The Doppler effect is observed because the distance between the source of sound and the observer is changing. If the source and the observer are approaching, then the distance is decreasing and if the source and the observer are receding, then the distance is increasing. The source of sound always emits the same frequency. Therefore, for the same period of time, the same number of waves must fit between the source and the observer. if the distance is large, then the waves can be spread apart; but if the distance is small, the waves must be compressed into the smaller distance. For these reasons, if the source is moving towards the observer, the observer perceives sound waves reaching him or her at a more frequent rate (high pitch); and if the source is moving away from the observer, the observer perceives sound waves reaching him or her at a less frequent rate (low pitch). It is important to note that the effect does not result because of an actual change in the frequency of the source. The source puts out the same frequency; the observer only perceives a different frequency because of the relative motion between them.
The
Doppler effect is observed whenever the speed of the source is moving
slower than the speed of the waves. But if the source actually moves at
the same speed as or faster than the wave itself can move, a different
phenomenon is observed. If a moving source of sound moves at the same
speed as sound, then the source will always be at the leading edge of the
waves which it produces. The diagram at the right depicts snapshots in
time of a variety of wavefronts produced by an aircraft which is moving at
the same speed as sound. The circular lines represent compressional
wavefronts of the sound waves. Notice that these circles are bunched up
at the front of the aircraft. This phenomenon is known as a shock
wave. Shock waves are also produced if the aircraft moves
faster than the speed of sound. If a moving source of sound moves faster
than sound, the source will always be ahead of the waves which it
produces. The diagram at the right depicts snapshots in time of a variety
of wavefronts produced by an aircraft which is moving faster than sound.
Note that the circular compressional wavefronts fall behind the faster
moving aircraft (in actuality, these circles would be spheres).
If you are standing on the ground when a supersonic (faster than sound) aircraft passes overhead, you might hear a sonic boom. A sonic boom occurs as the result of the piling up of compressional wavefronts along the conical edge of the wave pattern. These compressional wavefronts pile up and interfere to produce a very high pressure zone. This is shown below. Instead of these compressional regions (high pressure regions) reaching you one at a time in consecutive fashion, they all reach you at once. Since every compression is followed by a rarefaction, the high pressure zone will be immediately followed by a low pressure zone. This creates a very loud noise.
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