How can you demonstrate that sounds are waves produced by the vibration of mater…

How can you demonstrate that sounds are waves produced by the vibration of material objects? — TP, Huntington Park, California

I can’t think of an easy way to make sound waves visible while they travel through air, but it’s relatively easy to make sound waves visible as they travel through materials. If you choose a system in which the sound waves bounce back and forth many times through a material, you can sometimes see the sound waves as they move. For example, partially fill a crystal wine glass with water and then rub your wet finger gently around the rim of the glass. With some practice, you’ll be able to get the wine glass to emit a pure tone as your finger alternately sticks and slips its way around the glass rim. As this tone appears—the vibration of the crystal glass itself—the water will begin to exhibit beautiful ripple patterns. You should be able to see these ripples by looking at a bright light reflected from the water’s surface. The ripples are sound waves that are travel through the water, back and forth, as the glass vibrates.

Another system that makes the movement of waves visible is a stiff, thin aluminum plate that’s supported rigidly and horizontally at only one point. If you sprinkle fine sand lightly over the surface of this plate and then bow its edge with a violin bow, it will begin vibrating with a clear tone. As it vibrates, the sand will drift into places where there is very little surface motion—the nodes of the vibrating surface. Once again, sound waves are traveling back and forth across this surface and the up-down motions squeeze the sand into certain parts of the plate. In this case, the surface’s vibrations and the sound waves in that surface are the same thing—in example of the fact that vibrations and sound waves are intimately related and are in many respects exactly the same thing.

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