The Doppler effect happens because the moving source:

  1. changes its frequency
  2. emits successive wavefronts from different positions
  3. creates more waves
  4. changes the speed of sound
Answer

B) emits successive wavefronts from different positions

The spacing of wavefronts changes. SLO P-11-D-02 · Understanding

Physics MCQ · More Wave Intensity and the Doppler Effect MCQs · Practise