Doppler Model 
In this model, students can change the relative velocities of observer and
source and note the change in period of the waves. The graph is set in
comparative mode so the students can start with no relative velocity and then
compare that plot to one with relative velocity.
Perceived_Wave(t) = Perceived_Wave(t  dt) + (Wave) * dt
Time SpecsRange = 0  3; dT = 0.01; Integration Method = RungeKutta 4 
