Two articles on this topic:
Acoustics: the vocal tract and the sound of a didgeridoo. Nature 2005. Jul 7;436(7047):39. By A Tarnopolsky, N Fletcher, L Hollenberg, B Lange, J Smith, J Wolfe.
Vocal tract resonances and the sound of the Australian didjeridu (yidaki) II. Theory. Abstract available here. By N H Fletcher, L C L Hollenberg, J Smith, A Z Tarnopolsky and J Wolfe.