This paper describes the development of a Thevenin model of the human voice for mask speech intelligibility testing. The following steps were used to develop the Thevenin model: (1) the long-term average speech spectrum of ten male speakers was measured; (2) the corresponding average vocal tract shape was estimated using linear prediction; (3) a Thevenin model of the voice was determined based on the vocal tract shape. Electroacoustic analogs are used to predict the mask speech spectra produced by the Thevenin model and a mouth simulator consisting of a loudspeaker equalized for free-air speech. Comparing the two spectra allows us to examine how the interaction between the driving point impedance of the voice and the load impedance of a mask affects objective speech intelligibility measurements. The Thevenin model is used in a voice simulator that is described in a future paper.