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Tuba Physics

Tuba Physics

Justin Mitchell: So I’m going to try to explain a little about how tubas work, but before I do that I want to do two other things. I want to describe how sound works, and then I want to describe a little bit about how instruments work in general. Then we can get on to how tubas work, and how the whole brass/wind family kind of works. So to talk about sound, we have to realize that any kind of sound that we hear at all is just stuff moving around in some kind of way that we can hear it. So some object’s going to move, it’s going to move the air around, and then that air is going to end up moving your eardrum around, and that’s what you’re going to really detect as sound. So if I’ve got some plate, like this right here, just a square piece of metal, that I can shake up and down with this mechanical vibrator, then I can end up getting a sound to come out of that. Well, if I put some sand on top of this, we can see how that’s really an object that’s moving around. So we can see that the plate’s moving because the sand is moving. We can also see that the plate is moving in a certain pattern because I have a pattern with the sand. The whole plate seems to resonate with that one frequency. So that’s part of how instruments are able to be so loud is that they work in these certain tones and these certain frequencies.

So we’ve seen how sound works, and now we’re going to start to talk about how individual instruments can work. So instead of starting with a brass instrument like a tuba, it’s a little bit easier to see what a stringed instrument does, because you can see the thing that’s actually moving. So in this case I have a string lined up and on this side off of the table I’ve got a weight hanging down from the bottom. And I can adjust how fast I move this end up and down. So right now, I’ve got this really nice wave sitting here on the string. If this was a tuba, say, then this part moving up and down would be your lips going back and forth, right? But it’s only going to be able to pick out certain ones of those, just like we saw with the plates. So in this case, if this thing was now the length of the tuba, I’d only get this one kind of tone.

So the wave that we saw on the string just a second ago is going to be a lot closer to explaining this guy than it is to explaining a tuba, but if we can figure out how a guitar works then we’re going to have a much better chance of figuring out how a brass instrument works. So with this guy, I’ve got a bunch of different strings, they’re all different weights, and they’re all at different tensions, and I can change the length of that string. So as I go from one note to the next, I can do it by moving the length of the string itself. Now I can do that with other kinds of instruments, too. And the one where you can really see that happening is with a trombone. You don’t actually have to have a real trombone, because you can just make one out of two pieces of PVC for about four bucks. And you can see that as I change the length of this thing, I can get a slightly different tone. So as I make the thing longer, I get a lower note. As I make the thing shorter, I get a higher note.

So the difference between a trombone and a tuba is that on a tuba you hit a key and that’s going to then let your breath go through another loop of the brass instrument itself. On a trombone, you just move your arm to make the thing longer. So it’s the same kind of thing. We’re making the length of the instrument different so that we can get a different kind of note. Now the reason it’s so loud is because we’re just using those kinds of pitches the instrument wants to use. It’s just like on the string — we were using just those frequencies that were able to give us an entire wave — or on the plates how we were just trying to find those frequencies where we could get the patterns in the sand. Now if I just try to talk into this thing, it’s not very loud at all — but you can hear a noise. But if I try to use the pitches that the instrument wants to have, the ones that it resonates at, then it gets a whole lot louder — way louder than I could get just by talking and just using the same amount of breath coming out of my mouth.

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