Doppler effect. Animation of an object emitting sound waves that are closer together in the direction of its travel, and further apart in the opposite direction. This causes the sound of an travelling object, such as a siren, to be higher pitched as it approaches, switching to a lower pitch as it passes and begins to recede. This shift is known as the Doppler effect, after Austrian physicist Christian Doppler (1803-1853). The Doppler effect is also observed in other examples, such as water waves and light waves.