March 04, 2013

Why I don't like the "Doppler Rocket"

It's time for me to rethink how we look at the Doppler effect in class.

When I'm working with a class on the physics of sound, whether it is a general physics class or a physics of music class, the Doppler effect is always one of the topics in whatever textbook we are using. Time permitting, we will look at the Doppler effect.  I think it's an interesting topic, and it certainly has important and useful applications in other fields of science: astronomy, medicine, weather, etc.

What I can't stand, though, is the terrible quality of the standard acoustic Doppler effect demos. I'm talking about the demonstrations where you take a sound source and tie a string to it, then whirl the string around your head with the sound source making a circle around you. The class is supposed to hear the change in pitch alternating between getting higher (while the source is moving toward them) and going lower (while moving away from them).

Unfortunately, the demo often has two major flaws with it:

1.) The change is pitch is often within the just noticeable difference (jnd) for non-ear trained musicians.


b.) The change in pitches is almost always overwhelmed by the observation of the relative change in AMPLITUDE.  As the source moves away from the class, it seems to be less loud, and louder as it is approaching the class.

The situation is not much better when the demo is using a "Doppler Rocket" or "Doppler Ball" where a sound source is embedded in a soft ball then thrown or slid along a guide string across the room.  While it is usually the case that the frequency change can be more noticeable with these demos, since the speed of the ball can be high enough to make the frequency be outside the average jnd, my experience has been that the change in amplitude is even more dramatic with the Doppler Rockets.

What to do?  For me, it's interesting. I mean, one of the main ideas in science is that we only want to test one thing at a time. But in this experiment, we seemingly have two inextricably linked quantities that are changing.

I don't have a real good answer for what I want to do to get around these conceptual challenges. I've tried an Interactive Lecture Demo style presentation on the Doppler effect without the success I was hoping for.  I may try that approach again, but I'm sort of leaning towards making the Doppler effect a lab activity where students have to confront the two aspects of the experiment (frequency and amplitude) and tease them out separately.  I'm not sure if that will work at the introductory level, but I'm willing to try.

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