In natural speech, for a given speaker, vocal tract length (VTL) is effectively fixed whereas glottal pulse rate (GPR) is varied to indicate prosodic distinctions. This suggests that VTL will be a more reliable cue for identifying a speaker than GPR. It also suggests that listeners will accept larger changes in GPR before perceiving speaker change. We measured the effect of GPR and VTL on the perception of a speaker difference, and found that listeners hear different speakers given a VTL difference of 25%, but they require a GPR difference of 45%.
Bibliographic reference. Gaudrain, Etienne / Li, Su / Ban, Vin Shen / Patterson, Roy D. (2009): "The role of glottal pulse rate and vocal tract length in the perception of speaker identity", In INTERSPEECH-2009, 148-151.