The Doppler principle was described by Christian Doppler in the 19th century.1This principle indicates that motion, under certain circumstances, causes an apparent shift in the frequency of a wave front. To detect this frequency shift, a source of a wave front and a frequency receptor located away from the source are required. Many examples of the Doppler principle occur commonly; one familiar one will be described. If an automobile is in motion and the driver sounds the horn, a stationary observer will detect a change in the frequency of the horn, even though the transmitted frequency of the horn is constant. Two general properties of the apparent frequency change are important. First, if the automobile is traveling toward the stationary observer, the frequency of the horn will appear to increase. If the automobile is traveling away from the observer, the frequency will appear to decrease. Second, the change