[dunes rumbling]
- [Joe] That's not the sound
of the drone you're hearing.
It's the dune. Actually, can you turn that up a bit?
[dunes rumbling]
We're in Dumont Dunes, California,
one of 40 or so places in the world where dunes sing.
They're called booming sand dunes.
Kind of mysterious, right?
The truth is, we still have a lot more to learn
about dunes and sand, in general.
And those mysteries have very important implications
for all of us.
- [Nathalie] A sand dune is a hump of sand at first sight.
But actually, if you look within it,
there's so much more, and it's really a mystery.
- [Joe] That's Dr. Nathalie Vriend,
one of the original crew
who first solved the mystery of the booming dunes.
She still studies dunes, but from her lab in Cambridge.
- [Nathalie] So if you think about sand,
you can hold it in your hands.
As you pour it out of your hand, it acts as a fluid
because it flows very easily, and it falls on the ground
and it forms a heap that is more like a solid.
And now, if you throw it up in the air,
it actually blows away by the wind,
so then it acts as a gas.
So sand is a very interesting substance
because sometimes it behaves as a fluid.
Sometimes it behaves as a solid. Sometimes it is a gas.
- Granular flow is a pretty poorly understood
field of physics.
Wait, wait, I talked a bit about that
for an episode of "It's Okay To Be Smart".
Let's cut in some of that.
Okay here you go, Joe.
Thanks, Joe.
You see these ripples?
They form just about anywhere sand is moved by wind.
First, the sand grains are lifted into the wind.
When they land, they splash into other grains.
If a tiny pile randomly forms,
it catches more of this bouncing sand.
Behind that hump, a shadow forms where fewer grains land.
A little ways down, sand starts splashing
and piling up again.
This feedback loop continues on and on
to create those neatly organized ripples of sand,
and on the larger scale,
wind, sand and physics combine to create dunes.
When wind encounters a large bump,
it's deflected up, compressed, and it speeds up,
similar to air moving over an airplane's wing.
This accelerated wind erodes the sand on the upstream face
of the dune and deposits it near the top.
But sand can only be stacked so high.
When the downstream face of a sand pile
reaches 33 or 34 degrees, known as the angle of repose,
it spontaneously avalanches.
Sand avalanches have everything to do
with that booming we heard about earlier.
- [Melany] How can a sand dune generate a musical tone?
How can nature give us something that sounds like a cello?
- [Joe] We're with Caltech professor
of Mechanical Engineering, Dr. Melany Hunt.
- [Melany] This booming dune problem
is something that people have thought about for centuries.
- [Joe] Ancient texts from around the world,
some over a thousand years old,
guessed at the source of the booms.
Charles Darwin heard it in the Chilean desert.
There are accounts of booming in the Sahara.
Even Marco Polo apparently heard it.
- [Melany] When you take someone for the first time,
they are very surprised by what they hear.
People would look around
and think that there must be something else there,
a giant worm coming out of the sand.
- [Joe] To be clear, a giant worm rising from the earth
is not totally out of the question.
That's "Dune" the movie for anyone who's had their head
in the sand for the last year or 60.
Okay, no, actually turns out
we can now explain this with science.
Dr. Hunt, her colleague, Rob Clayton,
and her former PhD student, Nathalie Vriend,
remember her from earlier?
they took dozens of trips to the desert
to study the booming dunes.
They had a grasp of some of the factors
that enabled the booming to occur
but uncovered exactly what was going on, structurally,
to create the sounds
that people had been hearing for generations.
In order for dune booming to be heard,
the conditions have to be just right.
- [Melany] The sand has to be really, really dry.
- [Joe] Like summer in the desert dry,
and summer at Dumont Dunes is no joke.
It was a sweltering 118 degrees Fahrenheit
when we first showed up in the late afternoon.
The dune has to be big, like over 120 feet tall.
That's because, for the dune to boom,
the dune has to have the correct internal structure.
- [Melany] There is this upper layer of sand
that is maybe about a meter or a meter-and-a-half in depth.
The sound doesn't travel very well through that,
but that's where that frequency comes from.
It's that thickness of this loose layer
that lies above this hard reflector.
- [Joe] An additional component seems to be
that the grains themselves
have to be the right size and shape,
and the wind acts like a size separator.
- [Melany] The big sand grains
never make it to the top of the dune.
They're too large to be carried by the wind.
And small, fine sand has been lifted off
and transported away.
So what you find is that, at the top of the dune,
it's just much better sorted sand.
- [Joe] To understand this,
let's think about a cello for a second.
The bow of a cello provides friction energy,
which is then converted into vibration energy by the string,
and certain frequencies of vibration are amplified
by the wood of the body interacting with the air
and the size of the cello's cavity.
The sand avalanche provides energy, just like the bow.
The moving sand grains put vibrations into the air,
like the wood of the cello, and the thickness
of the sand layer determines which resonant frequencies
are reflected between the air
and the non-moving sand layer below,
just like the cavity of the cello.
To begin this process, Dr. Hunt and her students
scoot down the dune in a line.
- [Melany] Where you first start, you really hear nothing,
and then there's this low rumble that develops,
and the rumble can continue for 10 seconds,
but it can continue up to a minute or so.
[people scraping sand] [sand dune rumbling]
It's really quite remarkable what it does.
- [Joe] The waves actually move individual grains of sand
and exert force in the same way
that sound can move molecules of air.
- [Melany] It's this frequency
from about 75 hertz up to about 100 hertz.
- [Joe] Those frequencies can be tied to musical notes.
[note humming] What you're hearing now
is closest to an E-flat, but it's sometimes
an F-sharp or a G. [notes ding]
- [Nathalie] But this is a natural phenomenon
that people don't expect.
It's a mystery, and there's this natural curiosity
to explain the unexplained.
With my booming sand dunes research in my PhD,
I was able to explain the mystery,
but the sand dunes have so many more unanswered questions.
So from there, I went on a quest
to identify still open questions involving dunes.
It is essential that we understand how dunes are formed,
how they evolve and how they move.
20 years ago, a study was done, and they found
that an area of 120,000 square kilometers
of agricultural land was lost to desertification every year.
To put this in perspective, this is the size
of the state of Mississippi every year.
This is due to dune encroachment and desertification.
Because of climate change,
this only became worse in the last few decades.
- [Joe] The European Commission's
"World Atlas of Desertification" reported
that 75% of Earth's surface had already been degraded,
and that figure could jump to 90% by 2050.
Areas like the Sahel, the Gobi Desert
and parts of South America are experiencing
disproportionate effects of desertification.
- [Nathalie] And these are actual people
that have their livelihoods impacted.
- [Joe] The people that live in these so-called dry lands
face food insecurity due to loss of arable land
and have to deal with dangerous dust storms
that can damage their lungs.
- [Nathalie] I would really like
to contribute a small part in understanding
how we can reduce the mobilization of these dunes
and perhaps divert the path of dunes specific
and protect the people downstream.
In the last few years,
I've done some exciting work
with my PhD student Karol Basick
in the lab on sand dune migrations.
These large desert dunes, they take years to move.
So in the laboratory, we're using an overlaying fluid
instead of air, and because we're using fluid,
we can see these processes in a matter of minutes,
instead of years.
- [Joe] What Nathalie and Karol found
in using their experimental dune racetrack was surprising.
The miniature dunes communicated with each other,
negotiating for space.
When they put two dunes of the same size in the tank,
initially the front dune moved faster than the back dune.
But after a period of time, the front dune slowed down
until they were moving at about the same speed
and eventually settled at a distance of 180 degrees
apart from one another on opposite sides of the tank.
- [Nathalie] The interesting question, however,
is how does this relate to the real world?
Do we observe the same behavior in large dune fields,
and subsequently whatever we observe from satellite images
in large dune fields, can we actually mimic it in the lab?
That's what I'm hoping to continue
to research in the next few years.
The fundamental work that we're doing
is essential to add to our knowledge base.
It's just that we can protect livelihoods of people
and infrastructure downwind of these marching dunes.
- Through the most simplified lens,
these dunes may be nothing more than sand,
wind and time made tangible, but although they may be
merely a product of physics and geology,
they are more than that, too,
something more than dead that evolves and even moves,
a thing, though not quite alive, that can still sing.
[ethereal music]
And make sure you head over to "It's Okay To Be Smart".
We are gonna dig deep into the beauty
and physics of sand and wind.
[ethereal music]
♪