De·vo'cal·ize transitive verb To make toneless; to deprive of vowel quality. -- De*vo`cal*i*za'tion , noun « If we take a high vowel, such as (i) [ = nearly i of bit], and devocalize it, we obtain a hiss which is quite distinct enough to stand for a weak... Found on http://www.encyclo.co.uk/webster/D/54
1) To devoice 
• (v. t.) To make toneless; to deprive of vowel quality.
De·vo'cal·ize transitive verb To make toneless; to deprive of vowel quality. -- 
to devoice. · to convert (a speech sound) from a vowel to a glide.