An instructor can use the applet as a lecture/demo to illustrate the qualitative
aspects of the Doppler effect. To use the item quantitatively as homework,
specific assignments will need to be created. In addition, for self study, the
students will need to be told that the circular lines represent (e.g.) spherical
wave crests since there is no discussion of the translation from the
simulation to an actual moving wave source.
The lack of units in the applet is troubling because it caters to the well-known
tendency for introductory students to ignore units.