Cup phone

Have you ever noticed that it's harder to hear people when they are farther away?

The sound of their voice gets harder and harder to hear until eventually...You can't hear them at all.

But there is a way to talk with your friend from several meters and yards away without yelling and you can make it yourself.

Befor we start building though. Let's talk about how sound works.

All noises, whether is piano music, bird singing, or talking are made by vibrations. Vibrations are made when something moves back and forth. When things vibrate to make sound, they are usually so small and are moving so fast that you can't even see anything moving at all. But you can probably feel it.

Hold your hand up to throat and say your name. Can you feel that sort of buzzing in neck? That's a vibration, the start of the sound of your voice.

What's vibrating here? It's two little things in your throat called your cords. You can also feel and see, the start of a sound when you strum a guitar. Can you see the string of the guitar moving really fast? That's a sound just getting started!

Now, after something like a vocal cord or a guitar string moves, it makes the air move too. And we know the air is made up of very tiny particles. This particles carry vibration through the air, so sound can travel!

For example, when a guitar string moves, the particles in the air around the string start to move too, and then they bump into other particles, making those move. These particles carry the vibrations through the air, until it hits your ear and you can hear the music. But the problem is the vibrations start to go away pretty quickly. After traveling through the air for a bit, the particals don't bump into as hard, and eventually the vibration stops. And if that happens before the vibration makes it to your ears, you don't hear anything. 

So now we know that vibrations can make the air move to produce sound. Do you think they can make other stuff move, too? Let's try it out!

You need two paper cups, a bit of tape, a sharp pencil of a pair of scissors, and a long piece of string. Use the scissors to poke holes in the bottom of each cup. Thread the string through each hole, so that cups are facing away from each other. If you can, tie a knot at the end of each string inside the cup. You can put a little bit tape on the string, too. Just to make sure it doesn't come out.

Now give one of the cups to your friend, and take one for yourself. Walk away from each other slowly until the string is in a straight line between two cups. And this is important to make sure the string is tight. Now tell your friend to put the cup over their ear while you talk into your cup. Can your friend hear you? Now trying to put the cup over your ear and give your a turn to talk. Can you hear them?

Are the cup string helping us hear? Let's do a test. If you keep your voice at the same level, and try to talk your friends without the cups, can you hear you? Well, yes, but not as well. So what's going on here? When you use the string phone, your voice make the air particles in the cup start to vibrate. These vibrations make the bottom of the cup vibrate, and make the string move the way. And then the vibrating string make the bottom of the other cup move and the particles in the air in that cup until it reaches your friend's ear!

Ok, let's try another experiment. stand a little closer to your friend. so the string hangs loose between the cups. Can you hear your friend? No, that's not working at all. What's going on? when the string is loose, it can's vibrate nearly as well, which means sound cannot travel through it. To see what I mean, ask you friend to move farther away again, so the string is pulled tight. Now pluck the string with your finger. Can you see the string move back and forth? Now stand closer together again, so the string is loose, and pluck it again. The string moves a little, but is doesn't keep vibrating. The string has to be tight for your phone to work. So vibrations can travle it.