Answer: Doppler effect
Explanation: It is basically a change in the observed speed of sound due to the relative motion between the source and the observer.
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Due to motion, the change in sound wave's frequency due to motion between the observer and sound source is the cause of the doppler effect.
Doppler effect is the change of frequency of wave with respect to the observer who is moving relative to the wave source.
The very common example of the Doppler effect is the change in pitch of the approaching vehicle.
To know more about the doppler effect
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