I wonder schwa sound [ə] such as but, cut, sub in weak form etc. is voiced or unvoiced sound. Thank you!
A google search for "unvoiced schwa" turns up examples for both English and other languages. Mostly it is voiced but there are a few exceptions where it is unvoiced :)
I had never come across the concept of an unvoiced vowel, never mind an unvoiced schwa, but it turns out that it is possible to record such a phenomenon with a spectrogram.
Norman Lass in Contemporary Issues in Experimental Phonetics gives the word
commercial as an example: "The /k/ sound appears to be unusually long because of the unvoiced quality of /ə/". Figure 6.18 in this reference is particularly illustrative.
@StoneyB explains in the comments that the unvoiced schwa does not happen with stressed syllables and thus it doesn't happen in