Distance from source to observe Doppler effect?

  • #1
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Main Question or Discussion Point

I was just looking at a few animations and something that could be just visualized incorrectly or I may have interpreted correctly is that the wavelengths coming from the sources are initially smaller close to the source, but become larger as you get far away from the source. Is there any particular distance away from the source you have to be to observe a wavelength of particular size?
 

Answers and Replies

  • #2
Im not sure i understand you correctly, the frequency doesnt change, it is the apparent frequency the amount of apparent frequency is determined by velocity i believe. A helpfull formula to understand would be
F'=f(v/v+/-us)
For example when a police car or ambulance passes you on the road, you can hear the pitch of sound change from high to low frequency it is high when it is approaching and low when it is going away
 
Last edited:
  • #3
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I was just looking at a few animations and something that could be just visualized incorrectly or I may have interpreted correctly is that the wavelengths coming from the sources are initially smaller close to the source, but become larger as you get far away from the source. Is there any particular distance away from the source you have to be to observe a wavelength of particular size?
Do you have a link to one of these animations?
 
  • #4
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414
No, the OP mentions some animations and these seem to be source of some confusion. I think it will be helpful to see some of them.
I don't understand what video are you thinking about.
 
  • #5
No, the OP mentions some animations and these seem to be source of some confusion. I think it will be helpful to see some of them.
I don't understand what video are you thinking about.
My fault nasu, i ment to ask the op if he wanted some more example videos of the Doppler effect
 
  • #6
sophiecentaur
Science Advisor
Gold Member
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I was just looking at a few animations and something that could be just visualized incorrectly or I may have interpreted correctly is that the wavelengths coming from the sources are initially smaller close to the source, but become larger as you get far away from the source. Is there any particular distance away from the source you have to be to observe a wavelength of particular size?
I have a feeling that you could be trying to reconcile the gradual change in frequency of what you hear as a siren approaches and goes past. It is due to the geometry of the situation. If the vehicle is travelling at constant speed and you are (hopefully) to one side of its track, the distance between you and the siren does not change at the same rate that the vehicle is travelling. It's a result of Pythagoras. The frequency you hear will depend on the rate of change of the relative separation and not the speed over the ground of the siren. In fact, when you are abreast of the vehicle, the relative speed is quite low for a significant time. If you were directly in the track of the siren, the (higher) frequency would be the same as it approaches and (if you were alive to hear it) would change abruptly to the lower frequency as it receded.
It unlikely that an animation will show you this (animations and simulations are not reality). You may be better advised to think about it rather than relying on animations to help you here (and in many other cases).
 

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