halacha - Whispering prayers?

Can one whisper t'fila (prayer)? I'm thinking of obligatory t'filos like sh'mone esre and "Sh'ma".

I know one can speak t'fila quietly, even very quietly; in fact, sh'mone esre is said quietly. I'm not asking about loudness versus quietness but specifically about whispering. As Wikipedia notes:

In normal speech, the vocal cords alternate between states of voice and voicelessness. In whispering,… the vocal cords alternate between whisper and voicelessness (though the acoustic difference between the two states is minimal).

In other words, whispering is speaking without the use — or with minimal use — of the vocal cords. All consonants and vowels become largely or completely devoiced. This is independent of loudness: it can be done pretty loudly, and normal speaking can be done quietly.

(The words תִּזְכְּרוּ and תִּשְׂכְּרוּ thus come out almost the same — but not quite. One difference lies in the fact that whispering generally doesn't completely remove voice (as noted in the Wikipedia quotation, above). Another lies in the fact that for some people the place of articulation of a zayin is slightly different than that of a sin, or one is more closed than the other. Also, the preceding chirik may differ slightly, e.g. in length, between the two words. Finally, WAF notes in a comment, below, that people often lengthen normally-voiced consonants when whispering, and suggests that that might suffice to distinguish them from the normally unvoiced ones.)

Is there any source or argument that says that t'fila can or cannot be done in a whisper?