Explorers Club

In November 2009, the Explorers investigated the nature of waves.

This activity began with a simple demonstration using a tub full of water. The students correctly predicted that a wave in the water required some initial disturbance: a pencil dipped in, a thump on the side of the tub, whatever. They were less certain about Mr. Ramsey's next question: does the water in a wave actually move? Using a floating pencil gripper, the students observed that in fact the water does not move, at least not in the sense of going anywhere. The gripper merely bobbed up and down as waves passed through the water and bounced off the sides. If the water itself was moving, the gripper would have been carried along for the ride. From this, the students learned that a wave does not involve a material actually moving—instead, it involves a disturbance moving through a material. We call this material, whatever it might be, a "medium," and a wave is a disturbance that passes through a medium.

Of course, water is not the only medium through which a wave can move. Air is made of matter, and can be disturbed in a way that causes a wave to move through it. We call this disturbance a sound wave, and it is caused by a vibration. The students shook their hands back and forth, but no matter how fast they shook they could not create a sound wave, because a person cannot shake his or her hand fast enough. Putting a finger against their throat and humming, however, the students were able to feel—and hear—a vibration that can cause a sound wave, as their larynx (or voice box) is able to vibrate at a much, much higher rate than their hands. When someone hums or speaks or sings, their larynx vibrates and disturbs the air in their throat, thus creating a sound wave (with the pitch of the sound depending on the specific vibration rate). The sound wave then travels out of their mouth, through the air, and hits the ear of a listener. The ear is really just a collector for sound waves—it funnels the waves inside a person's head, where several small bones are in turn vibrated by the wave. These bones then pass a signal to the brain, which interprets the signal as sound.

Air is not the only medium through which sound waves can travel, though. Simply knocking on a door so that someone can hear it on the other side is proof of this. All a sound wave really requires is a material that has molecules packed close enough together to pass along the vibration. The Explorers tested this effect with some string and a few forks. Tying the fork into the center of a piece of string, then holding the tips of the string tight into one's ears while a partner taps the fork with another fork, is a nice test of whether a sound wave can travel through string. The students discovered that if the string was slack—for example, if it was lying on the desk—then nothing would happen, because the sound wave is dampened out by the slackness. When the fork is dangled in front of someone, though, and is therefore held taut, tapping it produces a very peculiar noise and sensation in the ears of the person holding the ends of the string—some students described it as a ringing, others as a "twang." Regardless of the description, it made a definite impression!

Next it was time to test how effective string could carry a vibration over a distance. The students moved out into the hallway with the string and some paper cups. Poking a hole in the bottom of two cups, each pair of students worked the ends of the string through the holes and secured them into the cups by tying them to paper clips. The students then moved apart by up to 50 feet or more and tested their "string phones" by having one partner speak into a cup while the other partner held the second cup to an ear. They found that if they allowed the string to hang loosely, they again heard nothing, just as with the forks. However, when the string was pulled tight, they could hear each other fairly clearly through the phone. The effect would probably have been even more clear, but many of the Explorers found themselves having a difficult time keeping quiet outside of the normal confines of the classroom—as they discussed with Mr. Ramsey once back in the room, hearing a faint vibration in a string phone can be a difficult proposition when there is a lot of yelling and laughing go on around you.

Still, the general concept was observed to work, and the students enjoyed experimenting with a couple different thicknesses of string. As a final experiment, one length of string that already connected two phones was linked with another piece of string leading to a third phone, creating a three-way conversation. This turned out to be a limited success, as the vibration being carried was split into a couple of parts and became weaker. Some of the students could hear a conversation in some directions but not others, while observing that the clarity of the words being spoken was much less distinct. Still, the extra string did manage to carry some of the vibration.

To see some photos from our sound wave activity, click on the word "Gallery" below.

Gallery