then the detected waves have a lower frequency than they had when they were
emitted by the source. The shift in frequency that occurs when the source and
detector are in relative motion is known as the Doppler effect. In general, relative
motion toward each other results in a frequency shift upward, and relative motion
away from each other results in a frequency shift downward.
Let fS be the frequency of waves that the source emits and fD the frequency that the
detector hears. To determine fD from fS, use the following equation: