Sound + Fire = Rubens' Tube

So Dr. Phil, uh, what's going on here?

Okay, what we've got here is a metal pipe with a whole lot of holes in it. We're pumping gas through it, and we've lit it up, as you can see. So we have like a whole lot of, uh, buns and burners all in a row—a whole bunch of little flames burning gas. And um, this is all very pretty, but what does it have to do with science? Just you wait. I've got a speaker on the end here.

Whoa! So there you go, you're pumping a sound through the pipe. Now, is that right?

That's right. It's quite an annoying sound, Phil. Let me change [Music] it.

Okay, a few changes happening.

Yeah, so there's these areas where the flame is higher and other areas where the flame is not as high. What's causing that?

Well, it kind of looks like a wave, doesn't it?

Yeah, it definitely does look like a wave. But I mean, what is it showing us? Why has it got these high points and low points?

Well, sound is a wave. When I speak, my vocal cords vibrate the air, which sends a wave to your eardrum, vibrates your eardrum, and that's how you detect sound.

Okay, so how is this sound wave changing the height of these flames?

Well, the wave is actually areas of high pressure and low pressure in air. So there's actually a sort of compression traveling through, and that's what we can see is the pressure in the pipe here. There's areas where there's high pressure and areas where there's low pressure, and that results—the higher pressure is obviously pushing the gas out, uh, faster, and so we're getting these higher flames.

That's right, in a simplistic way. Pretty complex situation. But what about if we change it?

So you made it a higher pitch now, and I noticed that these peaks are sort of getting closer together.

That's right, yeah! So we've actually changed the wave by having a different frequency. We've made a short, shorter wavelength. We've changed the sound wave—changed the sound wave; we've made a higher pitch, so that's a higher frequency. Yep! It's vibrating faster, and to get all those vibrations in, it has to do it in a shorter space—in a shorter wavelength.

Shorter wavelength.

Okay, but uh, this is all very interesting, but you know, it's killing my ears! So what are we going to do?

Let's go for something a bit more musical, shall we?

All right, let's try a fairly simple tone. So I'll just [Music] whistle.

[Music]

So that's, uh, not bad. Let's have a listen to someone singing— you know anyone who can sing a bit?

I don't know anyone who can really sing.

Oh jeez! All right, it's just that you and me are made of these.

Hey, do you know any Mozart?

I don't. I'm sure—I'm sure you know a bit of Mozart. Maybe you heard it in the womb! Here, go have a try of this.

Okay, I'll just warm the orchestra up. Ready, you guys?

[Music]

[Applause]

[Music]

Than [Music]

[Applause]

[Music]

Very good.