Using Sound Sound Waves: The Symphony of Physics

TWEETING BIRDS

BUZZING BEES

This is a familiar scene.

It's the Somerset countryside on a calm day.

And it sounds familiar.

I can hear the birds singing,

I can hear the wind rustling through the trees

and I can hear the insects around me.

This isn't just a landscape, it's a soundscape.

A constant flood of sound waves washing over me from all directions.

'No matter where we are or where we go, sound is always present.

'And each individual noise offers us information about our world

'from a moment in time and space.

'Every sound wave carries a story about where it's come from

'and the journey it's been on.'

And our evolutionary history has given us these two detectors

for tapping into those stories.

What we hear shapes our understanding of our world.

'In this programme, I'm going to explore how we exploit,

'manipulate and control sound.'

Just the quality of the sound says something's not right in here.

'I'll delve into the complex ways in which our own bodies

'precisely decode the information carried in sound waves.'

That's amazing.

When you take it off I can hear nothing.

It's incredible!

'And how the more we've come to understand sound,

'the more we've been able to use it

'to make remarkable discoveries about life...

'..our planet...

'..and even the solar system.'

'In our normal everyday lives, it's hard to really appreciate

'how much information sound carries.'

-Want to put the helmet on?

-OK.

-You need those, as well.

'Which is why acoustic engineer Professor Trevor Cox is taking me

'to a hidden location deep inside the hills of Scotland.

'Where, in the absence of light, hearing becomes my primary sense.'

I'm going to go in first, so I shall demonstrate.

It's ever so slightly sinister, this, isn't it?

-There's your helmet.

-OK.

-You want to put your gloves on.

I've probably have nightmares about doing something like this.

Slide yourself in.

Now, just be really careful as you get up. A bit further.

-OK?

-What have I arrived into?

I'm going to be slightly cruel and turn my head torch off

so we can't really see.

We're just really working with the acoustic here.

You have one of those. Shall we wander in just a bit further?

Very, very dark, isn't it?

-Watch where you walk.

-Urgh, that's horrid.

This is where the baddie turns up, right?

Two people walk into a dark space

and just the quality of the sound says something's not right here.

'Just from the way that sounds behave in this place,

'I'm beginning to piece together a picture of what it might be like.'

What do you think this space is?

So, it feels like it's gigantic.

I can't tell because I can't see anything but it feels as though

it could be enormous - the size of a cathedral or bigger.

Just because that's the only place

I've heard this sort of thing happen to my voice before.

I'm finding it hard to finish a sentence because I keep saying

a word and then stopping to listen to what it sounds like.

When you listen to a sound in a room you can get a lot of information.

You'll get the sound straight from me to you

and then all the walls are contributing reflections -

the sound's bouncing around the room.

All the time in a space we're listening for these sort of clues.

But we're not usually that, you know, conscious we're doing it.

'The ability of sound to reflect is one of the most critical ways

'it can carry information.

'But sound reflections can tell me more than the size of a place.

'I just need a different type of sound.'

-I've got a stopwatch for you there.

-OK.

-So, if you could wait for... hear the bang.

-Yeah.

And then just measure how long it takes the sound to decay to nothing,

which is actually how they first measured reverberation.

-I shall retreat to a safe distance.

-Yeah!

I just dropped it.

I can't see...

LOUD BANG

SOUND SLOWLY DIMINISHES

57 seconds.

Wow.

This place actually holds the world record

for the longest reverberation time,

which is what you kind of measured there.

What's going on to make that happen?

First of all, it's a very big place.

But there must be something more than that

because if you go into St Paul's Cathedral in London,

the sound would only last about ten seconds before dying away.

The sound is being contained and held in this giant space.

And that's because the walls here are incredibly massive.

You can tell that this must have hard, heavy walls,

whereas if you brought a lot of soft furnishings in,

which absorb sound, this place would go dead.

So, we're getting extra information

because sound reflects differently off different materials.

What is this place? After all that, where are we?

Well, let's put the lights on.

So, this is a massive space.

It's about a quarter of a kilometre long

so that's where a lot of the reverberations come.

-What's it doing here?

-Well, it's actually an oil storage depot

which was built in the run-up to World War II

to protect the Royal Navy shipping oil from bombing.

So, it's been made bombproof

and that's the reason it's got this huge reverberance.

They've made it out of half-metre-thick concrete

and behind it is the bedrock of Scotland.

So, this is really massive walls.

And the walls are covered in oil, as well. It's horribly sticky.

Sticky on your feet, everywhere. That's really useful acoustically.

Concrete's a bit porous so normally you get a little bit of absorption

but its pores have been gunked up with oil.

So, what's happening is that the sound is reflecting off the walls

really efficiently, it's not getting absorbed.

You can get a tremendous lot of information by looking at

the pattern of reflections, and, as an acoustic engineer,

that's what you do when you design a grand concert hall.

SAXOPHONE REVERBERATES

You try and design the pattern of reflections

to be just right to enhance the music.

SAXOPHONE REVERBERATES

'The reason that sound can carry so much information

'is because of its fundamental nature.

'It travels as a wave.

'And every time a sound wave reflects off a surface

'it's changed in subtle ways.'

Reflection is a way of redirecting sound

and that redirected sound carries information

about the obstacle it bounced off.

We use that acoustic signature to learn about our environment

in a general way, but there are animals that absolutely rely on it,

and they are the true masters of sound.

'For most bats, hearing is their primary sense.

'Listening to sound reflections is key to their survival.

'And their success has driven complex relationships

'with other creatures that live in and exploit this auditory world.

'Bats are one of the loudest creatures in the animal kingdom.

'We can't hear them because they mostly use frequencies

'our ears can't detect, making it quite difficult for bat experts

'like Dr Marc Holderied to study them.'

We have an acoustic camera that can pick up ultrasound

and we've just put it in one of my favourite research spots.

So, this is a commuting corridor

with loads of bats using it every night.

And this acoustic camera now shows me what is going on

as we look at this screen.

We've just seen two bats flying and there's a third one.

So, there's a whole group flying past.

You can see all these whitish yellowish blobs there.

As the bat was flying past

it was emitting these ultrasonic frequencies.

So, you're looking for patterns?

We can look at this spectrogram display down here

and try and find out which species we were looking at.

There's another one coming right now.

Now, if you look at that, they all ended about the same frequency.

They're around 45 kilohertz,

which tells us that this is a common pipistrelle.

And just now is a very different call.

And I can tell you that this is a Daubenton's bat.

So, you're painting this picture of all these bats whooshing past us,

making sounds that we can't hear.

If we could hear them, what would we hear?

What I've brought along here is a tiny bat detector.

It turns the ultrasonic frequencies into audible frequencies.

-That was a bat!

-There's one flying over right now.

We heard this very quick succession of calls there.

There it is again. Very good. It just whizzed over there.

So, they're very short and sharp and even though that sounds very quick

-to us, there's a lot going on between one pulse and the next.

-Yes.

They send out the high-intensity sound...

..and then they hit all the obstacles that are in the area.

These obstacles produce echoes

and the bat then waits for these to come back.

The further away an object is, the longer the echo takes to return

to the bat and this is how bats measure distance.

And that is an incredibly complex achievement.

There is so many different reflectors, like all the leaves,

you have the ground, you have all the branches,

and all of them produce echoes.

'Bats evolved the ability to use sound to see

'at least 53 million years ago...

'..giving them an enormous advantage when hunting for prey

'under the cover of darkness.'

So, we've got a moth here. What species is it?

It's a heart and dart.

It's got this beautiful gold sheen.

Yeah, yeah, yeah, they are quite beautiful.

And how's a bat going to find this moth?

So, a bat uses biosonar not only for navigation but also to capture prey.

So, when they are searching for insects, they want to look very far.

So, what they use is their lowest frequency calls that carry very far.

But as soon as they've detected the moth,

they add in higher frequencies to their calls.

BAT CALLS

Higher frequencies have shorter wavelength

and give them better resolution.

And better resolution means they can localise the moth very well.

BAT CALLS

And the bat sonar is giving it a brilliant tool

for finding these very fast-moving moths.

Do they have it all their own way?

Moths, of course, are fighting back.

All these moths had to do is evolve an ultrasound sensitive ear

that picks up the frequencies the bats emit, and they did.

-So, can this moth hear?

-This moth has ears, yes.

When they hear a bat that's far away,

they just steer out of harm's way.

And so, there's, sort of, one of these arms races going on

where one species makes a change that makes them more successful

and then their prey species also has to adapt.

So... Oh, it's going for a walk again.

And are there any other strategies that a moth could take

-to avoid this bat that's coming to get it for dinner?

-Yes.

Moths have taken the next step.

Moths have evolved a jamming mechanism

that helps them throw the biosonar off target.

You have a moth that knows it's under attack,

it produces ultrasonic clicks.

And these ultrasonic clicks are in the similar frequency range

as the echoes a bat is expecting.

But if it hears these clicks rather than the echoes

it can't really make out a full echolocation picture any more.

And that gives the moth the time to just whizz out of the way.

MOTH CLICKS

'This sophisticated interplay between bats and moths

'shows just how rich in information

'and how valuable reflecting sound waves can be.

'But reflections are not the only way sound waves help us

'understand our surroundings.

'There's another feature of sound that can provide us with

'even more information about the world.

'And it's particularly useful in warning us of approaching danger.

CAR HORN

AMBULANCE SIREN

LOUD TRAFFIC NOISES

I live in London and I cycle all the time

and it's easily the most dangerous thing I do on a daily basis.

There's so much traffic here.

Vans like that that overtake you when you're not expecting them.

What I'm conscious of is paying attention to light.

I can see what's in front of me, I look behind me,

that makes me feel secure.

But I'm getting a lot of extra information from sound.

Two things that really worry me when I'm cycling,

and they are big trucks and motorcycles.

Fortunately, both of them make a huge amount of noise.

That was a motorbike.

And you can hear them coming, even from around the corner.

LOUD MOTORCYCLE ENGINE

I certainly heard him.

'We can often hear things we can't see

'because, unlike light, sound can travel around corners.

'It's something made possible when a fundamental feature

'of the sound wave is just right - its size.'

It works a bit like this.

If we imagine we've got an obstacle in the way

and in this case that could be the corner of a building.

I'm going to draw a sound source over here.

Sound is spreading out in ripples, like the ripples on a pond.

So, as the sound travels away, those ripples spread out.

They can spread around the corner.

So, if I was standing here, I might not be able to see the sound source

but I would be able to hear the sound.

And this is called diffraction.

It doesn't work in the same way for all wavelengths

because diffraction depends on how the wavelength

is related to the size of the obstacle.

And a corner of a building is quite big.

So, this time I'm going to draw a higher frequency sound

which means the wavelengths are much shorter.

So, they'll spread out like ripples and they will diffract a little bit

as they go around the corner but not nearly as much.

So, sound that might be a wavelength of a few centimetres

are much smaller than the corner of the building

so I can't hear the high frequencies here

but I can hear the low frequencies.

'Most sounds can travel around objects

'because their wavelength is relatively big.

'Light, on the other hand, has a very short wavelength.

'Which means there are very few things in our world

'that it can bend around.

'Instead, light stops and casts a shadow.

'The ease with which sound can travel around the environment

'has played an important role in the story of our survival.

'Because it means we can hear the roar of a hungry lion

'or the rumble of a truck - even if we can't see them.'

The diffraction of sound does more than just let me know

that there's a sound source somewhere near me.

It helps me pinpoint exactly where that sound source is.

'This ability is called localisation.

'Every animal needs to know which direction danger is coming from.

'It works because sound doesn't just diffract around our environment,

'but also around the listener.

'Dr Jenny Bizley is here to show me the complex mechanisms

'we use to localise sound.'

So, I don't know where the sound's going to come from?

No, so if you face the front, I'll play a sound

and then you can maybe point to where you think it comes from.

No pressure!

LOUD MONKEY CHATTER Oh, it's loud, isn't it?

-Somewhere over there.

-Yeah, that's right.

We'll try another one.

GRUNTING Up there!

Yeah. And how about this one?

LOUD WHOOSHING Somewhere up there.

CRASHING Something broke over there.

'Although I'm not conscious of it,

'my brain is precisely locating each sound I'm hearing.'

So, the biggest bee in the world is over there!

Oh, it's moving.

'And it's not limited to fixed sounds.

'To understand how we localise sound,

'we need to look at the way it moves around our bodies

'and interacts with the two ears on opposite sides of our head.'

So, we're going to play the sound of the twig snapping

that you heard previously from one of the speakers over there,

and it was coming from the left of the head.

And we'll look at the input from the microphones on here.

So, we should see the sound waves coming in here.

TWIG SNAPPING

So, this is the signal from the first microphone,

which is on the left,

and this is the signal from the right microphone.

-And they look very different.

-Yes.

You can see that the left-hand microphone is picking up a signal

that's much louder than the signal on the right.

And it's also arriving sooner.

The timing difference, how long is that from there to there?

From there to there is about 500 microseconds.

So, just about half of a millisecond.

So, the sound reached my left ear

-half a millisecond before it reached my right ear?

-Yeah.

We can measure that difference

because sound moves relatively slowly, at least compared to light.

The difference in timing is useful for low-frequency sounds.

Because the low-frequency sound has quite a long wavelength,

longer than the width of the head,

the sound can diffract around the head to the far ear,

but it does so with a delay.

The other big difference here is the amplitude - the level of the sound.

What's the level difference between one ear and the other?

For this sound, we have a difference of the order of a few decibels,

5-10, depending on the frequency of the sound.

-So, that's quite a lot, is it?

-That's quite a large difference.

The amplitude difference is important really

for high-frequency sounds which have shorter wavelengths.

They are not able to diffract around the head

and they are shadowed by the head.

So, the signal will be louder in the near ear and quieter in the far ear.

These signals are kept within the brain and they're kept separately

until higher up, sort of, in the processing hierarchy,

when they're put together to give you a perception of space.

And that means that, within seconds, you can tell where a sound

comes from so that you can avoid it if it's going to eat you, or...

I definitely avoid sounds that are going to eat me!

'However, this system only works for localising sound

'in the horizontal plane.

'To know whether the sound is coming from above or below,

'we use a trick that depends on the shape of each individual ear.

'To show me, Jenny has kindly brought with her what looks like

'an awful lot of Blu-Tack.'

You know at school,

teachers were always taking Blu-Tack out of people's ears.

Somehow, you get older, and you become a scientist

and it works the other way around.

'The aim is to smooth out the folds of my outer ear.'

There you are. Take your finger out.

OK, now I've got ears full of Plasticine. Brilliant!

Sound can still go down there but it can't bounce off all of this.

I'm going to clap somewhere in front of you

and you should just close your eyes and then point at it.

OK, all right.

Erm, there?

No?!

OK, give me another try, give me another try. Go on.

-Down there?

-No.

There?

So, I'm rubbish at this with these in my ears.

I'm going to take these out because they're doing...

It does make the world sound very weird, actually.

When I've got them in, it's like there's less going on

and I take them out and suddenly the world opens out.

You're just missing that information that you're used to having.

'Ordinarily, sound waves will interact with my outer ear

'before travelling inside.'

When I clap, I make a broadband sound,

so it has many sound frequencies in it.

As the sound comes in, depending on where it comes from,

it'll hit different parts of your ear.

As it hits these complicated folds,

some sound frequencies are made louder and others are made quieter,

and your brain's learned over time how to interpret these changes

that occur, according to where the sound comes from.

You're listening for really subtle changes in the frequency composition

of the sound that are introduced by

the folded structure of the outer ear.

So, the ear here is not just guiding sound in, this outer bit,

it's actually changing it.

So, it's really clever. That's really complicated

and really clever at the same time.

It is really clever and you have to learn to do it.

Everyone's ears are different

and the peculiarities of your outer ear are special to you.

'The properties of sound waves and the way they travel

'carry important messages about our environment.

'But once those messages enter our ears, they need to be translated.'

In order to access this information that's all around us,

we need a detector.

Something that can convert these tiny vibrations of the air

into a signal our brain can understand.

'Most of us take hearing for granted,

'because it happens apparently automatically deep inside our ears.'

The reason that we can hear so much and so well

is that our ears are sophisticated detectors -

a series of different structures all working together.

If just one of the links in that chain is broken

the consequences can be devastating.

'I miss not hearing the birds.

'I lost my hearing very, very quickly.

'You can't believe it's happening.

'You think, "Oh, did I hear something?"

'But, no, you don't.

'It really is frightening.'

'This is Barbara.

'She lives with her husband, Tony,

'and they've been married for 53 years.'

LAUGHTER

-What's funny?

-Hm?

What's funny?

'But, for the past year and a half,

'they've not been able to communicate properly.'

Crashed on...the wires...!

'Because, very suddenly, Barbara became profoundly deaf.'

'I can't hear anything round out here.

'I just miss my old life in general, really.

'Yeah.'

Not, sort of, hearing people or knowing what they're talking about.

That's quite difficult.

Deafness is a lonely world.

'Barbara lost her hearing because just one small part of her ear

'stopped working.

'When sound enters a healthy ear, it gets funnelled through

'to a coiled up structure called the cochlea -

'a spiral-shaped cavity containing some 16,000 specialised cells

'called hair cells.

'As the sound wave moves through the cochlea,

'the cells' hairlike protrusions are displaced...

'..causing the cell to send electrical impulses

'along nerve fibres that are destined for the brain.

'But Barbara's hair cells are no longer working,

'which means that although the rest of her ear is healthy,

'her brain is completely starved of sound.'

'I miss my independence.'

What I try not to do is get down. I try to think positive.

How are you feeling about today?

-I'm OK. Yeah.

-OK?

How about you?

Bit nervous, I suppose.

'A month ago, Barbara was fitted with a cochlear implant.

'An array of electrodes has been threaded into her cochlea

'that will take over the role of her faulty hair cells.

'And today, at Southampton University,

'it will be switched on and tested for the first time.

So, I'm going to switch it on, OK?

-Can you hear anything?

-Not yet, no.

Just going to bring it up.

Nothing.

BEEPING

Very faint.

Very, very faint.

BEEPING

Very gradual, isn't it?

Yeah. Bit more?

Yes.

I'm going to keep talking as I bring it up, OK?

Just going to keep bringing it up.

How did you get here today, Tony?

I can hear... Can't understand.

I can almost hear my own voice again!

How's the volume now?

How's the volume?

-Yes!

-The volume?

-The volume. How's the volume now, you said, yes.

-Yeah.

What can you hear?

-Can you hear me?

-Yes, I can hear you.

Oh, dear.

No, it's good.

Yeah.

'For the first time in over a year,

'Barbara's brain is receiving sound signals.'

-OK?

-That's amazing.

When you take it off I can hear nothing.

Amazing, yes.

Don't make me cry!

Don't worry about a hanky.

-So, you're noticing the difference?

-It's incredible.

Stop it. You're going to make me cry.

Thank you.

Oh, dear.

I didn't think it would be this quick.

No, you're doing really well.

I thought for my birthday in July I might be able to hear then.

What are we going to have for dinner tonight, some champagne?

Stop it. You'll make me cry again!

'Barbara is no longer lost in silence.

'By translating sound into electrical signals,

'the implant replicates the cochlea's key job,

'returning Barbara to a world full of sound.

'The cochlea is a truly extraordinary structure,

'doing much more than simply translating noise.

'It's also able to discriminate the incredible variation of sounds

'in our environment.'

Even though it's quite quiet and calm where I am now, there's still

a huge richness of information in the sound around me.

And a lot of that richness comes in the frequency of the sound,

the number of times every second that air molecules are vibrating

backwards and forwards.

It could be a hundred times or a thousand times

and they're all overlaid on top of each other.

So, the singing birds and the distant road

are all creating an environment

that's full of different frequencies

and that is really useful information.

'Our cochlea has a really clever way of telling us

'which frequencies are coming into the ear.

'It exploits a phenomenon called resonance

'which can be demonstrated with these conkers.'

You can see if I push on one and I push on another one,

this one with the short string is going backwards and forwards

really quite quickly.

Whereas this one down here with a longer string,

you can see it swings much, much more slowly.

Each one has its own natural frequency.

And it's different for every conker

because the string is a different length.

Now, the clever bit comes when a frequency comes from somewhere else.

And I'm going to demonstrate that here with this apple.

If I swing the apple, what happens is that the apple

will gently move the string and that's forcing all the conkers

to oscillate at the same frequency as the apple,

however longer their string is.

And you can see that these ones are moving a little bit,

moving a little bit, little bit more, and this one,

this one is the one that's really responding.

And if you look at it from this angle, you can see that this conker

is the one that's got the same length of string as the apple.

The others are hardly moving at all and this one is swinging loads.

'And I can show you what happens

'when I change the frequency of the driving force.

'By shortening the string, I can make the apple swing faster.'

We can see that this time it's this one.

This conker is responding really, really strongly

and this is the one again that's got more or less

the same length of string as the apple.

It's got the same natural frequency as the oscillation coming in.

And now it's trying to hit me in the face!

This is the phenomenon of resonance.

This is very similar to what's happening in the cochlea.

'Just as the conker strings have a variety of natural frequencies,

'so do structures in the ear.

'The thousands of tiny hair cells that send messages to the brain

'sit along a structure called the basilar membrane.

'This stretched piece of elastic that runs through the cochlea

'has different natural frequencies as you go along it.

It's got one end which is narrow and taut

and it's got a very high natural frequency of oscillation

and the other end of the basilar membrane is wider and less taut

and that's got a lower frequency of oscillation.

So, when sound comes into our ear,

the whole basilar membrane will vibrate a little bit

but one part of it will really start to vibrate.

The one that matches the frequency of the sound coming in.

And it's the hair cells at that part of the basilar membrane

that are stimulated, that send the sound into our brains

and that's how our ears tell us which frequencies of sound

are coming in from the environment around us.

'This elegant and simple mechanism gives us the ability to detect

'and interpret an enormous range of frequencies.

'A far greater range of sounds than the spectrum of light waves

'we can see with our eyes.

'From low-sounding noises that go through 20 cycles a second

'and have wavelengths 17 metres long.

'All the way through to very high-frequency sounds

'that can exceed 18,000 cycles a second

'and have a wavelength of under two centimetres.

'The cochlea's a sophisticated structure

'that lets us detect a huge variety of sounds.

This story is interesting because it passed through

one of the most significant stages in evolutionary history.

When hearing and life first evolved, it all happened underwater.

'Which would mean that, one day, it would have to confront and overcome

'a physical law of nature.

'3.5 billion years ago, life began in the oceans.

'And as organisms became ever more complex,

'they developed increasingly sophisticated senses.

'Around 400 million years ago,

'fish became the first hearing animal,

'evolving structures that,

'although much simpler than the modern cochlea,

'worked in a similar way.'

Ears underwater were fluid-filled cavities

and so sound could easily travel from the water

into the underwater ear and it could easily be detected

because there was liquid on both sides of that boundary.

'But when that life came up into air,

'suddenly the sound was in the air

'but the ear was still filled with fluid

'and that was a problem.'

I've got a set up here that will show what happens when sound

tries to travel across a boundary from air into water.

I've got two microphones here. One's a normal microphone. This one.

It's set up for hearing sound in air.

And the other one is set up for hearing sound underwater, down here.

That's called a hydrophone.

I've got some tent pegs here.

I could hear that quite easily and so could the microphone,

so there's a great big spike on the microphone in air.

But the hydrophone in water heard almost nothing.

What's going on is that at the boundary,

when there's air up here and water down here,

and sound comes from the air and hits that boundary,

because air is less dense and much easier to squash than water,

instead of travelling through,

that sound wave just bounces straight off.

It doesn't get through the boundary.

And this is the problem that early life faced.

If you've got a fluid-filled ear, liquid-filled ear,

it works perfectly underwater because sound can travel

through the water into your liquid-filled ear

and you can hear the sound. But once you put that in air,

the sound comes in from the air

but it hits your ear and bounces straight off.

It can't get in to be detected.

'The way sound behaves at a boundary between two mediums

'hindered the ability of early land-based life to hear properly.

The process of evolution came up with a really elegant solution

to this problem, by moving around some very tiny bones.

And here they are. These are life-size casts of them.

And they're called the malleus, the incus and the stapes.

The ossicles, which means "tiny bones".

And they are the smallest bones in the body.

And two of them were part of the jawbone in our marine ancestors

but they moved into the middle ear and they do something very clever.

By working together,

they help move sound from the outside world into the cochlea.

'The ossicles sit just in front of the cochlea.

'And when sound hits the eardrum, these tiny bones are set in motion.

'Moving efficiently as a set of levers

'between the large eardrum and the tiny stapes.

'This increases the energy that's transferred to the cochlea.

'This sophisticated little mechanism acts as an amplifier

'and it's really efficient.'

What matters is the amount of sound energy

that gets into the fluid inside the cochlea.

And without this, it would be about 1%,

but with a middle ear like this, it's about 60%.

So, this is the crucial evolutionary step

that allowed land-based mammals to develop such good hearing.

'Hearing that allows us to detect a huge range of amplitudes.

'Everything from the thundering roar of an engine...

'..to the flapping of an insect's wings.

'And hearing the very quiet end of this range

'doesn't rely solely on the ear but also on what lies beyond it.

'To experience this, I need to find something extremely rare.

'Silence.

'It doesn't exist in the natural world

'so I've come here - the largest anechoic chamber in Britain.

'It's been meticulously engineered to be incredibly quiet.

'And it's here that I'll test my ears to their limit.'

The idea of all this clobber is that I have to be in there

completely on my own.

So, there's no sources of sound and nothing to reflect off.

So, this might be a moot point because I might decide I hate it

after two minutes and that's all right.

But if I'm all right after 20 minutes, is there any reason to...

Does it get worse as you go? Because some people don't seem to mind it.

I think it's completely individual and so you, kind of, see how it is.

'All on my own, I can feel myself adjusting to this new environment.

'I can't hear any sounds from outside.

'It's the quietest place I've ever been.

'And as I sit, the rustle of my clothes sounds strangely loud.

HEART BEATING

'I'm starting to notice the sounds of my own body.

'The regular beating of my heart.

'A background hiss, perhaps from the firing of my nerves.

'The soft whisper of my breath.

'Sounds that I don't ordinarily hear have now become dominant.'

Oh, they're opening the door.

I wonder what the outside world's going to be like now.

'After 50 minutes, Dr Peter Keating arrives to explain

'how I could hear so much in a place like this.

-So, how was that?

-It wasn't ever completely silent.

My brain was always telling me it was hearing something

but that something was very, very quiet.

When you take external sounds away, which is what's happening here,

then first of all you become more sensitive to the sounds

that are inside your body.

There's actually a little separate set of cells in your auditory nerve

which are responsible for hearing very quiet sounds.

So, in here, you were probably switching over to using those.

'A specialised type of nerve fibre

'carries very quiet sound signals from the cochlea to the brain,

'where our sensitivity to this type of sound isn't fixed.

The brain is constantly adapting,

and so, if you take away loud sounds and you only have quiet sounds,

the brain will get used to that over time.

So, the physical hearing apparatus is staying the same

-but our brains are what's doing the adapting?

-Absolutely.

So, when you came in here, in the first seconds to minutes,

there would have been some changes going on in your brain.

If you'd stayed in here for longer, if you'd stayed in for days, weeks,

more changes would have happened.

And if you'd stayed in here for months,

even more changes would have happened.

That's one of the things that we're finding out about the brain

is that you can adapt to these changes in sensory input.

Not just hearing, but in vision and all kinds of other sensory systems.

And these can happen at all kinds of different timescales.

'The processing power of our brain,

'together with the mechanics of our ears,

'forms an incredibly powerful and adaptive system

'to listen in to the world.

Understanding the physical properties of sound

and being able to decipher them to learn about the world around us

is a really powerful tool.

But we're not limited to just listening in

on what the environment sends to us.

We can create our own sound to send it out to probe the world.

And that can teach us about ourselves, our planet

and even what's beyond that.

'Sound has been especially useful in looking at things we can't see.

'Things that are hidden from the world of light.

'It began in the early years of the First World War,

'when submarines became a deadly weapon.'

EXPLOSION

'Almost invisible,

'these machines would drive the Allies

'to develop new detection technology.

'Sound can travel exceptionally long distances underwater

'and so acoustic echo ranging, or sonar, offered an obvious solution.

'And after the Second World War had come to an end,

'the rapid advancements of underwater acoustics continued.

'Our relationship with the oceans can be limited.

'Quite often you look out over the sea and what you see is this.

'It's grey and opaque, you can't see through the surface.

'It looks a little bit dull.'

But underwater acoustics changed all of that.

Once you can use sound to explore the underwater world,

you're not limited to looking for submarines.

'Today, even as we reach for the stars,

'we know less about this ocean than we do the surface of the moon.'

'By the 1950s, oceanographers across the world

'were using military sonar technology

'to look down at the deep ocean floor,

'which, for centuries, we could only imagine.

'They discovered an extraordinary underwater landscape

'of towering mountains and deep trenches.

'Sound played a key role in understanding

'the magnificent structures of our world.'

The oceans are one of the most important features of our planet

and they're not just the filler between the interesting bits.

Once you can see them properly,

you can see the oceans become a place.

'Today, sonar doesn't just show us large-scale structures,

'it can also reveal exquisite detail.'

-Welcome aboard.

-Thank you.

'Which, until recently, had been a job only our eyes could perform.

'This is the North Sea, off the coast of Suffolk.'

Looks just like an ordinary bit of ocean

but there is an archaeological site down there,

so I'm going down to have a look.

I have a lot of layers to put on.

Oops. The other way round.

Right, it's definitely cold in the North Sea!

-I can't actually see you.

-You can't see me at all?

-Unless you come in really close.

-So...

-Yeah, it's just so brown.

I've got my glove here.

And if I hold that out, in front of my face underwater, you can't

see anything, so I can't see this far in front of my face.

And the reason it's this brown, horrible colour,

is that the water is clearly full of sediment.

There's tiny little particles of silt and sand.

And so seeing anything...

is virtually impossible.

'Even though we're near the coast,

'where the water isn't particularly deep,

'the visibility is still appalling.'

That's... That's terrifying.

I was only going down a metre or two and it's completely black.

Like, absolutely dark.

'Since I couldn't see anything for myself,

'Professor David Sear explains what lies beneath us.'

When I was down there a little while ago I couldn't see anything.

So, what is down there?

Well, actually, down there is one of the largest archaeological sites

in the world, called Dunwich.

Dunwich, to a lot of people, is just a small village.

800 years ago it was the sixth largest international port

in the North Sea.

And the story of Dunwich is one of coastal erosion.

Coastal erosion driven by a series of very large storms.

So, this sounds like the perfect job for sonar.

What do you see when you look with sonar?

Sonar enabled us to cover a large area

and we were able to see that there were indeed structures.

The important thing was that we didn't know whether they were

geology or were they actually parts of churches and buildings?

So, what you ideally need is a technology

that is able to see through this turbid, muddy water

with the detail to enable you to see individual,

say, carved blocks or other evidence of it being made by people.

We came across a technology that is relatively new

and it does just that.

It uses sound to project... A bit like a torch beam, but sound.

And you don't do that from a boat?

You don't. You have to send a diver down

and that diver sees what the sound is illuminating, if you like,

in their visor.

'Sound waves from surface-based sonar

'can travel easily through the water, which provided David

'with the layout and general structure

'of this two-kilometre-squared site.

'Yet it was the much higher frequency sound waves

'from the sonar camera that gave David what he really needed.

'Although these sound waves can't travel as far,

'they can create much more detailed images,

'and showed that what lay beneath the waves

'were structures with sharp straight edges.

'Edges that could only have been made by man.'

The first time we saw this imagery,

looking at it in real-time as the diver saw it, it was fantastic,

because you could see great blocks of masonry, made of flints,

rubble, mortar, just like the churches today on land.

You see it on the seabed.

That nailed it for us. It was the evidence we needed

to move from the historical accounts to the reality of,

yes, these are the ruins of churches from medieval Dunwich.

'Sending sound waves through the ocean

'has unlocked marine archaeology,

'uncovering the human stories hidden beneath the sea.

'We're continually getting better at detecting and controlling

'the nuances of sound waves

'and at using them as tools for probing and manipulating our world.

'But there are other worlds out there.

'Even though sound can't travel across the solar system,

'every planet and moon is like a little bubble of sound

'isolated from us by the vacuum of space.

'And there's a huge amount to learn from those little bubbles of sound,

'if only we can listen in.'

'Three, two, one...

'And lift-off of the Cassini spacecraft!'

'In 1997, one of the largest spacecraft ever launched

'started its billion-kilometre journey.'

'We have cleared the tower

'and the Cassini spacecraft is on its way to Saturn.'

'In 2005, Cassini sent a probe called Huygens to Titan,

'the largest of Saturn's moons,

'A world shrouded by a thick, opaque atmosphere...

'..making it almost impossible to explore from a distance.

'So, for decades, this moon remained much of a mystery.'

Huygens is still the only probe to have successfully landed

in the outer solar system.

And as it deployed its parachutes and started this two-and-a-half-hour

drift down through the atmosphere of Titan towards the surface,

there was a suite of instruments on the probe measuring all sorts

of things about the environment and the conditions.

And some of those instruments were recording sound.

'Around 160km above the surface of Titan,

'Huygens deployed a microphone,

'which recorded the sounds of Titan's atmosphere.

SOUND OF STRONG WIND

And this is it.

This is what the microphone on Huygens heard

as it fell through Titan's atmosphere.

What you're hearing is the roaring of the wind going past the probe

and the probe falling down through the atmosphere.

This is the sound of an alien world, and this was only the start.

'Another instrument used sonar to detect the surface

'during the final 90 metres of the descent.

'It showed that Titan's terrain rises and falls.

'That the surface is relatively smooth,

'not dissimilar to gravel,

'and that this surface is likely to be damp.'

This is the landscape that Huygens landed on.

Sonar was one of the tools that helps us understand it.

Even if a planet or a moon hasn't got an atmosphere,

sound can still be generated and transmitted through its liquid

and solid layers, so potentially, if you sent an acoustic probe

to another world, you might hear the sound of thunder,

or hear meteorite strikes,

or the flow of rivers. Perhaps rivers of methane.

Or the sound of rain.

And as more and more missions are sent out into the solar system

to explore, acoustic probes are going to become more and more common

as a way of exploring not just our world but others.

'We live in a dynamic, pulsating world of sound

'and it touches our skin and our clothes and our lives every day.'

We can only tap into it because we have these two complex,

sensitive detectors on either side of our head,

but that's enough to sense the riches.

Sound is so important for our species.

It's deeply embedded in our culture

and it's allowing us to push our technological boundaries

to better understand our world.

And the best thing about it is that that world of sound is right here.

All you have to do is listen.

FIREWORKS

If you'd like to find out more about the science of sound

and how we hear sound, go to the BBC website on screen

and follow the links to the Open University.