Applied Underwater Acoustics

2.4. Source or Receiver in Motion

When a sound source and an observer are in relative motion in a medium at rest, the received frequencies are shifted upward on approach and downward on departure. Such upshifts and downshifts in frequency are a commonplace occurrence, exemplified by the sound of the whistle on a passing train, and are named after Johann Christian Doppler who was the first to propose the principle [65]. Fig. 2.9 shows a visual example of Doppler shifts, in this case, in the harmonics from the engine and propeller of a light aircraft as it flies over a microphone moored 10 m above the sea surface. As the aircraft passes the microphone, at time approximately equal to 18.5 s corresponding to the closest point of approach (CPA), the ...