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The context of the original question was related to celestial observations
and (ordinarily) we don't listen to the stars! I have already posted
sufficiently with respect to the doppler interpretation of wave fronts. If
the discussion is now restricted to audible demonstrations for large
audiences, I have to agree with William that most of the demonstrations are
unsatisfactory in that they are confusing for a neophyte audience, even
using the demo he proposes. A major difficulty is that the effect is not
uniform for all observers in a typical classroom or auditorium and that has
to do in great measure with the distance the moving source is from the
observer. A foam football with a sonalert and battery is a help if there is
time to listen to everyone be the passer and the receiver.  A sound source
moving directly toward or away from the observer conveys the best
information. If the sound source is passing the observer, the time it takes
for the up-doppler to become the down-doppler depends on the passing
distance. For my students, this was a common task to calculate and graph
this for an assortment of speeds and distances.

The most satisfactory endeavor evolved from an original attempt to record
the doppler shift of an automobile horn. It happened that this gave good
results on the approach only. When a separate 500Hz horn was rigged with a
switch to plug into the cigarette lighter socket, a passenger could keep it
aimed out the window at the stationary observer at the side of the road who
similarly kept the microphone aimed at the vehicle. Driving by at different
constant speeds and different passing distances gave data that could be
actually analyzed on a computer back in the classroom and compared with the
student graphs. My point here is that the recording itself was an excellent
demonstration tool, even without the analysis. If I were doing it again
today, I would do it with a digital video camera instead of just an audio
tape. This would make it easier to clean up the recording for analysis and
the playback would be more self explanatory. Asking people to "remember"
something they might have witnessed is dangerous to proper concept
formation, but using this form of playback would enable all listeners to
witness the same thing at the same time and be guided to distinguish
between changes in intensity (sound doesn't have "volume") and changes in
frequency or pitch.

Tom Ford


At 10:22 AM 8/12/04, you wrote:
>ISEN-ASTC-L is a service of the Association of Science-Technology Centers
>Incorporated, a worldwide network of science museums and related institutions.
>*****************************************************************************
>
>Unfortunately a number of classroom-sized doppler effect demos (for
>after-the-fact demonstrations) aren't that accurate.
>For example I recall the aluminum rod demo where the teacher moves his arm
>(with the singing rod) forward and backwards.  In that one, most people
>will buy that it's a pitch phenomena (because that's the topic you're
>talking about) unless they really have an ear for pitch in which case they
>will realize that the pitch doesn't change noticeably - what does change is
>the volume.  The primary factor there is actually one of volume (the speed
>you can move the rod forward with your arm isn't that appreciable, and if
>you leave your hand extended forward it still basically sounds the same).
>  However, using a rotational motion you can achieve substantial velocities.
>  If you really want to see if the doppler effect is achieved, use a sound
>source that clicks - you should hear the clicks coming more rapidly or less
>rapidly depending on the item's motion.  With a lecture hall/auditorium,
>you could arrange for a device to fly overhead with such a sound.  Everyone
>knows of the changing sound of a motorcycle, and can relate to that
>wavelength change partially because it doesn consist of clicks that turn
>from a whine to a low pitched clicking sound.
>
>-William
>
>William Katzman                    "The important thing is to never stop
>questioning." -A. Einstein
>Director of Exhibits               "Are you sure about that Al?" -His wife.
>Catawba Science Center
>ph: (828) 322-8169 x307
>fax:(828) 322-1585
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