Doppler Effect Question

[member232]

Hey guys,

I wanted to confirm something:

In the first example, say you have a stationary observer and a source moving at 30 m/s towards the observer. The frequency should be: f ' = f (343 / 343-30) or to simplify 343/313.

In another example, you have a stationary source and an observer moving at 30 m/s towards the source. In this case, the frequency should be: f ' = f (343 + 30/ 343) or to simplify 373/343.

SO, even though the doppler effect is all about relative motion, when comparing the two fractions above, it shows that when the observer is moving and source is stationary, you can get a greater perceived frequency than the opposite. Is this correct??

This was a discrete question on a TBR CBT and the answer explanation said that "because the relative motion is the same, the perceived frequency will be the same." Am I correct to think this is wrong?

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[MedPR]

No. It is the net movement toward/away. It doesn't matter who is moving though, so your numbers should have come out equal in both cases.

 

[member232]

Hmm... but 373/343 = 1.087 while 343/313 = 1.095

What am I doing wrong??

 

[milski]

You are doing it right. It does matter if the source or the receiver is moving. What breaks the symmetry is their relative speeds to the media in which the sound is transmitted are different.

 

[member232]

You are doing it right. It does matter if the source or the receiver is moving. What breaks the symmetry is their relative speeds to the media in which the sound is transmitted are different.

Thank you !

Its so frustrating when answers/explanations are incorrect...