Re: [Phys-L] Doppler Effect (sound)

[John Denker <jsd@av8n.com>]

On 11/30/2014 10:56 AM, Peter Schoch wrote:
> 2.  He took the observer as stationary, and increased the speed of the
> emitter by the combined speed of the two and got a different answer.
>
> If I'm not losing my mind, he should've gotten the same answer both ways.

Actually the answer should be different.

I assume you are instinctively applying the idea of Galilean
relativity.  That does not apply in this case, because the
/air/ has a reference frame of its own, which breaks the
symmetry.

To see immediately why it must be different, consider two
trains moving at 50% of the speed of sound.  If you give
all the speed to one train, speed /relative to the air/,
it is transonic, and nobody hears the whistle at all.
Consider it an infinite Doppler effect.

  In contrast, if you give half the speed to each train,
  speed /relative to the air/, you get a largish but
  finite Doppler effect.

Note that speed relative to the lab frame is not important,
except insofar as we assume the air mass is at rest in this
frame.




For light waves, where there is no medium, no ether, the
answer would be the same either way.