"Bob Cain" wrote in message
...


Porky wrote:

If you mean that a whistle riding on a moving train, or a speaker
swinging back and forth (or spinning round and round like in a Leslie
speaker system) in a repeating oscillation cycle will produce Doppler
shift,
but a stationary speaker reproducing a complex waveform containing a
mixed
LF and HF tone (or any multiple combination of tones, as would be the
case
in a complex musical waveform) won't produce Doppler shift, then, by
golly,
I think you're right!

I even finally agree that in many cases it has the
properties that can be called Doppler distortion, so it is a
real phenomenon in qualified conditions but not for the
reasons usually given and not from the systems for which it
has been claimed.


I would go so far as to say that anything generating sound and in motion
relative to the listener will generate Doppler shift, but in the special
case of a speaker reproducing a complex waveform consisting of more than one
pure tone, the velocity of the source relative to the listener is
effectively zero, and therefore no Doppler shift will be generated, because:
1) there is one complex waveform driving the speaker and producing the
energy necessary to generate the sound, not some higher frequency or
frequencies "riding" on some lower frequency.
2) the effective sound source is not the speaker cone, but some point or
plane which does not move relative to the listener. The moving speaker cone
just provides the mechanical energy which is transformed into acoustical
energy by it's interaction with the surrounding air. Each instantaneous
point in the complex motion of the cone is in direct co-relation to the
corresponding instantaneous point of compression or rarefaction in the
complex waveform and the notion that the source has motion relative to the
listener is an illusion. The sound wave can be pictured as standing still
with each bit coming into existance as the cone passes through that space.
The sound wave might be represented as:
-- - - -- - - --
with the space between the dots representing compression and rarefaction of
the wave, and the cone's motion producing the sound can be represented as:
]-- - - -- - - --
]- - - -- - - --
] - - -- - - --
]- - - -- - - --
]-- - - -- - - --
]- - - -- - - --
] - - -- - - --
]- - - -- - - --
Note that while the cone is moving, the apparent source is not moving, and
this applies no matter how complex the waveform is. This can actually be
measured with a sensitive pressure meter and graphed, showing that this is
really what is happening.
3) the effective sum of all velocities is zero relative to the listener
4) due to the nature of how a speaker produces sound, the listener is
effectively "riding on the train"
5) In order to produce Doppler shift, there must be a sound source in motion
relative to the listener and in the case of a stationary speaker there is a
complex sound source, but no motion relative to the listener
6) If a speaker did generate Doppler distortion, simply turning it so that
the listener was looking at the edge of the speaker would eliminate the
motion toward and away from the listener (the cone would move side to side,
but would stay at a constant distance from the listener), and thus would
eliminate Doppler distortion. I don't believe that this is the case in the
real world.
7) the empirical measurements I made indicated that there was no audible or
measurable Doppler shift when the Doppler equations predicted that there
would be audible and measurable shift, therefore the Doppler equations do
not apply to this special case.
8) I just have a gut feeling about it
9) none of the above
or
10) any of the above
Personally, I prefer number two.