Speech production model

I am implementing the speech production model,

First of all I gave impulse response to the system and I am making the glottis model by using filter function in matlab as second order IIR filter and making lip radation as first order FIR high pass filter,I am wondering which function can I use for vocal tract?by doing this experiment,I am going to hear "I" az sustain vowel.

Any suggestion and examples for the model?