Original subtitles

EXPLOSION

This is Stromboli, one of the most active volcanoes in the world.

And a few times every hour,

it sends out huge explosions of lava and ash.

And of course we expect noise to go with those explosions

but along with the sounds we can hear,

there are also sounds we can't.

Because this volcano, like many others,

is, in effect, a gigantic musical instrument.

Only now are scientists understanding

how strange and spectacular

the world of sound really is.

It is easy to take sound for granted.

Sound is noise...

it's music...

..it is the spoken word.

But it is far more than just a soundtrack to our lives.

CRACK!

The more we've discovered about the physics of sound...

BOOM

..the more astonishing the secrets it's revealed.

HIGH-PITCHED SQUEAKING I hear the word's angriest mosquito.

In this series, I'm going to investigate the nature of sound -

what it is...

BELL CLANGS

I can feel that through my feet. it's really cool.

'..what it tells us...'

Just the quality of the sound says something is not right here.

'..and how we use it...'

ENGINE ROARS

Certainly heard him.

'..allowing us to see the world and even the universe

'in new and exciting ways.'

Every sound is created for a reason.

Every sound has a story to tell.

BIRDS SING AND BEES BUZZ

CRACKLING

Listening to a tree seems like an odd thing to do

but this tree isn't silent.

Even through a stethoscope like this, I can hear creaking and

groaning as the branches move in the wind,

and there are other sounds in there that I can't quite hear with this.

Crackling, popping sounds.

POPPING AND CRACKLING

It happens because the tree

is drawing water up from its roots to its leaves

and, on a hot sunny day like this, as that water travels through

the tiny tubes round the outside of the tree,

bubbles form, and those are what are making the crackling noise.

So although you wouldn't know it by looking at it,

that crackling noise could tell you that this tree is thirsty.

Before we can unlock all the secrets of sound...

..we need to understand it at a fundamental level.

So in this programme,

I'm going to explore what sound is and how it's made.

First, it would help if I could turn a sound

into something we can actually see.

This is a very special space.

It is called a hemi-anechoic chamber,

and what means is that all the walls and ceiling have these funny shapes

on them that are absorbing sound so it's really quiet in here.

It's the perfect environment to isolate a pure sound

and observe its effects.

All I need is this small army of candles and a speaker.

To make this work, I need the sound to be really loud

so I'm going to wear ear defenders.

DEEP RUMBLE This is a really deep sound -

you can see the speaker going in and out.

And what you can see is that the candles are vibrating -

this very, very fast vibration.

'The individual candle flames are showing the movement in the air

'caused by the speaker.'

What's happening is that the speaker here

is producing enormous amounts of sound by pushing on the air.

And that push pushes on the air next to it

which pushes on the air next to it,

and it travels out across the candles.

'The candle flames are flickering back and forth 20 times per second,

'or at 20 hertz.

'This is the frequency of the sound we are hearing.'

I'm going to turn it up.

NOISE RISES IN PITCH

If I increase the frequency, the candle flames flicker even faster.

And what you can see is that the candles are all flickering

but they are all flickering together.

This is synchronised movement.

They are all moving forwards and backwards together.

'So what we're seeing is the sound.

'The movement of the speaker causes the air molecules to oscillate

'back and forth at a specific frequency.

'These oscillations travel through the air as sound waves

'and they are picked up by our ears.'

A loudspeaker is actually a very unusual way of making sound

because it's artificially manufactured

to generate any sound you like.

Most sound is much more interesting.

That's because, unlike the loudspeaker,

most objects create a specific sound that's unique to them...

..and this is ultimately at the heart

of why sound is such a rich source of information about the world.

To understand how an object produces its own unique sound...

ENGINE ROARS

..we need a clear and simple sound source.

For me, one of the most beautiful examples of this is a sound that has

been ringing out across our cities for centuries.

BELL CLANGS

The sound of church bells

is one of the most distinctive sounds of Britain.

And I learned to ring bells as a kid,

so I have certainly spent a lot of time in bell towers,

but there is one bell that I've never seen.

It's not only the most famous bell in this country,

but the most famous bell in the world.

It's just up there and it's the one we all know as Big Ben.

'The sound of Big Ben is instantly recognisable.

'It's an apparently simple sound

'but also one that's rich and melodious.

'Analysing how Big Ben's sound is created

'reveals something remarkable about the relationship between

'an object and the sound it produces.'

So this is it.

This gigantic bell is Big Ben.

And all sorts of things have changed in the 150 years

since the Victorians hung it here. But the sound is exactly the same.

And now I am up here, I can see it in action for the first time.

'Alongside Big Ben,

'there are four other smaller bells that hang in the belfry.'

BELLS CHIME

'These play the famous Westminster chimes.'

BELLS PLAY WESTMINSTER CHIMES

'It is only after this is finished that Big Ben itself is heard.'

LOUD CLANG REVERBERATES

It is an incredible amount of sound.

I could feel that through my feet.

That's really cool.

The way that the bell makes sound is that this huge 200kg hammer

hits the side and that sets the metal vibrating.

And as it pushes out, it pushes into the air,

sending pressure waves outwards.

And those are the sound waves.

But all of this doesn't just happen at one frequency.

The huge richness of the sound that Big Ben makes

comes from many frequencies all happening at the same time.

So how does one bell produce many different frequencies?

And what makes them sound so good together?

The first scientist to try and unpick the frequencies

within an object's sound

was the German physicist and amateur musician, Ernst Chladni.

Chladni devised a special experiment that enabled him

to study how even the simplest of objects

can produce a complex sound...

CYMBAL CRASHES

..made up of many different frequencies.

I'm going to do a modern-day version of Chladni's experiment.

This is a Chladni plate.

It's just a flat metal sheet that's held in the middle.

And if I hit it... FLAT CLANG

..it makes it a sound that doesn't sound very pleasant.

Certainly not nearly as nice as Big Ben.

But that sound has a lot in common with the sound of Big Ben

because it's made up of lots of different frequencies.

And Ernst Chladni came up with

a really clever way of picking apart

where that sound comes from. So he started with a plate like this.

And he sprinkled sand on top, so I'm going to do that.

And then he set the plate vibrating.

And I'm going to do that with a signal generator here

that's going to move the middle of the plate up and down.

And the number on the front here is the number of times every second

that vibration is going to happen - so at the moment it's 240.

So if I turn this on...

WHINING HUM

So it's not a pleasant noise.

You can see the sand is dancing about in the plate

but it's not too exciting so far.

But what happens if you turn the frequency up is quite different.

HUM INCREASES IN PITCH

And suddenly at this frequency here, 264 hertz,

you can see this beautiful pattern pops up in the sand

of the top of the plate.

And what this is giving away

is that the plate is vibrating in a shape

and the sand is showing us what shape that is.

What's happening is that the plate is bending like this,

and at the parts of the plate that are moving a lot,

the sand is getting bounced away.

And the parts of the plate that are between a bit that is going up

and a bit that is going down, don't move at all,

and so the sand accumulates in those places.

So what Chladni had found was a really clever trick

for seeing the shape of the vibration,

even though he couldn't see it with his eyes.

The vibration pattern revealed by the sand occurs at what is known

as a natural frequency of the metal plate.

This is a specific frequency at which the plate naturally vibrates

and produces sound.

And this is part of what's making up the sound when I hit the plate.

But it's not all of it, because if you keep turning the frequency up,

there's more to see.

SOUND INCREASES IN PITCH

And so here we are up at 426 hertz

and suddenly, out of that mess,

there's another pattern of vibration,

beautiful pattern on the plate here.

Chladni's experiment reveals how a simple object, this metal plate,

can produce a complex sound...

..because it doesn't vibrate at one frequency.

It has many natural frequencies...

HIGH-PITCHED RINGING

..each corresponding to a different pattern of vibration,

more elaborate than the one before.

When you hit the plate, what happens is that lots of those vibration

patterns all happen at the same time, one on top of the other.

Each one contributes their natural frequency to the mix,

and that combination is what makes up the sound that you hear.

Every object that vibrates

has its own combination of natural frequencies

determined by its physical characteristics.

And together, these frequencies form a unique acoustic signature.

So there's a beautiful relationship between an object and the sound that

it produces. When you hear sound,

you are hearing messages about the thing that created it.

Its size, its shape, what it's made from, even how the object was made.

And natural frequencies are the key to understanding

one of the most fascinating mysteries

about the sounds we encounter in our daily lives.

Why do some sounds seem rough and unpleasant,

whilst other sounds like Big Ben

seem more attractive to the human ear?

To find the answer,

we need a way to reveal the exact natural frequencies

of Big Ben.

Sprinkling sand isn't going to work for a bell.

But a team of scientists from the University of Leicester

are recreating Chladni's experiment using state-of-the-art technology.

Tell me about the measurements you're making here.

You've got these lasers around. What are they doing?

We've got two laser Doppler vibrometers

pointed at the surface of Big Ben.

That allows us to measure the motion of the surface,

the vibration of the surface, directly,

but without touching the bell.

So by a tiny change in the laser light,

you can find out how quickly

the surface of the bell is moving in and out.

That's right. We are going to characterise that,

and be able to show that for all of the natural frequencies of the bell.

BELLS CHIME

Across three hours, Martin's two lasers scan Big Ben as it chimes.

The aim is to discover the bell's different natural frequencies

and patterns of vibration.

BIG BEN CHIMES

Just as with Chladni's plate,

every time the hammer strikes the bell, the metal vibrates at many

different natural frequencies,

each corresponding to a different pattern of vibration.

Together, these make up

the distinctive, melodious sound that we hear.

Tell me what we're looking at.

We've got an average of the entire chime of Big Ben,

and from that you can see a number of different dominant frequencies,

and some subordinate frequencies that all go together to make up

the characteristic sound of Big Ben.

So those are some at the front here which are really obvious.

They are much bigger than the others.

Yeah, particularly 199 hertz and the 336 hertz

really dominate the character.

So each of these natural frequencies corresponds

to a different vibration pattern on the bell.

That's right. To give the note and the colour

that is the sound of Big Ben.

This animation is showing the lowest natural frequency of Big Ben.

95 hertz.

At the bell's higher natural frequencies,

the animation shows that it vibrates in more complex patterns.

It's this mixture of frequencies

that make up Big Ben's acoustic signature.

CLANG!

But knowing the frequencies reveals something else that helps explain

why we perceive this to be a melodious sound.

Because underpinning the difference natural frequencies

is a mathematical relationship.

The sound of Big Ben isn't random.

Some of its natural frequencies are lined up in a harmonic relationship,

and that's what gives the bell is harmonious sound.

Some of Big Ben's natural frequencies

are simple ratios of one another.

For example, this natural frequency is almost precisely half

of this one.

When frequencies are mathematically related like this,

in what's called a harmonic relationship,

the human ear finds them pleasant.

And in the UK, most bells are specifically tuned to be like this.

If we change the shape of the bell or the material it is made from,

the sound would change.

And so when we listen to something like a bell,

what we're hearing is its structure.

So far, the world of sound seems relatively simple.

An object vibrates to make a distinctive sound.

And if these vibrations are specially tuned,

then we can turn sound into something beautiful.

ORCHESTRA PLAYS A WALTZ BY JOHANN STRAUSS

But there is more to the beauty of sound than tuning an object.

There is often something else involved in the production of sound.

Something that adds complexity and richness.

Something that, exploited to the full,

can create sounds that stir the soul.

With the help of acoustics expert Professor Trevor Cox,

a violinist and a special camera,

we're going to explore the way that some sounds are produced

and how it can be more complex than it might first appear.

This is a fantastic toy.

It's an acoustic camera.

It's got a little camera right in the middle looking at me,

and then a ring of microphones around the outside.

And they are very directional.

And so if I clap up here you can see the sound is coming from up here,

the rest of the time you can see it coming from my mouth,

so you can identify where the sound is coming from.

In a musical instrument like a violin,

the initial vibration comes from the string...

..but although the string is vibrating,

it is not directly producing the sound that we hear.

Something else is involved, too.

When you look at a stringed instrument,

might think the string is making all the sound.

Well, it is starting the sound

but it is not what makes the sound so powerful and so strong.

The string determines the pitch of the sound.

Just the string - it would all be rather dull and quiet.

The acoustic camera shows that the loudest sound,

coloured in pink and red,

isn't coming from string but from the wooden body of the violin.

Tell me what happens to the sound after that.

Well, once you've made a sound, you've got the source of the sound,

it then has to be amplified.

So the sound goes through the bridge first of all,

which connects the string to the body of the violin,

and then the actual wooden plates are all vibrating

and they're amplifying the sound.

So the important thing is that the thin string

can't push on the air very much by itself,

but once you've got a great, big, large, wooden, flat plate,

that can push quite hard.

Yes. Every musical instrument

has resonances at heart and in the violin,

it's actually the wood body that is the resonator.

The wooden body of the violin is what's called a sound resonator.

It transforms the sound of the vibration from the string,

picking up and enhancing certain natural frequencies

whilst damping down others.

ORCHESTRA TUNES UP

Most musical instruments have a resonator.

The pipes of an organ, the bore of a clarinet

and the body of a cello.

It's what amplifies and sculpts the sound,

giving the instrument a far richer acoustic signature.

But the ultimate ability to shape sound doesn't belong to a musical

instrument. It belongs to us.

SHE SINGS: O Mio Babbino Caro by Puccini

It's the human voice.

As a professional opera singer,

Lesley Garrett has exquisite control over the sound her voice produces.

She can produce a range of sounds

far greater than any man-made musical instrument,

and at a volume that can compete with an entire orchestra.

To see how Lesley is able to create such extraordinary sounds,

we have come first to Harley Street in London to meet throat specialist

consultant surgeon John Rubin.

But this is so precious that I do have it checked regularly.

John has looked after me for many decades now and kept me going.

John is going to use a laryngoscope to allow us

to look at Lesley's larynx,

where the sound of her singing voice begins.

Just give me a nice, bright forward...

- SHE SINGS NOTE - Lovely, lovely.

Now, I'm going to ask you if I may, to show me the tip of your tongue.

Now, smiley face.

Get ready. Take a little...

HE SINGS A NOTE AND SHE REPEATS IT

THEY SING A HIGHER NOTE

Terrific.

The larynx produces vibrations in air,

the origin of the sound we hear.

So these are your vocal folds.

So these two white stripes down here.

These two white stripes are Lesley's vocal folds.

So we can see them of opening and closing as she sings.

Exactly.

It is the opening and closing that actually breaks up the air

and makes sound.

Now in Lesley's instance,

she can make her vocal folds vibrate

anywhere from about 80 times per second probably to over 1,000.

- Wow, I didn't know I could do that. - It's really amazing.

There are various sets of muscles and I have to, almost unconsciously,

arrange those muscles so that my larynx is in the perfect position

for the amount of pressure I'm choosing to exert upon it.

And that is what we call the onset of tone.

So if I was just going to move my larynx without air,

it would sound like this...

ALMOST SILENT BREATHS There is almost nothing there.

But then if I were to introduce air, it would sound like this.

SHE SINGS LOUDLY

Like that, you know.

You did that with so much volume, so quickly, it's astonishing.

Just that tiny little thing.

HE SINGS A NOTE AND SHE REPEATS IT

The vocal folds create the initial vibration in the air.

Yet, as remarkable as Lesley's vocal folds are,

just like the strings of a violin,

they are not producing the sound we hear.

It is her resonator that is the key to her extraordinary voice.

We've come to University College London

to meet Professor Sophie Scott,

who's going to reveal what makes the human resonators so special.

So what we're going to do today is I'm going to take you through to our

MRI machine and what we're going to do is use it to image

Lesley's vocal tract, and that should tell us something about

what is happening for you when you are singing so beautifully.

I cannot tell you how excited I am about this.

It's sort of like the answer to the ultimate mystery.

For 40 years I've been singing and never really quite understood

what is going on in my throat.

None of us can. None of us can see it.

It's not like we're pianists and we can see what is going on,

so this is so exciting.

The sound resonator of Lesley's voice is her throat and mouth.

And this is what the MRI machine is going to image as she sings.

Lesley, can you sing for me the vowels

ee, eh, ah, oh, euh?

LESLEY SINGS

The whole thing is moving.

It's quite extraordinary.

The MRI shows how, for each of the different vowel sounds,

Lesley's mouth and throat change shape,

amplifying the vibrations in air produced by her vocal folds

and sculpting them into the sound we hear.

So what we saw with the laryngoscopy right down here

is just the very beginning of making sound

and then there's all this shaping that goes on up here,

that actually determines what we hear.

Exactly, so

all the work being done above the voice box, the larynx,

is essentially changing the spectral characteristics of the noise that

you're making down there. You're making a noise here and then you're

continuously changing it up here.

Particularly, as you can see,

by exactly how the tongue has been positioned

and how the tongue is moving.

Lesley, that was absolutely beautiful.

Thank you.

- So, should we do "I Dreamed A Dream"? - OK.

# I dreamed a dream in time gone by... #

The MRI reveals the secret of our resonator.

And like the sound resonator of a musical instrument,

the vocal resonator isn't fixed.

It's incredibly flexible.

Through the movement of the tongue in particular

and the jaw, lips and throat,

it can be manipulated to form a myriad of different shapes.

Look how open that is. It's extraordinary.

And with a trained singer like Lesley,

the range of movement is truly amazing.

The tongue is basically like an octopus tentacle.

It just deforms in all these different directions.

This is such a flexible, adaptive instrument, isn't it?

That is a surprise to me, I must admit.

It takes you so long to coordinate all that to the level

that we can project a beautiful sound,

a sound that will hopefully make people cry or laugh,

to the back of a 2,000-seater auditorium without amplification,

it's something that requires massive training

and now I can see why it did.

# I had a dream my life would be

# So different... #

Sound begins as a simple vibration.

# So different now from what it seemed... #

But it is how this initial vibration are sculpted by the resonator that

lies behind our mastery and control of sound.

# The dream

# I dreamed. #

APPLAUSE

Music is an obvious way in which sound can have an impact on us.

But there's a type of sound

that makes an impact in a very different way.

It is a type of sound that doesn't play by the rules

of any of the sounds we've heard so far.

WHIP CRACKS

This is a thing that is entirely new to me.

It is a bullwhip, and Lila here is about to have a go at teaching me

how to crack it.

OK, so whip cracking.

- Safety goggles. - Good idea.

Right, so these are bullwhips.

This is the bit that makes the sound.

So behind you, turn sideways slightly.

Yes.

I hit myself in the head.

FAINT CLICKING

CRACK!

So I think I'm doing all right, and then you're coming along behind

with this enormous noise.

- Oh! - That was it, yeah. - We're in business.

Just try and get that...

That was a good one.

Shall we finish on a high?

The sound comes that comes from this whip is something special.

It's different. We're not hearing a shape.

It hasn't got specific frequencies associated with it.

And it's also fantastically loud.

All of that sound is coming just from that tiny bit on the end

and yet it echoed around this entire space.

Right at the point this sound forms, it isn't even a wave.

This is something different.

The key to what makes this type of sound different and so loud

is how it's generated.

And to see how that happens,

we need the help of physicist Dr Daniel Eakins.

- This is Lila. - Hi, nice to meet you.

So what have we got here?

What does the set-up do?

This is known as a Schlieren imaging set-up,

and what it allows us to do is detect very small, minute changes

in the way light refracts through gas as it is heated, for example.

- There you go. Yes. - It's pretty, isn't it? - Yes.

The Schlieren camera is able to detect distortions in light

created by changes in air temperature and pressure.

What we are going to try to do is have it

so that when the whip, or when the end of the whip

is at its highest speed,

that that's within the field of view of the Schlieren camera.

She's got to hit that toothpick thing there?

Yes, she has to be in the vicinity of this,

probably within about 50 mil if you can manage, yes.

- Can you do that? - Sure.

Wow. If only we had that one.

It's amazing.

Oh, my goodness.

This is it.

- Oh, wow. You've done it. - OK.

You owe me a cocktail stick. OK.

We'll just go and have a look at the data, then.

This slow-motion footage shows the disturbance in the air

created by the tip of the bullwhip.

The dark lines show where the air has been compressed together to form

concentrated pressure fronts.

The strands of the bullwhip create pressure fronts that travel

at phenomenal speed.

This is what creates the sound.

It looks like it is moving at around 364 metres per second.

So the speed of sound in air is about 343 metres a second,

so this is going faster than the speed of sound.

It is a supersonic disturbance.

The reason this sound can travel at supersonic speed

is because it's not a wave but a shock front.

For a fraction of a second,

it has enormous energy that punches through the air

with such force that the air molecules can't oscillate

back and forth as a wave.

The one distinguishing feature of a shock is that it is like an impulse.

It is an instantaneous change in pressure.

So the reason that such a tiny thing can make such a loud sound

is because it's barrelling into the air and so there's

far more volume given out.

- That's right. - So you've been breaking the sound barrier, Lila.

So cool!

THUNDER RUMBLES AND CRASHES

From the crack of a lightning bolt...

..to the bang of a gunshot...

..and the blast of an explosion,

the loudest sounds on the planet all originate as shock fronts.

Nasa scientists have used the same Schlieren technique

to image the shock fronts created by supersonic aircraft,

by filming the aircraft flying in front of the sun.

Three, two, one, mark.

The aircraft is moving faster

than the speed at which sound waves travel.

Because of this, the air molecules in front of the aircraft

get shoved out of the way with such ferocity

that there's no time for normal sound waves to form.

Instead, a pattern of shock fronts are created.

This is the origin of the sonic boom.

BOOM

SIREN, ENGINES AND CHURCH BELLS

For all of the fascinating science

behind the sounds we are familiar with in our daily lives,

these are only a tiny fraction of the sounds that fill our planet.

There are entire worlds of sound that remain hidden from us.

Places where sound can behave in very different ways.

And perhaps the most intriguing of these is the ocean.

Two-thirds of our planet is covered by water.

And yet apart from the sound of the waves,

it's a world that appears to us here on land as silent.

When I put my hand in the water here,

I'm touching a different acoustic world.

And that's because both sides of the water surface act like

an acoustic mirror. Sound coming from beneath bounces off the air

and goes back into the water and all the sound up here

bounces off the water and goes back into the air.

So I can put my hand into this acoustic world,

but I can't hear it.

The acoustic mirror effect ensures

that sound travelling in water can't escape into the air.

So the only way to experience

how sound behaves differently in the ocean,

and to see the profound effect this has on life,

is to enter the underwater acoustic world.

- Hello. - Hello.

- How are you doing? - I am all right.

'I have come to meet Dr Steve Simpson, who is a marine biologist

'and he's going to reveal

'just how differently sound behaves underwater.'

- So what have we got here? - So here we have got...

The plastic bucket of science.

The plastic bucket of science, absolutely.

- So we've got a hydrophone here. - So that's our underwater microphone.

This is our ear, basically, underwater.

And then we have a recorder that allows us to be able to take the

recordings through the whole of our snorkel and have I have wired up

a speaker inside a cup.

So while we're snorkelling about on the surface of the water,

we'll be able to hear what's going on below.

- Exactly, yeah. - All right. Let's give it a go.

'Part of the reason that sound is so different in water compared to air

'is that water is 1,000 times more dense.

'One consequence is that it takes more energy to start a vibration

'in the first place.

'Sea creatures have evolved specific means to create sound

'in this much denser medium.'

- Here you go. You take this. - So this is the listening device?

- There's your ear and here's a hydrophone. - OK.

CRACKLING

- How was that? - I can hear popcorn.

It sounds like snapping shrimp to me.

It is the soundtrack of the ocean, that's right.

Snapping shrimp overcome the difficulty of producing sound

in water by snapping their claws together really fast...

..causing bubbles to implode.

So it is kind of a grating, scraping noise.

And this is the sound

of a sea urchin scratching seaweed off the rock.

SCRAPING

Water transmits sound much more effectively than air.

In fact, sound travels much further in water than light does,

something that life under the waves takes full advantage of.

I've got a recording of a soldier fish. So this is a coral reef fish.

Spends its day living in a cave

then goes out at night looking for shrimp

that come out of the sand to feed.

And when it finds the food...

BOOMING GRUNTS

It's a very big, deep noise, isn't it?

It's like a deep trumpeting sound.

- How big is the fish? - So the fish would be about this sort of size.

- It's quite a small fish. - A small fish to make

- a lot of noise, that's right. - That's really impressive.

What sort of distances are these sounds travelling underwater?

So with a hydrophone like this, if you're out in the open ocean,

you'd hear a coral reef from up to 25km away.

So it really is a cacophony of noise.

And we think that fish can hear the sound from hundreds of metres,

some species for kilometres.

So it's almost in the ocean as though sound and light have swapped

places. Sound is much more useful underwater than light is.

Yes. So you might be able to see 30 metres in really clear water,

but you can hear for hundreds of metres or kilometres.

So it becomes an information channel

that works over much larger distances.

The distances over which sound can travel underwater are truly amazing.

The sounds made by whales can carry for thousands of kilometres...

..travelling across almost entire oceans.

Yet because these sounds remain locked beneath the water surface,

they never reach our ears.

We can't hear underwater sounds

because we are not a part of that acoustic world.

However, there is a whole class of sounds that we don't hear

for a completely different reason,

because their frequency lies outside our range of hearing.

And yet it is these sound that turn out to deliver

the most fascinating insights.

It is easy to take the huge range of human hearing for granted

but it is worth spending a moment on.

The piano is a really good way to demonstrate it.

This is middle C here and that is at 262 hertz,

which means 262 cycles every second.

And the lovely thing about a piano is that you can go up in octaves,

and every octave involves a doubling of frequencies.

So the highest note in the piano, this C here is 4,186 hertz.

It doesn't stop there.

If we were to build our piano outwards

to the edge of the human hearing range,

we come all the way up here, which is 19.9 kilohertz -

a gigantic number.

And it also carries on down the other end.

The lowest C on the piano is this one,

with a frequency of 32 hertz.

And if we were to carry on our piano to the limit of human hearing,

we would get down here. This one is 20.6 hertz.

So this piano, with all its extra keys,

represents the full range of human hearing.

'This is our rich but ultimately limited experience of sound...

'..because the full spectrum of sound frequencies

'extends way beyond what we can hear.'

These sounds that lie outside our range of hearing

hold the key to a world where sound gives life extraordinary powers,

and opens new windows onto our planet and even the universe.

I'm in the middle of a huge pod of dolphins.

There must be hundreds of them out here.

These dolphins are hunters.

They're using high-frequency sounds to locate their prey.

Most of the clicks and whistles that these dolphins produce

are way beyond the range of our hearing.

HIGH-PITCHED WHISTLING

This is the realm of ultrasound -

sound at frequencies above what we can here.

What I can hear are whistling noises but they are calls.

Most of them are at higher frequencies than I can hear.

So I'm just hearing a tiny, tiny bit at the bottom

and it's still really loud.

Ultrasound is key to the dolphin's hunting ability.

Because ultrasound has a very high frequency and a small wavelength,

it reflects off small, fast-moving objects

that audible sound waves would pass over.

The dolphin creates short pulses of ultrasound and then listens

for the echoes and, from this,

creates a detailed image of its surroundings,

enabling it to catch its prey.

These animals are operating in a sound range that is outside

what we can perceive

and it really highlights how much more there is out there.

Dolphins are not alone

in using ultrasound as a second form of sight.

Bats use it for their version of echolocation...

..and we use ultrasound for medical imaging.

Pulses of ultrasound can penetrate the skin and reflect off

different tissues.

Fluid, muscle and bone.

And these echoes are recorded and displayed as an image,

enabling us to see the foetus inside the womb.

At the other end of the sound spectrum

lies an even more mysterious and unfamiliar group of sounds.

This is the realm of infrasound -

sounds that are too deep for us to hear.

And as we learn to decode these sounds,

they give us a greater understanding of our planet

an offer us the potential to save thousands of lives.

Infrasound lets us listen in on the geological world,

and, if you want to listen to infrasound,

this is the place to come.

This is Stromboli,

one of the most active volcanoes on the planet.

It has been erupting almost continuously for over 1,000 years.

I've come here to meet some scientists

whose research has helped reveal

that this volcano, although we can't hear it,

creates an extraordinary sound.

The sound is created in a two-stage process

that starts with the spectacular

explosions of magma from within the volcano.

And this is what Dr Jacopo Taddeucci

and Dr Jorn Sesterhenn are studying.

So basically, we use a high-speed camera to take footage

of what happens at the vent of the volcano.

So can we see some of these videos?

Yeah, sure.

OK.

This is the eruption.

And then you see the bombs...

that are these particles flying here.

- So these big lumps flying up into the sky. - Exactly.

And how fast are they going?

They can go up to 400 metres per second.

So that's very, very fast.

It is faster than sound in the air.

It is a supersonic eruption.

In this processed image,

the dark lines travelling ahead of the molten rock are the sound waves

created by the supersonic eruption.

So there is a rush of gas and particles.

Coming out very fast, even supersonic.

- This makes the sound. - There is a very powerful eruption of gas and

particles and it is just pushing on the air around it and sending out

- sound waves. - Exactly.

The eruption creates a supersonic shock front...

BOOM

..that we hear as an explosion.

So far, so conventional.

But this is just the first stage

of the creation of a far more surprising sound -

an infrasound that is well below our range of hearing.

Detecting it isn't easy.

- Hello. - Welcome.

So this is Stromboli.

'The way that this infrasound is created depends on how the sound

'of the eruption is shaped by the crater.

'This is what Dr Jeffrey Johnson has been studying.'

It's loud, isn't it?

That second reverberation,

that is effectively a sound wave

oscillating back and forth in this giant, giant pit.

So a load of sound just washed past us that we couldn't hear

but that was what you were measuring.

Right. We could hear a component of that but not all of it.

- And I would like to show you what the signals look like. - Cool.

Fractions of a second after the explosive supersonic eruption,

a second sound carries on -

a pure tone of infrasound.

Since we can't hear it directly,

we need the help of a bit of audio trickery.

I would like you to put these on and tell me what kind of sound you hear.

SQUEAKING

I hear the world's angriest mosquito.

That's what it should sound like.

This box produces a 700 hertz tone that is being frequency modulated by

infrasound produced by the volcano.

So what you should be hearing is a constant tone and there when

there is an infrasound signal,

it deflects that tone to higher and lower frequencies.

'We can't hear the infrasound directly.

'Instead, Jeff's apparatus is set up

'so that when the infrasound passes by,

'it changes the pitch of the constant buzzing sound.

'Whenever the angry bee sound wobbles,

'it's because it has been hit by infrasound.'

There we go.

And you can see a huge deflection corresponding to that explosion,

and that was about a 2-3 hertz tone that I just observed.

This distinct 2-3 hertz tone is part of the unique

infrasound signature produced by Stromboli.

It's created when the sound of the explosion

from the base of the crater reverberates around the walls

of one of the volcano's cavernous vents.

This vent acts as a sound resonator,

sculpting the noise of the explosion into a single tone.

So the whole volcano is a giant musical instrument.

The moment of explosion is like the hammer hitting a bell.

That's what starts everything but then the shape of the musical

instrument itself means the sound goes on for a little bit longer.

That's right. And the size of that vent,

how deep it is, how wide it is,

will dictate the tone that is produced by that crater.

Because Stromboli's craters are so big,

the sound they produce is incredibly low-frequency infrasound.

Scientists believe all active volcanoes like Stromboli

have their own unique infrasound signature...

..determined by the shape of the volcano vent acting as a resonator.

And just as for a musical instrument,

if the resonator changes shape,

for example, because lava rises up within the vent,

then the volcano sings a different sound.

This means that we could listen to volcanoes around the world

and, by monitoring their infrasound, better forecast a major eruption

and that would buy precious time for people living nearby to escape

with their lives.

You might think that by the time we've explored the deep notes of

Stromboli, the story of infrasound would have reached its limit.

And yet it hasn't.

To explore the extreme limits of infrasound,

we need to leave our planet behind.

It's long been assumed that

in the emptiness of space there is no sound,

because there's nothing for sound to travel through.

'But, as impossible as it seems,

'infrasound could be playing a fundamental role

'in shaping the structure of the universe.'

We're used to the idea of our busy bustling world down here

being noisy.

But when we look up at the night sky, we assume it's silent.

No-one has ever heard sound from space.

But in this building, there is a man who thinks he has seen it.

'Professor Andrew Fabian

'is an astronomer at the University of Cambridge.'

He uses telescopes to study galaxy clusters,

the largest structures in the universe.

And he's trying to solve a mystery concerning how they grow.

His research has led him to make a surprising discovery.

So, Andy, where is it that you think you've seen sound in space?

We're looking in the consolation of Perseus at what is known

as the Perseus cluster of galaxies.

When you have a cluster like this, which has got an enormous mass,

it tends to...

drag all the matter in and squeeze it and it makes it very hot

and this hot stuff is known as the intra-cluster medium,

is what we study in X-rays with an X-ray telescope.

So in between all the bright galaxies here there is other stuff.

Exactly.

'And it turns out there is more than space than meets the eye.'

Let's go to an X-ray image.

It is completely different.

So it is definitely the same bit of sky we're looking at.

It is the same bit of sky but what we're seeing here is

the gas between the galaxies.

'This intra-cluster medium, shown here in orange,

'is a cloud of gas that blankets the entire Perseus cluster.

'At one particle every few centimetres,

'the gas is far too diffuse to carry sound that we can hear.

'But infra-sound can boldly go where no other sound can.'

What makes you think there is actually sound there?

Well, now we are going to look at the same region

with a specially adapted image from the X-rays.

And what we see is a whole set of ripples.

And they are really clear.

Really clear shapes.

Yes. Where they are bright is where the gas is denser and it looks

very much as though we've got

a pressure wave which is propagating outwards.

In other words, a sound wave.

'If Andy is right,

'what we're looking at is a snapshot of a wave of infrasound,

'travelling through the intra-cluster gas

'of the Perseus cluster.'

So what is the scale of this image?

The spacing between the ripples

is about the diameter of our galaxy.

- So gigantic. - So it's gigantic.

And if you were to wait on one ripple,

sit there and wait for the next ripple to come past you,

- how long would that take? - Ten million years.

So you need patience for this game.

- Indeed, yes. - What could possibly cause ripples of sound that big?

Well, I think it is coming from the centre,

and there there's a massive black hole.

It generates an enormous amount of energy in the material

just before it's swallowed,

and that energy is pushing out into the surrounding gas.

So we think of black holes sucking stuff in,

but the way that material moves around them,

sometimes they can also spit it out.

Indeed. And this could solve one of the problems,

a puzzle that is associated with the centre of these clusters.

These galaxies we're looking at here are the biggest galaxies

in the universe. And they would be yet bigger,

they could be up to ten times bigger

in terms of numbers of stars, if this process was not operating.

These ripples would be the lowest frequency sound

ever detected in the universe - a pure tone of infrasound,

one million billion times lower than the limit of human hearing.

If Andy's theory is correct,

infrasound plays a significant role in controlling the size of galaxies.

The mysterious sounds of a black hole

and the unique voice of a volcano...

..are a fascinating glimpse into a new world of sound,

beyond our human experience.

As we explore more of these exciting soundscapes,

it's clear that sound will become an even more powerful tool for

understanding our world and even our universe.

Next time, I will be investigating

the incredible ways in which we use, control and manipulate sound...

..helping us to survive...

..to explore the world around us...

..and to make the invisible visible.

If you want to find out more about the science of sound

and how we hear sound, go to...

..and follow the links to the Open University.