This moving emission point causes the air compressions to be closer together on one side and farther apart on the other. Each compression of the air moves out in a sphere from the point at which it was emitted, but the point of emission moves. If the source is moving, the situation is different. If the source is stationary, then all of the spheres representing the air compressions in the sound wave are centered on the same point, and the stationary observers on either side hear the same wavelength and frequency as emitted by the source (case a). Each disturbance spreads out spherically from the point at which the sound is emitted. What causes the Doppler shift? The figure below illustrates sound waves emitted by stationary and moving sources in a stationary air mass.
The Doppler effect is an alteration in the observed frequency of a sound due to motion of either the source or the observer. Doppler’s effect explains the perceived increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move toward (or away from) each other.