The Doppler Effect

The Doppler effect is the change in the observed frequency of waves produced by the motion of the wave source and/or the wave receiver. In both parts of the diagram, the circles represent wave crests that travel at a constant speed. (a) When the wave source is stationary, the distance between waves is identical for all waves that are produced. The wave frequency (the pitch of the sound) at any point in this diagram is the same. (b) Here the source is moving toward the top of the page. Whenever the source (or the receiver) moves, the wave pattern becomes distorted. Wave 1 was produced when the source was at point 1; wave 2 was created when the source was at point 2; and wave 3 was emitted at point 3. Notice that even though the frequency emitted by the source remains the same as in part (a), a listener at location X would experience a higher frequency (higher-pitched sound) because each wave has a shorter distance to travel and therefore arrives at location X more frequently than would occur if the source were not moving. Conversely, a listener at location Y would experience a lower frequency.