends to map unfamiliar sounds onto the closest native equivalent — a process that produces the characteristic "accent" of a second-language speaker. Learning to hear and produce these sounds as distinct requires focused ear training, not just repetition.
Conversely, MBUM has 13 sounds not used in Fɔn. Native Fɔn speakers learning MBUM will face the mirror-image challenge with these sounds.
Phoneme inventories from PHOIBLE. Data reflects one documented inventory per language; some variation exists across dialects and sources.