 They are widely used for process and quality control in industrial production processes.The Doppler effect (or Doppler shift) is the change in frequency of a wave for an observer moving relative to the source of the wave.The wave has a frequency f and propagates at a speed c When the observer moves at a velocity of v relative to the source, they receive a different frequency f' according to

The above analysis is an approximation for small velocities in comparison to the speed of light which is fulfilled very well for practically all technically relevant velocities.