Decoupling consonant phoneme
Consonants come in various types and classes. One major differentiation is voiced vs unvoiced (in German: "stimmhaft" vs. "stimmlos", not sure I have the proper English words here). Voiced means that the vocal cords are moving which usually also results in a pitch perception (such as "mmm"). Unvoiced means the vocal cords do not move and there is not pitch ("s", "t" for example).  

It looks to me that most unvoiced consonants should do the trick. Not sure though whether your language has 12 of those. 

For further classifications of consonants and vowels see for example: http://www.zompist.com/kitlong.html