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I guess it's time for me to weigh in on this one. The way this was demonstrated in  the late 19th century (other than the classic train demo done with musical horns) was to use a device known as the scientific siren. Helmholtz, Tyndall, et al, used a siren running at a constant speed, producing a known pitch and then they moved the nozzle plate in relation to the spinning siren wheel. This works very well (I've built one of these recently) and it gets right at the heart of the frequency shift question because the output frequency does shift as the nozzles are moved either in the direction of rotation or against it. You can easily hear it shift and the shift is directly proportional to the speed of the wheel and the nozzles.

Check out John Tyndall's classic work on sound entitled "Sound" to learn more and to see an illustration of the device used.

Hope this helps.

John Bowditch





> ----------
> From:         William
> Reply To:     Informal Science Education Network
> Sent:         Thursday, August 12, 2004 10:22 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.
> *****************************************************************************
> 
> 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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