Physics for IGCSE & O level - Waves And Sounds (Section 3 - No. 47)
What causes the 'Doppler Effect'?
Reflection of sound waves.
Relative motion between the source and the observer.
Changes in the speed of sound.
Interference of sound waves.
Explication
The Doppler effect is a change in frequency due to the motion of the source, the observer, or both.