subvocal
/ˌsʌbˈvəʊkəl/adjective
1- (of a word or sound) barely audible（话语, 声音）几乎听不到的：
a subvocal sigh.
轻轻的叹息。
1.1
- Psychology & Philosophy relating to or denoting an unarticulated level of speech comparable to thought【心理, 哲】（与）无声（有关）的：
almost all of what is called 'thinking' is subvocal talk.
差不多所有所谓的“思考”都是种无声的说话。