RUBEN MEERMAN

I've been running up and down beaches all my life and one thing I've always wondered is why do some beaches squeak and others don't?

NARRATION

This is Hyams Beach, the Guinness World Record holder for the whitest sand in the world, and also rating pretty highly on the squeakometer.

RUBEN MEERMAN

This is whale beach, just 225km north but no matter how hard you try, this sand just won't squeak.

NARRATION

Sandman Dr Tim Senden is an expert on how fine particles like sand behave.

RUBEN MEERMAN

So Tim what is happening under my feet?

DR TIM SENDEN

Look that's a really great question, why don't we take a sample and get back to the lab.

RUBEN MEERMAN

Great idea. A bit of squeaky sand in the bottle and back to the lab we go.

RUBEN MEERMAN

So Tim, this is the whitest sand in the world?

DR TIM SENDEN

That's right. From Hyams Beach. It's almost pure quartz. The grains are very well rounded, there's no sharp edges and there's no shell material.

RUBEN MEERMAN

Yeah. It's absolutely beautiful. And what about the um non-squeaky sand that I've picked up?

DR TIM SENDEN

Right. Let's have a look. This is from Whale Beach.

RUBEN MEERMAN

Ah.

DR TIM SENDEN

Now, by contrast you can see that there's the essence of quartz but lots of shell material.

RUBEN MEERMAN

Right. And it's all different shapes.

DR TIM SENDEN

So it's a very different type of granule material. It won't flow like the other sand.

RUBEN MEERMAN

So squeaky sand is smooth and round a bit like marbles, so the most obvious question is, is the squeak coming from the grains just rubbing together?

NARRATION

While friction is one theory, it's still unproven.

DR TIM SENDEN

I don't think it's about friction. I think the key thing is with these grains, they're small - they tend to roll out of each other's way. It's more about how it flows. Imagine that these are the grains in our squeaky sand.

RUBEN MEERMAN

Ok. What, what are going to do to them? Well you'll find pretty quickly that some of these are weight bearing and some of them are not, and at some point the whole system will flow and we'll see an Avenlanch.

So if I take that out... Right.

DR TIM SENDEN

And that's a flow.

NARRATION

But insert a few long plates to represent those shell fragments and the sand will just slip, and lock. It won't flow like squeaky sand.

RUBEN MEERMAN

So squeaky sand flows more like a liquid than non squeaky sand... but now you might be wondering what that's got to do with the noise?

NARRATION

A big clue might lie in another rare, sand phenomenon. These fun loving scientist are trying to work out the mechanism of the booming noise. Like squeaky sand, booming sand is smooth, and flowing. There's a lot of debate, but it's thought that when the sand avalanches it sets up a series of vibrations between the grains.

DR TIM SENDEN

In booming sand you have a, a situation where a lot of grains avalanche together. They all flow together in unison. That gives a very deep rumble. I think that the squeaking sand is just the same thing on a smaller scale. And so the pitch changes as well.

NARRATION

But what actually makes the sand squeak? Tim thinks the answer might be lurking in yet another strange property of hard granular materials.

DR TIM SENDEN

So Ruben you know when you're walking along the beach and you step on wet sand it goes dry around your footprint?

RUBEN MEERMAN

Yeah.

DR TIM SENDEN

Have you ever wondered why that is so?

RUBEN MEERMAN

Yeah. I thought it was just your foot pressure squishing all of the water out of the way.

DR TIM SENDEN

Yeah. Actually it's not that simple, When you press onto grain - specially your regular grains, like sand - they rotate against each other and push each other out of the way slightly and it opens up the space between them. The water rushes in to fill that new space and so the sand goes dry around it.

RUBEN MEERMAN

I never knew that.

DR TIM SENDEN

So in dry sand, when you press on dry sand, not only do the grains have to get out of the way, but the air that's between the grains also has to escape. And I think that's very important. There's two ways a sound can be made. It can be made by the grains hitting each other and rubbing or it can be made by the way the air escapes from the space between the grains. And I'm with the latter idea. That's my bet. But we still don't know. We don't have enough information and ah it's an open topic.

NARRATION

And it's not just Tim who doesn't know for sure... there has s been a surprisingly large amount of research into the topic. We know that for sand to squeak, it must be dry, made of smooth, round grains of quartz and not have too much shell material. But there's still no definitive answer.

RUBEN MEERMAN

So there you go. We can land a remote controlled robot on Mars but we don't know why sand squeaks here on Earth and to be honest I kinda like that.