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Quick example of student framing

September 19, 2013

In Physics III, one of our experiments involved measuring the amplitude of pressure variations at several different distances from a small speaker. The data wasn’t real clean, but students used it to take a quick look at how the intensity dropped of with distance.

On the next homework assignment, I asked the following question:

Equation 17.13 in the book says that the intensity of a sound wave falls off as 1/r^2. However, in lab you found that the intensity from a speaker fell off slower than 1/r^2. Why might this be? (You should probably draw a sketch for a point source and a speaker to help you answer the question.)

We hadn’t talked intensity in class so I attempted to point students in the correct direction by suggesting that they make a sketch. To my surprise, almost every student talked about various possible sources of experimental error in their answer and almost no one drew a sketch. It seems that the mere mention of an experiment from lab cued students to frame this question as an experimental error question rather than a question about intensity and about point sources vs. directed sources.

I will try to remember next semester to ask the same question but referring to a generic speaker rather than the speaker in lab and see what kind of responses I get.

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