The Collaborative International Dictionary of English v.0.48:

Devocalize \De*vo"cal*ize\, v. t.
   To make toneless; to deprive of vowel quality. --
   De*vo`cal*i*za"tion, n.
   [1913 Webster]

         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 (jh). --H.
                                                  Sweet.
   [1913 Webster]