Vocal timbre, or voice quality refers to a particular multidimensional quality or tone color of vocal sound production that is perceived by a listener. It is the perceptual correlate of spectral characteristics of vocalized sound shaped by the regularity of the vibration cycle (period) of the vocal folds. Vocal timbre is often referenced in terms of what it is not — that is, the aspect of vocal sound which is not frequency (pitch), not amplitude (volume), and not time (duration). As Kreiman and Gerratt demonstrate, “voice quality is an interaction between an acoustic voice stimulus and a listener; the acoustic signal itself does not possess vocal quality, it evokes it in the listener. For this reason, acoustic measures are meaningful primarily to the extent that they correspond to what listeners hear” (1998: 1598).
Kreiman, Jody, and Bruce R. Gerratt. 1998. “Validity of rating scale measures of voice quality.” The Journal of the Acoustical Society of America 104 (3): 1598-1608. (See also related works by these scholars.)