A Audio and hi-fi forum. AudioBanter

Go Back   Home » AudioBanter forum » rec.audio » Pro Audio
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

Doppler Distoriton?



 
 
Thread Tools Display Modes
  #531  
Old September 2nd 04, 01:17 AM
Chris Hornbeck
external usenet poster
 
Posts: n/a
Default

On Wed, 01 Sep 2004 16:40:43 -0700, Bob Cain
> wrote:

>> Isn't this (and the case of coupled infinitely extended
>> planes) just a special case with no relative motion?

>
>Relative motion as we usually think of it is just the DC
>component of the soundfield created by the Tx.


But for our purposes wouldn't it be a better model to
think of the relative motion as a separate thing, and
outside of the Tx and Rx soundfields?

Within those soundfields, especially if they're rigidly
coupled, nothing interesting happens.


> That it is
>fully coupled in the tube (or infinite plane) means it is
>received as is by the Rx without giving rise to any Doppler
>effect. Art's step function derivation shows that. That it
>is not coupled at all to an Rx in the far, free field is
>what allows simplification to the standard Doppler equation
>(which is only valid for a constant v and a single frequency
>in the Tx spectrum.)


Yeah, this is the part that Scott calls reciprocity failure.
It's a photographer's pun, but it works.

Chris Hornbeck
Ads
  #532  
Old September 2nd 04, 01:18 AM
Bob Cain
external usenet poster
 
Posts: n/a
Default



Chris Hornbeck wrote:

>
> I only post to rec.audio.pro and don't crosspost.


Just as well. From the technical competence of its regulars
I've come to realize that there is a _very_ good reason why
alt.sci.physics.acoustics is in the alt. subtree.


Bob
--

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

A. Einstein
  #533  
Old September 2nd 04, 02:35 AM
Bob Cain
external usenet poster
 
Posts: n/a
Default



Chris Hornbeck wrote:

>>Relative motion as we usually think of it is just the DC
>>component of the soundfield created by the Tx.

>
>
> But for our purposes wouldn't it be a better model to
> think of the relative motion as a separate thing, and
> outside of the Tx and Rx soundfields?


I don't thing so because it is a general phenomenon and
where people have gotten into trouble with it is precisely
for thinking there is something intrinsically definitive
about constant motion and then erroneously applying that
simplified special case solution to the more general case.

>
> Within those soundfields, especially if they're rigidly
> coupled, nothing interesting happens.


Yes.


> Yeah, this is the part that Scott calls reciprocity failure.
> It's a photographer's pun, but it works.


Yeah, I like the intuitive feel it gives while at the same
time being accurate, which as we've just found out is not
true in general with intuition.


Bob
--

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

A. Einstein
  #534  
Old September 3rd 04, 04:49 AM
Bob Cain
external usenet poster
 
Posts: n/a
Default


I withdraw the proof. It is flawed. In considering a
challenge by Art Ludwig, I realized that my proof begins
with an unrealistic assumption which allows no conclusions
to be drawn, much less a proof of anything. The flaw is
that I began with:
http://www.silcom.com/~aludwig/Physi...collisions.htm

having a value of zero for delta-t. For any medium with
non-zero mass density that requires infinite force to
achieve a step change in velocity. Obviously, nothing that
follows from the assumption of a step change in velocity can
be valid for a system that disallows that.

Back to the drawing board. There are heuristic proofs
involving reciprocity, conservation of energy and time
reversal (which are actually more satisfying to a physicist)
but I want to find a more direct one.


Sorry,

Bob
--

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

A. Einstein
  #535  
Old September 3rd 04, 04:50 AM
Bob Cain
external usenet poster
 
Posts: n/a
Default



Bob Cain wrote:


> Did you see my proof of why a piston in a tube will evidence no such
> behavior with any signal including constant translation of the piston?
> I believe it is a refutation of that (unless, of course either I still
> don't understand what you are saying or someone can refute my proof.)


I just refuted it myself. Only the particular method of
proof, not what I lamely attempted to prove. Tryin' again.


Bob
--

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

A. Einstein
  #536  
Old September 3rd 04, 05:39 AM
Chris Hornbeck
external usenet poster
 
Posts: n/a
Default

On Wed, 01 Sep 2004 18:35:20 -0700, Bob Cain
> wrote:

>> But for our purposes wouldn't it be a better model to
>> think of the relative motion as a separate thing, and
>> outside of the Tx and Rx soundfields?

>
>I don't thing so because it is a general phenomenon and
>where people have gotten into trouble with it is precisely
>for thinking there is something intrinsically definitive
>about constant motion and then erroneously applying that
>simplified special case solution to the more general case.


I've just finally realized that you're after the general case.
Sorry, never was too bright. But I'll make it up to you by
sending you my spare copy of Beranek, 1954. If your address
of a year ago isn't good, let me know quick.

You'll appreciate it's complete non-mention of Doppler effect!


>> Yeah, this is the part that Scott calls reciprocity failure.
>> It's a photographer's pun, but it works.

>
>Yeah, I like the intuitive feel it gives while at the same
>time being accurate, which as we've just found out is not
>true in general with intuition.


Makes life worthwhile.

Chris Hornbeck
  #537  
Old September 3rd 04, 07:38 AM
Bob Cain
external usenet poster
 
Posts: n/a
Default



Chris Hornbeck wrote:


> I've just finally realized that you're after the general case.
> Sorry, never was too bright. But I'll make it up to you by
> sending you my spare copy of Beranek, 1954. If your address
> of a year ago isn't good, let me know quick.


Actually it has so hold on a bit.

>
> You'll appreciate it's complete non-mention of Doppler effect!


Ah, virgin territory!


Bob
--

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

A. Einstein
  #538  
Old September 5th 04, 05:26 AM
Bob Cain
external usenet poster
 
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
  #539  
Old September 5th 04, 05:26 AM
Bob Cain
external usenet poster
 
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
  #540  
Old September 5th 04, 09:47 AM
Bob Cain
external usenet poster
 
Posts: n/a
Default



Bob Cain wrote:

> 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.)


Ouch! That's dead wrong. Compass drift. With this problem
it is really difficult staying in the correct frame of
reference and when I wrote that I'd partially stepped off a
stationary one onto one that was moving, one foot still
firmly in each. :-)


Bob
--

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

A. Einstein
 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Stereophile Tries To Come Clean About The DiAural Fiasco Arny Krueger Audio Opinions 9 November 23rd 04 05:21 PM
Experimental Evidence for Dynamic Doppler Shift The Ghost Tech 100 October 19th 04 07:14 AM
Bob Cain Is In Convulsions: A Doppler Piston Just Got Shoved Up His Tube The Ghost Tech 42 September 29th 04 02:52 AM
Doppler Distoriton? Arny Krueger Tech 627 September 8th 04 03:14 AM
Doppler Distortion - Fact or Fiction Bob Cain Pro Audio 266 August 17th 04 06:50 AM


All times are GMT +1. The time now is 04:44 AM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Copyright 2004-2019 AudioBanter.
The comments are property of their posters.