Doppler effect

A police siren but by a constant frequency, you hear the sound of a different height each time it is stopped, when approaching, when moving away.I called the Doppler effect this.
Preview below expresses the Doppler effect.
    

Official of the Doppler effect

Official of the Doppler effect, early difference is speed of sound
Next,When moving away to the observer,
          

It is a constitute.