From Scientific American:

Introduction
Have you ever tried making “walkie-talkies” using a long piece of string and two tin cans? If you have, you know that they work surprisingly well—at longer distances you can hear people better through the cans and string than you can through the air!

In this activity we’re going to use the same concepts to build a personal bell, one that makes sounds that only you can hear!

Background
If you’ve ever been near a speaker with a loud bass (or heard a car drive by with the radio turned up), you may have experienced the “buzzing” feeling in your body caused by the loud noise. This isn’t your imagination; the sounds we hear are actually vibrations traveling through the air—or through other materials, as we’ll observe in this activity.

Most of the sounds we hear come to us through the air. When your friend calls your name, your friend’s vocal chords cause vibrations in the air, which travel through it as a sound wave and arrive at your ears. Sound waves, however, can travel through other materials, too, and in fact many materials are much better than air at transmitting sound! You can experience this for yourself by gently tapping a metal fork or spoon against a countertop, and listening to the sound. Next, put your ear to the countertop and tap the counter again with the fork or spoon. The sound should be much louder, because the counter is better than air at transmitting the sound vibrations caused by the tapping!

The difference in how well a material can transmit sound is determined by the material’s density, or how closely packed the molecules (that make up the material) are to one another. Imagine a row of dominoes. If the dominoes are far apart, one or two of them can fall over …

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