## Ride the wave to learn how sound is transmitted

Imagine that you are carefully submerged in a pool of water to the level of halfway up the pupil of your eye. Half of your view is above water and the other half below. Imagine that the water is as smooth as glass.

Then imagine what happens if someone leans over and drops a perfectly round pebble directly in front of your nose.

All of the water molecules surrounding the pebble will be pushed to either side. However, there are innumerable water molecules pushing in from all directions. So the water molecules that are displaced by the pebble can’t move very far. Since there are more molecules to push back against those displaced by the pebble, the water level must rise in front of your pupil and depress in front of your nose.

As soon as the pebble has fallen past your nose, the water molecules that have been crowded together fall back into their original position. What you see in front of your eye is the water level rise and then fall back. We call this oscillating motion a wave, and we illustrate it with a wavy line. I think I feel a little queasy.

Molecules make waves in the same way whether they are in liquids, gases or solids. If we push on molecules, they bunch together and then slip back, creating a wave. In fact, the molecules of a solid are closer together than they are in liquids or gases, so waves often travel better in solids.

At our house we can hear the dishwasher turn the water on and off clear in the back room by the sound of the valves closing. The sudden closing of the valves pushes water molecules together, which pushes pipe molecules together, which creates a wave in the pipe the sound of which travels across the house. The vibrating pipe pushes air molecules together and creates a wave that we call a sound wave. I wonder if a person can have imaginary sea sickness.

Anyway, events that seem complex are often simple events put together in complex ways. In this case, a wave creates a wave. Here is an example of a wave creating a wave, which creates a wave, which creates a wave: A violin player creates sound by drawing a bow string across a violin string. The bow strings are covered with a sticky substance in order to make the violin strings stick to the bow string. The sliding bow string causes the violin string to travel with the bow for a time before slipping in the other direction. Then the bow string catches the violin string again a moment later. These movements make the violin string vibrate in a wave, our first wave.

The violin strings themselves move very little air, but the violin string vibrations are transmitted to the violin body through a piece of wood called a bridge that stands on the violin body. When a string is bowed, a force is created in the direction of the bow’s motion and the force on the bridge increases in the direction of bowing. Then when the string slips, the force reverses direction. The oscillating string causes the bridge to oscillate in a “second wave.”

The vibrating bridge causes the top of the violin to vibrate in yet a “third wave.” The oscillating top of the violin pushes air molecules together, and it’s this motion that causes a sound wave to form. The sound wave is created both inside the hollow violin as well as outside it. The holes in the top of the violin are made to let the inside sound waves out to increase volume and tone.

The sound waves strike your ear causing your ear drum to vibrate in, yes, another wave. Then ... No, let’s stop all the wave action right here. I’m definitely feeling kind of sick.

# # #

Gary McCallister is professor of biology at Mesa State College.

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