Patterns in superposition
Classroom Activity for 14-16
What the Activity is for
Hearing superposition, seeing a pattern evolve.
The idea here is to connect the somewhat abstract idea of superposition with a real physical experience on a human scale.
What to Prepare
- a pair of loudspeakers
- a signal generator
What Happens During this Activity
The loudspeakers should be connected to the same signal generator and facing a wide open space. If you have a large enough unobstructed space, this experiment can be very impressive. Some have even worked out of doors in a playground. It's important that you choose a suitably open space to avoid too many reflections. The loudspeakers should be several metres apart, even if in a laboratory. Choose a suitable frequency – several hundred hertz.
As the students move in a line parallel to the loudspeakers, so they will move through areas where there is constructive superposition and then into areas where there is destructive superposition. They will therefore hear a loud noise, followed by a somewhat quieter noise as they move along this line. Having a circulating queue is one technique that works well in the laboratory. Another is to use a higher frequency and simply to have the students move their heads approximately 1 metre from left to right, still parallel with a line drawn through the pair of loudspeakers. In the playground you might choose such a frequency that you are able to get a visual impression of the pattern stretching out over space by asking the students to choose a point to stand where they can hear lots and then, after a pause, where they can hear very little. If you're very lucky, you'll have an upstairs window from which to operate a digital camera in order to record these two states.