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Old Jan 17th 2015, 07:07 AM   #1
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Finding the frequency of a wave.

A siren rings with 1200Hz frequency. A wind blows with 55m/s from the siren toward a man 1km away. In what frequency will the man hear it if the speed of sound is 330m/s? I found the wavelength first. Then the speed which is sound + wind velocity. Next I calculated the frequency which is 1400Hz. Is my answer correct and why was the distance 1km given?
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Old Nov 24th 2015, 11:18 PM   #2
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If both the siren and the man aren't moving, shouldn't the frequencies be equal since there is no Doppler's effect?
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Old Nov 25th 2015, 01:30 PM   #3
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It looks to me like one of those nasty questions specifically designed to confuse.
I agree that the 1km distance seems to be superfluous information,
(perhaps it is part of a subsequent follow up question).

My thinking is as follows:
The frequency of the siren will be transferred to the moving air,
the velocity of the sound in the moving air with respect to the siren will be greater than the velocity of sound in still air, thus while the frequency will be the same, the wavelength will be different.
However, when it gets to the man, the effect will be reversed, thus the man will still hear 1200hz, the sound will just reach him quicker.
Effectively there is a Doppler shift between the siren and the moving wind, but then a reverse Doppler shift between the wind and the stationary man, which cancel each other out.

When presented by a series of questions it is often useful to at least scan the subsequent questions because they can sometimes provide a clue to the sort of answer required.
If the answer to the early question clearly does not fit with the later questions, there is probably an issue with the early answer...
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Last edited by MBW; Nov 26th 2015 at 01:03 AM.
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