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?
A monitor equipped with a speedometer emits ultrasonic waves with a known frequency to the moving vehicle, and at the same time measures the frequency of the reflected wave, and the speed of the vehicle can be known according to the frequency change of the reflected wave