ac- This prefix is a variant of ad- . Its pronunciation varies according to context. (1) When followed in spelling by ce, ci, it is usually BrE AmE ək (ac ˈ celerate) , but ¦æk if stressed because of a suffix ( ˌ acci ˈ dental) and in a few two-syllable nouns ( ˈ access). (2) Otherwise, it is ə , but ¦æ if stressed because of a suffix (ac ˈ claim; ˌ accla ˈ mation).
Meaning of AC- in English
Longman Pronunciation English Dictionary. Английский словарь произношений Longman. 2012