Sound Waves: The Symphony of Physics

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...

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.

You try and design the pattern of reflections

to be just right to enhance the music.

'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.

Higher frequencies have shorter wavelength

and give them better resolution.

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

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