Meaning of AD- in English

AD-

ad- BrE AmE əd, (¦)æd —This prefix is strong ¦æd when stressed, (1) because of a suffix ( ˌ adap ˈ tation), and (2) in some two-syllable nouns ( ˈ adverb). Otherwise, in Received Pronunciation and General American, it is usually unstressed and weak (a ˈ dapt), although some speakers use a strong vowel if the following stem begins with a consonant (ad ˈ mit). Before a stem with initial d or dʒ the prefix regularly loses its d (ad ˈ diction). Note the irregularly stressed word ˈ adjective.

Longman Pronunciation English Dictionary.      Английский словарь произношений Longman.