Differential Doppler effect

The differential Doppler effect is a special kind of sound emitted by an object moving very quickly in relation to a listener. Imagine standing on a sidewalk and hearing a car drive past you. As the car moves closer, the sound of its engine gets louder, then quieter as it moves away. The same thing happens with the differential Doppler effect, except that the sound you hear is different depending on whether the object is getting closer or farther away. When the object moves closer, the sound shifts to a higher pitch, and when the object moves away, the sound shifts to a lower pitch.