- With a few exceptions Sommerfeldt and Strong (1988); Keefe (1990b), most wind instrument simulations have assumed a constant or slowly varying pressure in the player's mouth and otherwise ignored possible upstream influences.
- Under these assumptions, the upstream system can be considered a large reservoir driven by a zero-frequency (DC) current source.
- An electrical circuit analog for such a system is shown in Fig. 23. The current source
*U*_{l}represents the player's lungs, while flow resistance in the lungs and trachea is characterized by*R*_{l}. - In general, the lung impedance varies over time based on the vocal fold configuration.
- The cavity impedance is given by
, where is the mass density of air,
*c*is the speed of sound in air,*V*is the volume of the cavity, and is the radian frequency. - The upstream resistance parameter
*R*_{u}characterizes losses in the player's windway. - The impedance seen by the reed looking upstream is infinite for steady flow but relatively small at higher frequencies.
- Under these conditions, the reed is controlled by the oscillating pressure on its downstream side and the DC upstream pressure only.

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