Only in Acehnese 29
What this means for learners
Acehnese and Javanese share 26 sounds — roughly 47% of Javanese's inventory overlaps with Acehnese. Shared sounds are ones a speaker already knows from their native language and will generally produce and perceive accurately without explicit training.
The 7 sounds found only in Javanese represent the greatest pronunciation challenge for Acehnese 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, Acehnese has 29 sounds not used in Javanese. Native Javanese speakers learning Acehnese 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.