Doppler

Physicist Christian-Johann Doppler (1803–53) noticed this effect first with sound and then also with light. The Doppler-Fizeau effect describes a relative anamorphose of the original signal. This perceptive modification is due to a relation of movement between the sound source and the listener that provokes either a compression or an expansion of the sound wave. A sound signal that moves closer is perceived as being higher than it actually is, whereas that same signal moving away will be perceived as being lower. This phenomenon comes from a combination of the sound's speed of propagation and the movement of the sound source. When both the sound wave and the sound source move in the same direction, the perceived frequency rises. When they move in opposite directions, the perceived frequency drops. When there is a sudden change of direction of the source in relation to the listener, the Doppler effect can be accompanied by a complementary effect of approach and distancing.