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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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