Any time you hear a sound, it is because something is vibrating. Place your fingers against your throat and hum. You will feel your throat vibrate. We usually hear sounds from vibrations that travel through the air, but solids can be much better at carrying vibrations.
For this experiment, you will need:
- 2 metal spoons
- about 4 feet of string
OK, this experiment will look a bit silly, but it really is impressive. Tie one end of the string to the handle of one of the spoons. When suspended by the string, you want the spoon to hang with the bowl hanging down. Tie the other spoon to the other end of the string, in the same way. Hold the string in the middle, and swing the spoons so that they bump into each other. Notice the sound that they make.
Now comes the part that looks silly. Bend slightly forward. Place the string over your head, so that the middle rests on the back of your head, and the spoons hang down on either side. Stick your index fingers into your ears, as if you were trying to block out a loud sound. Move the string so that it hangs across your index fingers, just outside your ears. Move your head from side to side, to get the spoons to swing. You want them to bump into each other. When that happens, notice the sound. It is very different, sounding more like large bells chiming.
Why? When you held the string in your hand, the bumping spoons made a clinking sound. The spoons were vibrating, and that vibration was transferred to the air. The vibrations in the air spread out in all directions. When the vibrations were transferred from the air to your ear drums, you heard the sound.
When you had the string over your fingers, some of the vibration could travel from the spoons to the string. The string vibrated, and since it was across your fingers, your fingers vibrated too. The vibration moved from your fingers to your ears, again letting you hear a sound, but this time the solid string and your solid fingers carried much more of the vibration to your ears, so you hear a much louder, richer sound.
Because solids are denser than gases, they often pass along the sound vibrations much better and faster. That is why you can hear a train approaching by listening to the rails long before you can hear it through the air.