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I agree with most of what Tom is saying on this but I still think it's worth messing about with sound demos. Here's another idea which may or may not work: If you have a fairly large auditorium (we don't) string a loop of tight wire from the front to the back with two pulleys, one at the front and one at the back. (The whole thing should be like an old-time laundry line used in tenements.) and attach a little "car" holding a horn to this wire. Then use the pulley at the front of the room to move the car quickly to the back. My guess is that this would do quite nicely, at least for those sitting more or less in line with it.

By the way, I saw a re-creation of the original train experiment on a PBS show years ago where they had a group of trumpeters standing on a flatcar being pulled by a steam locomotive of the era. The whole "band" played a set note (I think "A") and proceeded down the track past the observer. All the time the whole ting was very well explained. You might want to contact PBS to see if you can track this video down. It was fun and great!

John Bowditch

> ----------
> From:         Tom Ford
> Reply To:     Informal Science Education Network
> Sent:         Friday, August 13, 2004 9:49 AM
> To:   [log in to unmask]
> Subject:      Re: doppler shift demo
> 
> ISEN-ASTC-L is a service of the Association of Science-Technology Centers
> Incorporated, a worldwide network of science museums and related institutions.
> *****************************************************************************
> 
> 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
> >[log in to unmask]
> >
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