Sound Waves: The Symphony of Physics

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.

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

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

I hear the word's angriest mosquito.

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

what it is...

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...'

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.

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.

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.

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.

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...

..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.

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.'

'These play the famous Westminster chimes.'

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

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...

..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...

..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...

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.

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.

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...

..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.