-------- Original Message --------
Subject: Experimental Evidence for Dynamic Doppler Shift
Date: 23 Aug 2004 16:32:06 -0700
From: (The Ghost)
Organization: http://groups.google.com
Newsgroups:
alt.music.home-studio,rec.audio.tech,rec.audio.pro,alt.sci.physic s.acoustics
References:




"Karl Uppiano" wrote in message
.. .


That's why my initial reaction to your experiment was positive. My mistake
(and others also) was in jumping to the conclusion that your results could
be applied directly to Doppler FM in a loudspeaker. It cannot, and you never
claimed they could, which is a "Good Thing".

My measurement results can be applied to a loudspeaker but only to the
extent that they demonstrate that dynamic Doppler shift can be
produced by a loudspeaker. Whether or not Doppler shift produced by a
loudspeaker can be measured is another issue. That is going to depend
on the relative amount of intermodulation distortion that the
loudspeaker also produces because the Doppler FM components and the
intermodulation distortion components are at the same frequencies.
Personally, I wouldn't waste my time trying to separate the two.


................... AM and FM sidebands look fairly similar at low
FM modulation indices. They might be hard to distinguish, although IIRC,
upper and lower FM sidebands are opposite in phase.

The FM demodulator that I used was a precision frequency-to-voltage
converter which operates on zero crossings and, as such, is completely
insensitive to amplitude variations of the signal being demodulated.


The low frequency radiation is much less efficient for the reasons you
state. A loudspeaker, on the other hand, radiates all frequencies in its
usable frequency range with nominally equal efficiency. That is not the case
with your experimental setup, as you point out.

I not only believe that it is not only relevant, *but crucial* whether the
low frequency component is radiated, to the issue of dynamic Doppler shift.
The entire energy equation changes. Instead of a high frequency sound source
moving back and forth in free air, we are instead summing two acoustic
waves. It's entirely different.

.............. It isn't sufficient to
get a louspeaker to produce something that "smells like Doppler FM". You
need a model that predicts how much, at what freqencies, in what phase,
etc., and then you need to get good experimental agreement with your
prediction. Otherwise, you could be measuring anything -- other forms of
distortion, environmental noise, sampling errors, etc. In the end, it must
all be accounted for if it's the *truth* you're looking for. Unsubstantiated
claims are much less demanding :-)

Not surprisingly, we again mostly disagree. In the early stages of
research, where the issue is whether or not a phenomenon exists, the
first order of business is to demonstrate experimentally the existence
of the phenomenon. After the existence of the phenomenon has been
demonstrated, model development and quantative predictions follow.
For whatever reason, you seem to refuse to distinguish between the
existence of a phenomenon and the accurate quantitative description of
a phenomenon. The absence of a quantitative description of a
phenomenon is not a justifiable excuse for denying its existence. In
the history of science, there are many phenomena that were
demonstrated first and quantified later.


You haven't proved anything until you have a high degree of agreement
between your prediction and your measured results, and you can account for
any remaining errors to the satisfaction of all reviewers. That's what's
meant by "reproducible results".

If you believe that, then you have a lot to learn. It has been my
experience over many years that models go up in flames, more often
than not, becasue of fundamental phenomenological
inconsistencies/inadequacies rather than because they did not produce
a sufficiently high degree of agreement between prediction and
measured results.