Re: speed of sound

["Paul Camp" <pjcamp@coastal.edu>]

Considering the relative velocity (as you would do with an 
electromagnetic problem) as the source of the Doppler shift is a 
common error. The sound problem is different in that moving source 
and moving observer are physically distinct processes. A source may 
move faster than the waves it is emitting and, so to speak, perceive 
them out of order. A moving observer cannot. The distinction can be 
traced to the fact that there is a preferred reference fram for this 
problem, namely the medium through which the sound wave propagates. 
It matters which one is doing the moving relative to that medium (and 
it is rather annoying that most texts don't point this out since it 
is a rather significant difference between the sonic and optic 
problems).


> A student posed this question, and I have not come up with a definite 
> answer. Any takers?
> A source of sound is moving 343 m/s in air (speed of sound = 345 m/s) 
> emitting a frequency of 500 Hz. An observer passes the source at a speed of 
> 344 m/s. What will be heard by the observer?
> My initial thought was to simply consider the 1 m/s separation of the 
> source/observer causing a doppler shift reduction in the observed 
> frequency (498.6 Hz). But, if I consider both of their motions with respect 
> to the speed of sound I get an observed frequency of 250 Hz. Somebody 
> straighten me out please!
>  
> Darren
> watskyd@mail.firn.edu
Paul J. Camp                   "The Beauty of the Universe
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