A simulation will only respond to keyboard control when it is "in focus". If not you can click on the simulation area once to bring it to focus.

Doppler Effect

Drag on the simulation below to change the velocity. The more you drag the faster the wave source moves. When the bungee changes to blue, the velocity is within 2% of the speed of the wave. When it is green, it is within 10%. You can see the difference in wavelength and frequency when the wave sources is moving toward you versus when it is moving away.