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Q & A: stationary Doppler?

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Most recent answer: 03/07/2009
Q:
(This question has been making me crazy for years. I can't find the answer anywhere and an experiment would be much too involved.) An observer who is passed by a rapidly-moving cart equipped with a blaring tone generator will hear a Doppler-induced shift in tone as the cart passes. Will the observer hear a similar shift if he or she is at the mid-point of a long series of tone generators as each emits a short "tweet" (at the same tone) in very rapid succession (like falling dominoes)? Will your answer apply to transverse waves as well as longitudinal ones?
- Derek (age 43)
Colorado Springs, CO
A:
The basic Doppler effect is exactly the same for longitudinal and transverse waves. It comes from the shifting location of the source. That makes successive wave fronts bunched closer together in one direction and spread farther apart in the other direction.

The tweets from each stationary tone generator will have a frequency associated with them that won't change as they are transmitted. They'll all sound like the same frequency to you, with no Doppler shift. The source of each of the many wave crests in each tweet isn't moving nearer to you than when it emitted the previous crest. Any interference between successive tweets, whose crests might not match up synchronously, won't take the form of a systematic frequency shift.

Mike W.


(published on 03/07/2009)

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