Sound and Music - Mission SM4 Detailed Help

The Doppler effect is observed ____.
Definition of the Doppler Shift:
The Doppler shift is the alteration of the frequency observed of waves that occurs whenever the wave source is moving towards or away from the observer (or the observer is moving towards or away from the wave source).

The Doppler shift can be observed of any type of wave (sound wave, light wave, etc.) as long as there is motion of the wave source relative to the observer. While it is most commonly discussed in its applications to sound and light waves, the effect can be observed for any type of wave.