Given
The frequency of a sound wave changes if the source of the sound is moving relative to the listener.
To find
What is this called?
Step-by-step explanation
For an observer, when any source of sound is moving towards it or away from it then the sound becomes higher and lower respectively.
Thus the moment of the source changes the frequency of the sound. This is known as doppler effect.
Conclusion
The given effect is known as Doppler effect.