How do vibrations make sound?
Sound is caused by a vibration. Now, some of you might be saying – What's a vibration? Well, vibration is just something that goes back and forth, back and forth. Now, if the object vibrates fast enough, it's going to disturb the air. If the air vibrates, our ears are sensitive enough to pick that up. Now, to show you what a vibration actually does, we have a tuning fork – and this tuning fork will vibrate at 426 times every second. So, when I hit this, these two ends will vibrate. Now, it's vibrating in the air, and we can't see what the air is doing to us, but our ears can pick it up. So, let me just hit this and listen. (Sound) Now, when it's vibrating, you might be able to see these ends are a little blurry, but you're not going to see what's really happening to the air. But, when I take this and touch this to the water – water's a lot thicker than air, so we're going to see what affect this actually has on water compared to the air. (Sound) And, that vibration actually disturbs the water enough for it to splash out. So, we have lots of vibrations going on here – 426 vibrations every second – moving the air – we can't see it, but when it touches the water, we can see what the vibration does to the water. Many, many things cause vibrations. One of the interesting things is this cup. In fact, when you look at this many of you might remember this as the old telephone. If we have another cup on this end, what we might have is a telephone. Now, when your voice vibrates – your voice actually vibrates the air and you talk into the cup, your voice vibrated the air, the air vibrates the cup, the cup vibrates the string, and if we had a cup on this end, this would also vibrate and the person could hear it – so, you could talk back and forth to each other. Now, I also want to show you this. This is just a cup with a string in it. You look at it and you say – well, this is just a cup with a string. But it was actually invented – in fact; this was invented in two places of the world many, many years ago. In South America, when this was invented, they made it with a wooden cup, they had a wooden stick on here, and when they pulled on the stick, it vibrated and made a sound. And, they still use this in South American music. It's called a cuaga. Now, with this one, we can pull on the string and actually get the string to vibrate, then the cup to vibrate, and we can hear it. So, instead of using it as a telephone, we're going to use this to create sound. So, if I take the string, and I pull on it like this, it almost makes a breathing sound, like Darth Vader. Now, we can change how this vibrates because my hand is sliding pretty smoothly on the string. Now, we can change how this vibrates by using a wet sponge or just making the string wet. And, when I put this on here, and pull on here, it makes a different sound. So, the vibration now is caused by water on here and water lets our fingers slide and it catches and it slides and it catches. So, if I pull on this again, you can hear what it does. Now, we can make cups vibrate differently. This cup has a special string on the bottom. In fact, it's not even a string – it's a ribbon of plastic. On the ribbon of plastic is actually a lady's voice recorded. And, there's little bumps on this plastic. In fact, I can see the bumps in this area and what this plastic ribbon will say is “Science is fun”, so let's listen. Did you hear that? Science is fun. Isn't that cool? So, I am vibrating this by pulling my thumb down. It's going over the bumps and actually reproducing that lady's voice. It works like a record.
