<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\nFranz Heymann wrote:\n\n> "Bob Cain" <arcane@arcanemethods.com> wrote in message\n\n>>No, I wouldn\'t skewer them. I am one. Just one with a bit\n>>more of a technical education.\n>\n>\n> But not quite enough to get to grips with the problem.\n\nNo argument there or I wouldn\'t be here. :-)\n\n>\n>>The simple argument I\'ve been using to disprove the\n>>phenomenon boils down to whether or not the principle of\n>>reciprocity applies to acoustic systems,\n>\n>\n> Your simple argument is, alas, incorrect.\n\nWould you agree that it is correct in the toy context of the\ninfinite tube?\n\n>\n>\n>>whether the motion\n>>of an ideal test piston (zero mechanical impedence) in an\n>>infinite waveguide, a tube, with a plane wave traveling in\n>>it will be the same motion as that of a piston creating that\n>>wave but with a delay. Seems obvious to me and I think, but\n>>am not sure, that this reciprocity can be proved by an\n>>appeal to the conservation of energy.\n>\n>\n> This is essentially where you are quite wrong.\n\nOk. Even in the toy tube?\n\n> The radiation impedance of a piston in a tube of the same diameter is\n> identical with the acoustic impedance of a sound wave propagated in\n> the tube. Such a piston will not cause a high frequency to be\n> modulated by a low frequency vibration of the source.\n\nThis is what I claimed also (eventually, in the argument\nthat rages elsewhere), so I understand that much. This\ncompletely invalidates the vernacular argument that is\nusually employed to justify Doppler distortion, however,\nwhich is what I was trying to do from intuition that it was\nwrong to eventual understanding of why.\n\nMy argument was couched in terms that my opponents could\nunderstand and that came to me in that context. I said that\nunder those conditions, the velocity wave that would be\ncreated would be of the same magnitude and shape as that of\nthe piston\'s velocity and since the piston is _in_ the wave\nit is generating, it is always in the correct instantaneous\nposition for the instantaneous velocity it is imparting to\nthe wave.\n\nIs that nonsense or is it equivalent to your much simpler\nstatement? I guess your principle is hidden in the\nassumption I made that the velocity wave would be of the\nsame magnitude and shape as that of the piston. Without it\nthat wouldn\'t be true.\n\n\n>\n> On the other hand, the radiation impedance of a piston, either free or\n> mounted in a baffle, is strongly frequency-dependant. In particular,\n> this results in the circumstance that the piston has to vibrate with a\n> much larger velocity at a low frequency than at a high frequency, if\n> equal sound pressures are to be produced at a large distance from the\n> piston. In a freely propagating plane wave, these two velocities are\n> identical. The nett effect is that when, on this piston, a high\n> frequency vibration is superimposed on a low frequency one, it\n> effectively finds itself produced by a source moving at a velocity\n> larger than that required by the superposition principle, which in\n> turn will then produce a frequency-modulated output.\n> The effect is quantitatively calculable. It turns out to be\n> negligibly small for all except determined audiophiles in the case of\n> speakers operating in normal listening conditions.\n\nOk, this is substantially different than my explanation for\nwhy, in those conditions, Doppler distortion was in\nevidence. My explanation was that Doppler distortion was\nonly in evidence when there was a difference in the coupling\nof the piston to the air as a function of frequency, that is\nwhen the pressure in the far field was a frequency dependant\nfunction of the velocity of the piston. I think that is\nequivalent, if using less appropriate and less exact\nterminology, correct?\n\n\nMany Thanks,\n\n\nBob\n--\n\n"Things should be described as simply as possible, but no\nsimpler."\n\nA. Einstein\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>Franz Heymann wrote:
> "Bob Cain" <arcane@arcanemethods.com> wrote in message
>>No, I wouldn't skewer them. I am one. Just one with a bit
>>more of a technical education.
>
>
> But not quite enough to get to grips with the problem.
No argument there or I wouldn't be here. :-)
>
>>The simple argument I've been using to disprove the
>>phenomenon boils down to whether or not the principle of
>>reciprocity applies to acoustic systems,
>
>
> Your simple argument is, alas, incorrect.
Would you agree that it is correct in the toy context of the
infinite tube?
>
>
>>whether the motion
>>of an ideal test piston (zero mechanical impedence) in an
>>infinite waveguide, a tube, with a plane wave traveling in
>>it will be the same motion as that of a piston creating that
>>wave but with a delay. Seems obvious to me and I think, but
>>am not sure, that this reciprocity can be proved by an
>>appeal to the conservation of energy.
>
>
> This is essentially where you are quite wrong.
Ok. Even in the toy tube?
> The radiation impedance of a piston in a tube of the same diameter is
> identical with the acoustic impedance of a sound wave propagated in
> the tube. Such a piston will not cause a high frequency to be
> modulated by a low frequency vibration of the source.
This is what I claimed also (eventually, in the argument
that rages elsewhere), so I understand that much. This
completely invalidates the vernacular argument that is
usually employed to justify Doppler distortion, however,
which is what I was trying to do from intuition that it was
wrong to eventual understanding of why.
My argument was couched in terms that my opponents could
understand and that came to me in that context. I said that
under those conditions, the velocity wave that would be
created would be of the same magnitude and shape as that of
the piston's velocity and since the piston is _in_ the wave
it is generating, it is always in the correct instantaneous
position for the instantaneous velocity it is imparting to
the wave.
Is that nonsense or is it equivalent to your much simpler
statement? I guess your principle is hidden in the
assumption I made that the velocity wave would be of the
same magnitude and shape as that of the piston. Without it
that wouldn't be true.
>
> On the other hand, the radiation impedance of a piston, either free or
> mounted in a baffle, is strongly frequency-dependant. In particular,
> this results in the circumstance that the piston has to vibrate with a
> much larger velocity at a low frequency than at a high frequency, if
> equal sound pressures are to be produced at a large distance from the
> piston. In a freely propagating plane wave, these two velocities are
> identical. The nett effect is that when, on this piston, a high
> frequency vibration is superimposed on a low frequency one, it
> effectively finds itself produced by a source moving at a velocity
> larger than that required by the superposition principle, which in
> turn will then produce a frequency-modulated output.
> The effect is quantitatively calculable. It turns out to be
> negligibly small for all except determined audiophiles in the case of
> speakers operating in normal listening conditions.
Ok, this is substantially different than my explanation for
why, in those conditions, Doppler distortion was in
evidence. My explanation was that Doppler distortion was
only in evidence when there was a difference in the coupling
of the piston to the air as a function of frequency, that is
when the pressure in the far field was a frequency dependant
function of the velocity of the piston. I think that is
equivalent, if using less appropriate and less exact
terminology, correct?
Many Thanks,
Bob
--
"Things should be described as simply as possible, but no
simpler."
A. Einstein