Doppler effect: refers to the phenomenon in which the observer feels the frequency changes when the wave source and the observer move relative to each other. When the wave moves from the wave source to the observer, the receiving frequency becomes higher, and when the wave source moves far away from the observer, the receiving frequency changes lower.
According to the above definition, among the following phenomena, which does not belong to the usage of the Doppler effect?
Experienced soldiers can judge from the sound of the flying cannonball whether the flying cannonball is approaching or leaving