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Illustration 18.2: Molecular View of a Sound Wave

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A sound wave is a longitudinal wave. Restart. In a longitudinal wave, the waving of the medium (here the air molecules) is in the direction of the propagation of the wave. In show wave/hide molecules mode, we see a speaker, which is the source of the sound wave, and wave fronts propagating toward the detector (the man's ear). This is the way we usually think of sound waves: originating from a source and propagating toward a detector. But what is really going on in the medium as the sound wave goes by?

In show wave/show molecules mode, we see the individual air molecules, which are the medium in which the wave travels, and the waving of the medium. Consider the motion of the red air molecule. The molecule oscillates back and forth about its equilibrium position. If we were describing the sound wave in terms of the individual molecules, we would call the wave a displacement wave. It turns out that the amplitude of the displacement wave is only on the order of 10-6 m! The other way to describe the sound wave is in terms of the pressure wave that travels to the right. The pressure wave fluctuates ever so slightly about atmospheric pressure.

Illustration authored by Morten Brydensholt, Wolfgang Christian, and Mario Belloni.
Script authored by Morten Brydensholt, Wolfgang Christian, and Mario Belloni.

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