if a source of sound moves, the frequency of the sound waves
it propagates gets modified and the sound will be perceived
at a pitch either higher (when the source is approaching)
or lower (when it is going away) than the original one
      when the sound emitted by a fixed source hits a surface
it is reflected and echoed at the same frequency
however, if it is reflected by a moving surface,
its frequency will be modified
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the source itself is moving
 

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the source is fixed; the sound is reflected
by either a fixed or a moving surface (blood)

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