Deriving Doppler Effect Frequency w/ Stationary Person & Moving Source

  • #1
annamal
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Can you derive the formula for frequency observed from doppler effect with stationary person and moving sound source away from the person like this:
where is the total velocity observed by stationary person from moving sound, v is velocity of sound and is velocity of sound source.

where is frequency observed and is wavelength observed
 
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  • #2
annamal said:
... from doppler effect with stationary person and moving sound source away from the person ...
If moving away, the wavelength will be longer so the frequency will be lower.
Maybe you need to change the sign to (v - vs).
 
  • #3
Baluncore said:
If moving away, the wavelength will be longer so the frequency will be lower.
Maybe you need to change the sign to (v - vs).
I guess my question is not very clear. What I am asking is why the velocity of sound is constant regardless of whether the source be moving. Wouldn't the velocity of sound be added to the speed of source? I am likening it to a person running on a moving train. To a stationary viewer the velocity of runner = velocity of runner on train + velocity of moving train.
 
  • #4
annamal said:
What I am asking is why the velocity of sound is constant regardless of whether the source be moving. Wouldn't the velocity of sound be added to the speed of source?
Because sound is dependent only on the air. The speed is determined by the medium.
annamal said:
I am likening it to a person running on a moving train. To a stationary viewer the velocity of runner = velocity of runner on train + velocity of moving train.
If you put some air in a box and move the box, the then speed of a sound wave to an external observer IS the speed of the box plus the speed of sound in the box.
 
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