Uh-oh. It’s a musical How. Who gave it to Gareth? Own up!

Oh heck, he’s actually going to play, too. At least you can’t hear this on the website.

When a guitar string is plucked, it vibrates – you can just about see its blurred motion in this pictures (click it for a larger view). The pattern the string makes is called a ‘standing wave.’

Electric toothbrushes are extremely useful for looking at standing waves. Here are some, with lengths of string tied to them.

When Gareth turns the toothbrush on, it starts to wobble the string, and that sets up a standing wave just like plucking the guitar string did. You can see that at the end, where it’s tied to the toothbrush, the string isn’t moving. That’s called a ‘node.’

In the middle, the string is wobbling from side to side – the standing wave.

…and at the bottom, where the string is tied off, there’s another node.

Now, if Gareth plucks his guitar string again, but this time he lightly touches the string about half-way down, you hear a distinctive ringing sound – he’s played a harmonic.

You can see that with another toothbrush:

Here’s the node at the top.

Here’s a second one in the middle. That’s new.

And here’s a third node, at the bottom. What the other two toothbrushes were for we’ll never know, because the How was ‘How can you see a standing wave with gas?’

Gareth has a gas supply connected to a long pipe, which has small holes all the way along the top.

As the gas comes out of the holes it burns, so there’s a neat row of flames, all the same height because the pressure in the tube is the same all the way along.

But plugging one end of the tube is a loudspeaker, which is connected to a device that generates musical notes. And when that’s turned on:

Look at that! The note – the sound wave – from the loudspeaker changes the pressure down the pipe. That changes the speed of the gas jets, and hence the heights of the flames. So you can see the pressure wave of the sound in the heights of the jets.

The jets are very small wherever there’s a node in the standing wave. So if you change the note…

…you change the positions of the nodes, and hence change the heights of the jets. So that’s how you can see a standing wave with gas.