A Moving Source: The Siren That Changes Pitch

An ambulance approaches: its siren is high-pitched. It moves away: the sound becomes lower. Yet the siren always emits the same note. This apparent change in frequency due to motion is the Doppler effect, described by Christian Doppler in 1842.

The explanation lies in the geometry of the wavefronts. A stationary source emits regular concentric waves. But if the source moves forward, each new front is emitted a little farther along, in the direction of motion: the fronts get squeezed in front, and stretched behind.

   stationary source        source moving forward ->
      ( ( • ) )                ((( • )    )     )
   regular fronts           in front: squeezed  behind: spread out
   same frequency everywhere -> high pitch      -> low pitch

In front of the source, the squeezed fronts arrive closer together in time: the perceived frequency increases, the sound is higher. Behind, the spread-out fronts arrive less often: the frequency decreases, the sound is lower. At the precise moment the ambulance passes you, you hear the switch from high to low.

The same reasoning applies if it is the observer who moves: moving toward the source, they meet the fronts faster (higher frequency); moving away from it, more slowly (lower frequency). What matters is the relative motion between the source and the observer. The effect is all the more pronounced as the speed is large compared with that of the wave.