View Single Post
  #539   Report Post  
Bob Cain
 
Posts: n/a
Default



Ben Bradley wrote:

fd = f*c/(c + v),


Randy, that equation is only defined for a static v.



So if you start changing v, the doppler effect stops until you
leave v alone for a while? How does the doppler effect know to stop
and start up again?


C'mon, Ben. Where did I imply that it stops and starts?
That equation just can't tell the whole story. Consider
that for v constant none of its motion is being imparted to
the wave that reaches the Rx but if it is oscilating, some
of it is. That has to make some difference in the net
effect beyond the predicted warble. That difference is
missing from the equation because it is a term which drops
out for dv/dt=0. Can I derive that yet, no. Am I sure
there are additional terms dependant on rate of change, or
multiplied by w if two tones, yes.


Seriously (or you can answer the above question seriously if you
like), do you have any reference for the equation being defined only
for v being static?


I'm still awiating Pierce's book wherin it is claimed that
it is derived for the fully dynamic case giving the same
result. All the derivations I somewhat remember from long
ago university freshman physics definitely assumed constant
v as a premise.

The main reason I'm working out the proof of why Doppler
mixing doesn't happen with a piston in a tube is that the
equation above will thus be violated. After it has ramped
up from a stationary position to where it is oscilationg
with a constant motion superimposed on it, and after that
ramping up has passed an observer at some distance from the
piston, he will see no change in frequency but instead the
same oscilation superimposed on a constant air velocity
(until the piston smacks him up 'long side the head if the
constant motion is toward him.)

I'm pretty sure I now have that proof but am sitting with it
a while instead of possibly jumping the gun again and I've
asked a few folks to sanity check it. Would you care to?


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein