How can water make such musical sounds? And, how does anything make sound at all?

The forest is a deep verdant green and thickly wooded. On the sloping banks, it gives way to shorter plants, and then pebbled rocks, until right at the bottom you find a river, clear and pristine, babbling along on its way. The only sounds are crickets…and a soft percussion beat.

You look around, hoping to find its source. Who’s playing drums in the jungle?

But no. It isn’t drums at all. It’s water.

Ëtëtung, or water music, is a longstanding and revered tradition among the women of Vanuatu. It’s based on many of the activities they already perform in the water, such as cleaning, washing, and fishing, and consists of three basic motions: the long cymbal-like plough, the low-bass plunge, and the short, high-frequency slap.

But you still wonder, how is more water able to make those musical sounds? And come to think of it, how does anything make any sound at all?

Since we’re into water, let’s begin with a Zen story about a fish. This fish was of a sceptical nature, and he wasn’t convinced about the existence of water: What is this ‘ocean’ that people keep talking about? he asked. We can’t see it, and we can’t feel it, but yet people say that it’s everywhere and all around us. How can this be?

We may laugh at the fish, who doesn’t notice the water its life depends on, but we often make the same mistake ourselves. Going through our daily lives, we often ignore things that are there all around us. Before COVID-19, we thought nothing of our ability to actually go and meet people. And now, we still don’t often think of that life-giving substance: air. Air is essential to us in many ways — we breathe it, after all. But apart from everything else, air is also the thing that carries sound to our ears.

In outer space, you won’t hear anything. Even if there’s an explosion just feet away from you, it’ll be perfectly silent. That’s because waves travel in air, which we can’t see, but is there all around — at least for us non-astronauts down here.

When something moves swiftly, it sends ripples through the air, some of which end up drumming on our eardrums. That vibration gets transmitted into neuronal impulses that move on to the brain. And then, in the usual way of the human body, a complicated mechanism will do its complicated job and tell you that that sound you just heard was a bus and you better jump out of its way real fast.

What we’re more interested in here, though, is what kind of ripples those are and how they work. And the answer lies in waves.

You might have heard that light is a wave, or sound is a wave. But what does that mean? A wave can also be the water moving in the ocean, or water rippling outwards after getting hit by a pebble, or water —

Well, maybe that’s enough water examples.

The point is, it’s hard to say what all these things have in common, and what makes them different from a wave of farewell.

Technically speaking, a wave is defined as something that transmits energy without itself moving. Now, that’s a little bit of a complicated statement, so let’s break it down.

Since we began by talking about women and sound, let’s pick up another example: Emilie du Chatelet.

Du Chatelet is not a name many have heard — certainly not as much as, say, Newton. But du Chatelet was Newton’s contemporary, and indeed, dedicated her life to improving upon his work. Her piece de resistance was a full translation of Newton’s Principia into mathematical calculus, a feat she completed less than a week before she passed in childbed.

But du Chatelet’s significance here is slightly different. Here, we want to know about conservation of energy. Energy, du Chatelet realised, can never be created or destroyed.

You can transfer energy from one place to another. You can transform energy from one form to another. But you can never ‘create’ new energy or ‘destroy’ an old one. Incidentally, that’s why people need to eat food. What they’re doing is absorbing energy from, ultimately, plants, who get it directly from sunlight, which gets it from the nuclear fusion reactor that is our Sun. And the Sun can trace its energy back through the generations right until the Big Bang.

While energy remains constant, it can take on many forms. When you spend an hour stacking books neatly in your shelf, that transfers energy from you to the books. When the books wobble and come crashing to the floor again, that transfers energy from the books back out to the air, where it’s used to make a loud noise. Not very useful, but it’s still being used.

The amount of energy moved is related to ‘displacement’, among other things. When you take books from your desk to your shelf, that’s less displacement than bringing them in from the sofa down the hall — and the energy taken from you is less as well.

What’s annoying about energy is that it usually gets used both ways. When you displace a book from your desk to the shelf, that uses energy. When you bring it down again to read, that uses energy too. You can’t save up energy when you put away the book and collect it back when you bring it down. (Unless it’s a very inspirational book, but that’s a different kind of energy).

What makes waves so interesting is that, for them, the trick actually works. Waves use energy to move, but they get the energy back when they’re done.

In physics, ‘displacement’ is used to describe movement, but it’s not to be confused with distance. Displacement is distance, yes, but it’s distance with respect to a particular point.

Let’s say we take the place where you’re sitting right now (or standing, although I can’t imagine why you might want to stand and read) is our starting point — point 0. And let’s say you shuffle two bums to your right. Then you’ve just moved +2 bums. So far, so good.

Now let’s say you shuffle two bums back to where you were before, and this is where it gets tricky. You’ve actually just moved -2 bums with respect to your starting point. (Think of the line you’re moving along as a number line; that usually helps with visualization.)

Now, to get your total motion, we’re going to add up both movements and we get this:

0 + 2 bums — 2 bums = 0 bums

No, that isn’t a typo. You actually have not, according to displacement, done anything at all.

Now apply the same principle to a wave that’s made when a stone falls into a pond: the water moves a little, pushing along some ripples, but in the end, everything goes back roughly to where it started. The reason this works is, water molecules are much smaller than you and books, so the little energy from risen water falling back down actually makes a difference.

And that’s why we say energy transferred without displacement.

Now, let’s get back to my original definition.

A wave is defined as something that transmits energy without itself moving.

Makes more sense now, doesn’t it? The ‘bit that’s displaced’ moves, because different bits gets displaced in sequence, but there’s no one bit that’s moving. They move, but then un-move. Energy is absorbed, but then un-absorbed, as it hops onward to displace the next bit.

So, now that we know what a wave is, how do we draw one? You’ve actually probably seen a diagram like this before, sometime in your distant, hazy, high school past, but we’ll do a refresher anyway.

We’re going to back to the stone in the pond image. Imagine a grain of sand sitting on the water, bobbing up and down as waves pass it. That’s the basis of what these drawings are. On the horizontal axis is the passage of time, against the relative height of the grain over it.

So far, we’ve been looking only at the surface of the water. Sure, things bob up and down on top, but what goes on down below?

Imagine yourself inside a swimming pool. You duck under the surface and push forward with both your hands. What happens? You create a wave rumbling over the surface, sure, but there’s a lot of push happening down below as well.

As your hands move forward, they compress the water droplets right in front of them. That water spreads out again as the wave passes through, just as a crowd parts to let someone pass through and then reforms itself once more. By this time, the next bit of water has been compressed by the water behind it. And when it expands it compresses the water further on. And so it continues, right till the end of the swimming-pool.

If you think a bit, you’ll realise we can use the same graph to represent this. But this time, the diagram represents not the height of the sand-grain, but the density of the water — how compressed it is, in other words.

Generally speaking, there are two kinds of waves. The displacement of ‘transverse’ waves happens at right angles to the way they’re moving: think of the sand bobbing up and down while the wave travels from left to right. In ‘longitudinal’ waves, the displacement is in the direction of motion: your hand pushes water back and forth, which push the next round back and forth, and so on as the wave travels forward.

Sound is a ‘longitudinal’ wave. It works the same way as our previous example, except that, instead of being pushed by your hands, the water is pushed by your vocal chords.

Of course, people don’t usually speak underwater, because that’s terribly inconvenient. But the same thing applies in air as well! Air is in many ways similar to water, so it’s up to us not to be like the oversceptical fish.

The human ear can hear a full ten octaves. But all that sound is a product of two parameters—which is why the Vanuatu water musicians can get so much variety in such a simple way.

The first parameter is amplitude. For a transverse wave it would mean how large the displacement was when it happened — how many bums you moved, in other words. For sounds it’s the same, except the bunching happens horizontally. So at the peak there’s a huge fist of sound slamming onto your eardrum.

As you can guess, high amplitude gives you high volume sound, and lower amplitudes give you lower ones.

Volume isn’t the only thing that makes for sound — there’s also pitch. We’ve all known that moment when we’re singing along to a song — and suddenly, we just can’t anymore. That’s because they’re going a little too high , or a little too low.

Pitch depends on the frequency of a wave — that is, how many waves go past a point in a given amount of time. So, for example, if ten waves pass a point in a minute, you could say that its frequency was ten waves per minute. Scientists generally prefer to take it in terms of seconds, because seconds are a more fundamental unit; the number of waves that pass a given point in one second has a unit, and its called the ‘hertz’.

Waves with higher frequency have more energy, because they cover more ground in the same amount of time. That’s why shrill screams travel further, and Bianca Castafiore’s voice is powerful enough to shatter glass, when yours and mine aren’t.

When you hear a musical note, what you’re hearing is a vibration at a specific frequency. It doesn’t matter what it is that’s vibrating: a violin string, the air inside a trombone, the wings of a cricket, or your vocal chords. If they’re vibrating at the same frequency, they’re all producing the same note.

This is what helps musicians, such as those in an orchestra, keep in tune with one another. Middle C, for example is 256 Hz, and the A above it is 440 Hz.

(Words and other sounds are made from frequency too — although there are other complicating factors as well, which we won’t get into right now).

If you’ve ever sung in a large group — say Christmas carols —then you’ve probably felt that moment when everyone is in sync and it all feels like the sound is soaring together, right?

Well, then, it looks like you’ve been experiencing resonance.

Resonance is like a child being pushed on a swing, gathering energy as she swings higher and higher. You have to be careful to give the push at exactly the right time, or the strength falls away pretty quickly. But if you do, then she’ll go pretty high pretty quickly — she’ll be increasing her amplitude.

That’s what guitar strings do, too. When you pluck the A-string, for example, the frequency of 440Hz is the swing, gaining energy as it’s pushed further and further, while all the rest die down very fast.

People have been using sound since time immemorial. In the beginning, it was one useful way to quickly sense movement. Then came language which allowed for communication, and the sharing of resources and techniques that allowed to create our evermore sophisticated world.

Along with practicalities, there also came music, which could be practical in a a different sort of way. Rhythm could change your mood, or help motivate a group of people working on a field, or rowing a boat. Before the coronavirus, we had the luxury of attending performances, but we’ve found a way to evolve those too — moving over onto Zoom and YouTube for distanced concerts. Everyone’s home with headphones and Spotify.

Whatever the medium, there’s no doubt that sound plays a great role in peoples’ lives.

As the evening draws to a close in Vanuatu, people wade out of the river and return to their chores and leisurely evenings. But even in the absence of human ears, the river splashes on by itself.