This model demonstrates the Doppler effect with a plane flying over a control tower. As the plane flies over the control tower, it emits rings of yellow turtles, which can represent the peaks of the sound waves dispersed by the plane's engines. In this model, it may appear that the sound is being emitted in pulses or bursts, but in reality, it is a continuous emission of waves, and the rings only signify the peaks of the sound waves.
For detailed instructions on using the program, take a look at the info window from the Doppler model.
Click on one of the pictures to see a quicktime movie of the model: