Musical Bottles is Sound Science

Hello, I'm Bruce Janney, and today on Mate Science, I want to take a look at making sounds using glass and plastic bottles. Now we have some great demonstrations, so let's get started. Now I think most of us have blown across the top of a water bottle, and it has a pretty nice tone to it. And of course, if we take a few drinks out of it and we try it again, you'll notice that the pitch has dropped. And as we continue to drink, the pitch will get lower until it gets empty, it would sound something like this. And if we want to go lower, we simply go to larger bottles. So this leads to the idea that if we have a set of bottles, we can adjust the water level in each one so that we can make up some type of musical scale. Now I really like doing this with bottles because there's a second demonstration we can do with it. As we go from this end to this end, we notice that the pitch drops because it's a change in the air column. But let's take a look at the bottles themselves. Now the sound's coming from the vibration of the bottle itself. The water actually has a dampening effect on the vibrations of the bottle. The more we add, the lower the pitch. It's just the opposite with the air column. Now this behavior is an example of a Helmholtz resonator, and the air goes in and out of the bottle and behaves somewhat like this spring. In this case, we could say it's the springiness of the air. Adding water to the bottle speeds up the movement as the air compresses more quickly. By blowing across the opening, some air goes into the bottle, compressing the air inside. Pressure inside the bottle forces the air back out, forming a sound wave as the air exits the bottle. More air goes back into the bottle, building up pressure again, and is again pushed out. This process continues at the resonant frequency of the bottle, producing the sound we hear. How quickly that air vibrates is determined by the length of the column, but it's also determined by the density of the air itself. So if we change the density of the gas inside, we'll also change the pitch. Now we can make the air denser by cooling the bottle. That should lower the pitch. Now let's see if there's any difference. That worked pretty well. Now going in the opposite direction, I put a bottle on the dashboard of my car so that it heated for a few hours. Ouch! Now let's try it again. This bottle was at room temperature, this one was in my car. Now we can also change the type of gas. I'm going to generate some carbon dioxide by mixing some baking soda and vinegar. I'll then pour the heavy gas into the soda bottle. Air. Carbon dioxide. So a gas that's denser than air will lower the pitch. Now let's see what happens with helium. Helium. Regular air. Helium. Now without changing the volume, it's also possible to change the pitch of the bottles by changing the density of the liquid inside. Alright, we'll start with rubbing alcohol. Now rubbing alcohol is less dense than water, but it's more viscous, so I'm not sure what we're going to get. Here's our water. Rubbing alcohol. Now let's try it again, this time with salt water. Adding salt to water should make it denser. And once more we'll try it with dish soap. So if the liquid is too viscous, it stops the ringing altogether. So we can change the pitch of the bottles simply by changing the volume of liquid inside or changing its density. Now we can do the same thing with these plastic bottles. So I think there's a better way to get the bottle skin to change its pitch, and that's simply to seal it and then compress the air inside of it. Now here we have eight of them. Now they're all pressurized, except for this first one. Now let's take a look at the setup. It's a quick build. All you need is a hole in the cap, a tire stem, and you pull it through to get a tight fit. Pressurizing the bottle will help give it a good seal. All right, my last two demonstrations today are my attempts to make musical instruments out of some plastic bottles. In the first case, I connected two plastic bottles with a plastic hose. They're held in place by friction and a little bit of sealant. As I lower the one, you'll notice the water drops in the other one. If I raise it, the water increases. And so I don't need several bottles. I can simply adjust it. Now I don't think this will ever catch on as a musical instrument. Now I think the second piece works a little bit better. It is simply a bottle that was cut at the bottom. It has a screw going through a washer, through the bottom, and then into the handle. The bottom has a few cuts in it to help it fit back inside. And to play this piece, I simply push the bottom up into the bottle. Well, there it is. I hope you enjoyed this video. And as always, I want to thank you for watching. Come back and see us again. Okay, bye!