0:00:07 > 0:00:08EXPLOSION
0:00:10 > 0:00:14This is Stromboli, one of the most active volcanoes in the world.
0:00:14 > 0:00:16And a few times every hour,
0:00:16 > 0:00:20it sends out huge explosions of lava and ash.
0:00:20 > 0:00:23And of course we expect noise to go with those explosions
0:00:23 > 0:00:26but along with the sounds we can hear,
0:00:26 > 0:00:28there are also sounds we can't.
0:00:28 > 0:00:30Because this volcano, like many others,
0:00:30 > 0:00:33is, in effect, a gigantic musical instrument.
0:00:34 > 0:00:37Only now are scientists understanding
0:00:37 > 0:00:39how strange and spectacular
0:00:39 > 0:00:41the world of sound really is.
0:00:47 > 0:00:49It is easy to take sound for granted.
0:00:51 > 0:00:53Sound is noise...
0:00:53 > 0:00:55it's music...
0:00:57 > 0:00:59..it is the spoken word.
0:01:03 > 0:01:07But it is far more than just a soundtrack to our lives.
0:01:07 > 0:01:09CRACK!
0:01:09 > 0:01:12The more we've discovered about the physics of sound...
0:01:12 > 0:01:13BOOM
0:01:13 > 0:01:16..the more astonishing the secrets it's revealed.
0:01:18 > 0:01:21HIGH-PITCHED SQUEAKING I hear the word's angriest mosquito.
0:01:24 > 0:01:28In this series, I'm going to investigate the nature of sound -
0:01:28 > 0:01:30what it is...
0:01:30 > 0:01:31BELL CLANGS
0:01:31 > 0:01:34I can feel that through my feet. it's really cool.
0:01:34 > 0:01:36'..what it tells us...'
0:01:36 > 0:01:39Just the quality of the sound says something is not right here.
0:01:40 > 0:01:42'..and how we use it...'
0:01:42 > 0:01:44ENGINE ROARS
0:01:44 > 0:01:46Certainly heard him.
0:01:46 > 0:01:49'..allowing us to see the world and even the universe
0:01:49 > 0:01:51'in new and exciting ways.'
0:01:52 > 0:01:55Every sound is created for a reason.
0:01:55 > 0:01:57Every sound has a story to tell.
0:02:09 > 0:02:11BIRDS SING AND BEES BUZZ
0:02:19 > 0:02:21CRACKLING
0:02:28 > 0:02:30Listening to a tree seems like an odd thing to do
0:02:30 > 0:02:32but this tree isn't silent.
0:02:32 > 0:02:35Even through a stethoscope like this, I can hear creaking and
0:02:35 > 0:02:38groaning as the branches move in the wind,
0:02:38 > 0:02:41and there are other sounds in there that I can't quite hear with this.
0:02:41 > 0:02:43Crackling, popping sounds.
0:02:43 > 0:02:45POPPING AND CRACKLING
0:02:54 > 0:02:55It happens because the tree
0:02:55 > 0:02:59is drawing water up from its roots to its leaves
0:02:59 > 0:03:02and, on a hot sunny day like this, as that water travels through
0:03:02 > 0:03:05the tiny tubes round the outside of the tree,
0:03:05 > 0:03:08bubbles form, and those are what are making the crackling noise.
0:03:08 > 0:03:11So although you wouldn't know it by looking at it,
0:03:11 > 0:03:15that crackling noise could tell you that this tree is thirsty.
0:03:17 > 0:03:20Before we can unlock all the secrets of sound...
0:03:21 > 0:03:24..we need to understand it at a fundamental level.
0:03:26 > 0:03:28So in this programme,
0:03:28 > 0:03:32I'm going to explore what sound is and how it's made.
0:03:36 > 0:03:39First, it would help if I could turn a sound
0:03:39 > 0:03:42into something we can actually see.
0:03:46 > 0:03:48This is a very special space.
0:03:48 > 0:03:50It is called a hemi-anechoic chamber,
0:03:50 > 0:03:54and what means is that all the walls and ceiling have these funny shapes
0:03:54 > 0:03:58on them that are absorbing sound so it's really quiet in here.
0:03:59 > 0:04:02It's the perfect environment to isolate a pure sound
0:04:02 > 0:04:04and observe its effects.
0:04:04 > 0:04:08All I need is this small army of candles and a speaker.
0:04:09 > 0:04:12To make this work, I need the sound to be really loud
0:04:12 > 0:04:15so I'm going to wear ear defenders.
0:04:18 > 0:04:20DEEP RUMBLE This is a really deep sound -
0:04:20 > 0:04:23you can see the speaker going in and out.
0:04:25 > 0:04:29And what you can see is that the candles are vibrating -
0:04:29 > 0:04:31this very, very fast vibration.
0:04:33 > 0:04:37'The individual candle flames are showing the movement in the air
0:04:37 > 0:04:39'caused by the speaker.'
0:04:39 > 0:04:41What's happening is that the speaker here
0:04:41 > 0:04:46is producing enormous amounts of sound by pushing on the air.
0:04:46 > 0:04:48And that push pushes on the air next to it
0:04:48 > 0:04:51which pushes on the air next to it,
0:04:51 > 0:04:53and it travels out across the candles.
0:04:54 > 0:04:59'The candle flames are flickering back and forth 20 times per second,
0:04:59 > 0:05:02'or at 20 hertz.
0:05:02 > 0:05:05'This is the frequency of the sound we are hearing.'
0:05:05 > 0:05:06I'm going to turn it up.
0:05:06 > 0:05:09NOISE RISES IN PITCH
0:05:11 > 0:05:17If I increase the frequency, the candle flames flicker even faster.
0:05:17 > 0:05:20And what you can see is that the candles are all flickering
0:05:20 > 0:05:22but they are all flickering together.
0:05:22 > 0:05:24This is synchronised movement.
0:05:24 > 0:05:27They are all moving forwards and backwards together.
0:05:33 > 0:05:37'So what we're seeing is the sound.
0:05:37 > 0:05:41'The movement of the speaker causes the air molecules to oscillate
0:05:41 > 0:05:44'back and forth at a specific frequency.
0:05:44 > 0:05:48'These oscillations travel through the air as sound waves
0:05:48 > 0:05:50'and they are picked up by our ears.'
0:05:52 > 0:05:55A loudspeaker is actually a very unusual way of making sound
0:05:55 > 0:05:57because it's artificially manufactured
0:05:57 > 0:05:59to generate any sound you like.
0:05:59 > 0:06:02Most sound is much more interesting.
0:06:03 > 0:06:06That's because, unlike the loudspeaker,
0:06:06 > 0:06:10most objects create a specific sound that's unique to them...
0:06:11 > 0:06:14..and this is ultimately at the heart
0:06:14 > 0:06:18of why sound is such a rich source of information about the world.
0:06:24 > 0:06:27To understand how an object produces its own unique sound...
0:06:27 > 0:06:29ENGINE ROARS
0:06:30 > 0:06:33..we need a clear and simple sound source.
0:06:36 > 0:06:40For me, one of the most beautiful examples of this is a sound that has
0:06:40 > 0:06:43been ringing out across our cities for centuries.
0:06:43 > 0:06:45BELL CLANGS
0:06:45 > 0:06:46The sound of church bells
0:06:46 > 0:06:48is one of the most distinctive sounds of Britain.
0:06:48 > 0:06:51And I learned to ring bells as a kid,
0:06:51 > 0:06:53so I have certainly spent a lot of time in bell towers,
0:06:53 > 0:06:56but there is one bell that I've never seen.
0:06:56 > 0:06:59It's not only the most famous bell in this country,
0:06:59 > 0:07:02but the most famous bell in the world.
0:07:02 > 0:07:05It's just up there and it's the one we all know as Big Ben.
0:07:07 > 0:07:10'The sound of Big Ben is instantly recognisable.
0:07:11 > 0:07:13'It's an apparently simple sound
0:07:13 > 0:07:16'but also one that's rich and melodious.
0:07:17 > 0:07:20'Analysing how Big Ben's sound is created
0:07:20 > 0:07:24'reveals something remarkable about the relationship between
0:07:24 > 0:07:27'an object and the sound it produces.'
0:07:31 > 0:07:32So this is it.
0:07:32 > 0:07:34This gigantic bell is Big Ben.
0:07:36 > 0:07:39And all sorts of things have changed in the 150 years
0:07:39 > 0:07:43since the Victorians hung it here. But the sound is exactly the same.
0:07:43 > 0:07:47And now I am up here, I can see it in action for the first time.
0:07:49 > 0:07:51'Alongside Big Ben,
0:07:51 > 0:07:54'there are four other smaller bells that hang in the belfry.'
0:07:54 > 0:07:57BELLS CHIME
0:07:58 > 0:08:00'These play the famous Westminster chimes.'
0:08:00 > 0:08:02BELLS PLAY WESTMINSTER CHIMES
0:08:07 > 0:08:12'It is only after this is finished that Big Ben itself is heard.'
0:08:17 > 0:08:20LOUD CLANG REVERBERATES
0:08:31 > 0:08:34It is an incredible amount of sound.
0:08:34 > 0:08:36I could feel that through my feet.
0:08:36 > 0:08:37That's really cool.
0:08:38 > 0:08:42The way that the bell makes sound is that this huge 200kg hammer
0:08:42 > 0:08:46hits the side and that sets the metal vibrating.
0:08:46 > 0:08:48And as it pushes out, it pushes into the air,
0:08:48 > 0:08:50sending pressure waves outwards.
0:08:50 > 0:08:52And those are the sound waves.
0:08:52 > 0:08:56But all of this doesn't just happen at one frequency.
0:08:56 > 0:09:00The huge richness of the sound that Big Ben makes
0:09:00 > 0:09:04comes from many frequencies all happening at the same time.
0:09:10 > 0:09:13So how does one bell produce many different frequencies?
0:09:13 > 0:09:16And what makes them sound so good together?
0:09:22 > 0:09:25The first scientist to try and unpick the frequencies
0:09:25 > 0:09:27within an object's sound
0:09:27 > 0:09:31was the German physicist and amateur musician, Ernst Chladni.
0:09:33 > 0:09:36Chladni devised a special experiment that enabled him
0:09:36 > 0:09:39to study how even the simplest of objects
0:09:39 > 0:09:41can produce a complex sound...
0:09:41 > 0:09:43CYMBAL CRASHES
0:09:43 > 0:09:45..made up of many different frequencies.
0:09:50 > 0:09:54I'm going to do a modern-day version of Chladni's experiment.
0:09:55 > 0:09:57This is a Chladni plate.
0:09:57 > 0:10:00It's just a flat metal sheet that's held in the middle.
0:10:00 > 0:10:02And if I hit it... FLAT CLANG
0:10:02 > 0:10:04..it makes it a sound that doesn't sound very pleasant.
0:10:04 > 0:10:06Certainly not nearly as nice as Big Ben.
0:10:06 > 0:10:09But that sound has a lot in common with the sound of Big Ben
0:10:09 > 0:10:13because it's made up of lots of different frequencies.
0:10:13 > 0:10:14And Ernst Chladni came up with
0:10:14 > 0:10:16a really clever way of picking apart
0:10:16 > 0:10:21where that sound comes from. So he started with a plate like this.
0:10:21 > 0:10:25And he sprinkled sand on top, so I'm going to do that.
0:10:28 > 0:10:30And then he set the plate vibrating.
0:10:30 > 0:10:32And I'm going to do that with a signal generator here
0:10:32 > 0:10:35that's going to move the middle of the plate up and down.
0:10:35 > 0:10:38And the number on the front here is the number of times every second
0:10:38 > 0:10:42that vibration is going to happen - so at the moment it's 240.
0:10:42 > 0:10:44So if I turn this on...
0:10:44 > 0:10:46WHINING HUM
0:10:47 > 0:10:49So it's not a pleasant noise.
0:10:50 > 0:10:53You can see the sand is dancing about in the plate
0:10:53 > 0:10:54but it's not too exciting so far.
0:10:54 > 0:11:00But what happens if you turn the frequency up is quite different.
0:11:00 > 0:11:01HUM INCREASES IN PITCH
0:11:08 > 0:11:12And suddenly at this frequency here, 264 hertz,
0:11:12 > 0:11:15you can see this beautiful pattern pops up in the sand
0:11:15 > 0:11:17of the top of the plate.
0:11:17 > 0:11:18And what this is giving away
0:11:18 > 0:11:21is that the plate is vibrating in a shape
0:11:21 > 0:11:24and the sand is showing us what shape that is.
0:11:24 > 0:11:28What's happening is that the plate is bending like this,
0:11:28 > 0:11:30and at the parts of the plate that are moving a lot,
0:11:30 > 0:11:32the sand is getting bounced away.
0:11:32 > 0:11:35And the parts of the plate that are between a bit that is going up
0:11:35 > 0:11:37and a bit that is going down, don't move at all,
0:11:37 > 0:11:40and so the sand accumulates in those places.
0:11:40 > 0:11:42So what Chladni had found was a really clever trick
0:11:42 > 0:11:44for seeing the shape of the vibration,
0:11:44 > 0:11:47even though he couldn't see it with his eyes.
0:11:49 > 0:11:53The vibration pattern revealed by the sand occurs at what is known
0:11:53 > 0:11:56as a natural frequency of the metal plate.
0:11:57 > 0:12:01This is a specific frequency at which the plate naturally vibrates
0:12:01 > 0:12:02and produces sound.
0:12:04 > 0:12:08And this is part of what's making up the sound when I hit the plate.
0:12:08 > 0:12:11But it's not all of it, because if you keep turning the frequency up,
0:12:11 > 0:12:12there's more to see.
0:12:12 > 0:12:15SOUND INCREASES IN PITCH
0:12:35 > 0:12:37And so here we are up at 426 hertz
0:12:37 > 0:12:39and suddenly, out of that mess,
0:12:39 > 0:12:41there's another pattern of vibration,
0:12:41 > 0:12:44beautiful pattern on the plate here.
0:12:44 > 0:12:49Chladni's experiment reveals how a simple object, this metal plate,
0:12:49 > 0:12:51can produce a complex sound...
0:12:52 > 0:12:55..because it doesn't vibrate at one frequency.
0:12:55 > 0:12:57It has many natural frequencies...
0:12:57 > 0:12:59HIGH-PITCHED RINGING
0:13:00 > 0:13:03..each corresponding to a different pattern of vibration,
0:13:03 > 0:13:05more elaborate than the one before.
0:13:10 > 0:13:13When you hit the plate, what happens is that lots of those vibration
0:13:13 > 0:13:17patterns all happen at the same time, one on top of the other.
0:13:21 > 0:13:25Each one contributes their natural frequency to the mix,
0:13:25 > 0:13:28and that combination is what makes up the sound that you hear.
0:13:34 > 0:13:35Every object that vibrates
0:13:35 > 0:13:39has its own combination of natural frequencies
0:13:39 > 0:13:42determined by its physical characteristics.
0:13:42 > 0:13:46And together, these frequencies form a unique acoustic signature.
0:13:49 > 0:13:53So there's a beautiful relationship between an object and the sound that
0:13:53 > 0:13:56it produces. When you hear sound,
0:13:56 > 0:14:00you are hearing messages about the thing that created it.
0:14:00 > 0:14:06Its size, its shape, what it's made from, even how the object was made.
0:14:09 > 0:14:12And natural frequencies are the key to understanding
0:14:12 > 0:14:14one of the most fascinating mysteries
0:14:14 > 0:14:16about the sounds we encounter in our daily lives.
0:14:19 > 0:14:23Why do some sounds seem rough and unpleasant,
0:14:23 > 0:14:25whilst other sounds like Big Ben
0:14:25 > 0:14:27seem more attractive to the human ear?
0:14:29 > 0:14:31To find the answer,
0:14:31 > 0:14:34we need a way to reveal the exact natural frequencies
0:14:34 > 0:14:35of Big Ben.
0:14:37 > 0:14:41Sprinkling sand isn't going to work for a bell.
0:14:41 > 0:14:44But a team of scientists from the University of Leicester
0:14:44 > 0:14:48are recreating Chladni's experiment using state-of-the-art technology.
0:14:50 > 0:14:52Tell me about the measurements you're making here.
0:14:52 > 0:14:54You've got these lasers around. What are they doing?
0:14:54 > 0:14:57We've got two laser Doppler vibrometers
0:14:57 > 0:14:59pointed at the surface of Big Ben.
0:14:59 > 0:15:02That allows us to measure the motion of the surface,
0:15:02 > 0:15:03the vibration of the surface, directly,
0:15:03 > 0:15:05but without touching the bell.
0:15:05 > 0:15:07So by a tiny change in the laser light,
0:15:07 > 0:15:08you can find out how quickly
0:15:08 > 0:15:11the surface of the bell is moving in and out.
0:15:11 > 0:15:13That's right. We are going to characterise that,
0:15:13 > 0:15:17and be able to show that for all of the natural frequencies of the bell.
0:15:17 > 0:15:19BELLS CHIME
0:15:20 > 0:15:25Across three hours, Martin's two lasers scan Big Ben as it chimes.
0:15:25 > 0:15:29The aim is to discover the bell's different natural frequencies
0:15:29 > 0:15:31and patterns of vibration.
0:15:31 > 0:15:34BIG BEN CHIMES
0:15:36 > 0:15:38Just as with Chladni's plate,
0:15:38 > 0:15:42every time the hammer strikes the bell, the metal vibrates at many
0:15:42 > 0:15:45different natural frequencies,
0:15:45 > 0:15:48each corresponding to a different pattern of vibration.
0:15:49 > 0:15:51Together, these make up
0:15:51 > 0:15:54the distinctive, melodious sound that we hear.
0:15:54 > 0:15:56Tell me what we're looking at.
0:15:56 > 0:15:59We've got an average of the entire chime of Big Ben,
0:15:59 > 0:16:02and from that you can see a number of different dominant frequencies,
0:16:02 > 0:16:06and some subordinate frequencies that all go together to make up
0:16:06 > 0:16:07the characteristic sound of Big Ben.
0:16:07 > 0:16:10So those are some at the front here which are really obvious.
0:16:10 > 0:16:12They are much bigger than the others.
0:16:12 > 0:16:16Yeah, particularly 199 hertz and the 336 hertz
0:16:16 > 0:16:18really dominate the character.
0:16:18 > 0:16:21So each of these natural frequencies corresponds
0:16:21 > 0:16:24to a different vibration pattern on the bell.
0:16:24 > 0:16:27That's right. To give the note and the colour
0:16:27 > 0:16:28that is the sound of Big Ben.
0:16:31 > 0:16:34This animation is showing the lowest natural frequency of Big Ben.
0:16:35 > 0:16:3795 hertz.
0:16:40 > 0:16:42At the bell's higher natural frequencies,
0:16:42 > 0:16:46the animation shows that it vibrates in more complex patterns.
0:16:48 > 0:16:50It's this mixture of frequencies
0:16:50 > 0:16:54that make up Big Ben's acoustic signature.
0:16:54 > 0:16:56CLANG!
0:16:57 > 0:17:01But knowing the frequencies reveals something else that helps explain
0:17:01 > 0:17:04why we perceive this to be a melodious sound.
0:17:05 > 0:17:08Because underpinning the difference natural frequencies
0:17:08 > 0:17:10is a mathematical relationship.
0:17:11 > 0:17:13The sound of Big Ben isn't random.
0:17:13 > 0:17:17Some of its natural frequencies are lined up in a harmonic relationship,
0:17:17 > 0:17:19and that's what gives the bell is harmonious sound.
0:17:21 > 0:17:24Some of Big Ben's natural frequencies
0:17:24 > 0:17:26are simple ratios of one another.
0:17:26 > 0:17:31For example, this natural frequency is almost precisely half
0:17:31 > 0:17:34of this one.
0:17:34 > 0:17:37When frequencies are mathematically related like this,
0:17:37 > 0:17:40in what's called a harmonic relationship,
0:17:40 > 0:17:42the human ear finds them pleasant.
0:17:43 > 0:17:47And in the UK, most bells are specifically tuned to be like this.
0:17:48 > 0:17:52If we change the shape of the bell or the material it is made from,
0:17:52 > 0:17:53the sound would change.
0:17:53 > 0:17:56And so when we listen to something like a bell,
0:17:56 > 0:17:58what we're hearing is its structure.
0:18:04 > 0:18:07So far, the world of sound seems relatively simple.
0:18:08 > 0:18:11An object vibrates to make a distinctive sound.
0:18:13 > 0:18:16And if these vibrations are specially tuned,
0:18:16 > 0:18:18then we can turn sound into something beautiful.
0:18:18 > 0:18:22ORCHESTRA PLAYS A WALTZ BY JOHANN STRAUSS
0:18:22 > 0:18:26But there is more to the beauty of sound than tuning an object.
0:18:26 > 0:18:29There is often something else involved in the production of sound.
0:18:29 > 0:18:33Something that adds complexity and richness.
0:18:33 > 0:18:35Something that, exploited to the full,
0:18:35 > 0:18:37can create sounds that stir the soul.
0:18:48 > 0:18:51With the help of acoustics expert Professor Trevor Cox,
0:18:51 > 0:18:54a violinist and a special camera,
0:18:54 > 0:18:58we're going to explore the way that some sounds are produced
0:18:58 > 0:19:01and how it can be more complex than it might first appear.
0:19:02 > 0:19:04This is a fantastic toy.
0:19:04 > 0:19:05It's an acoustic camera.
0:19:05 > 0:19:07It's got a little camera right in the middle looking at me,
0:19:07 > 0:19:10and then a ring of microphones around the outside.
0:19:10 > 0:19:13And they are very directional.
0:19:13 > 0:19:16And so if I clap up here you can see the sound is coming from up here,
0:19:16 > 0:19:19the rest of the time you can see it coming from my mouth,
0:19:19 > 0:19:22so you can identify where the sound is coming from.
0:19:24 > 0:19:27In a musical instrument like a violin,
0:19:27 > 0:19:29the initial vibration comes from the string...
0:19:32 > 0:19:34..but although the string is vibrating,
0:19:34 > 0:19:37it is not directly producing the sound that we hear.
0:19:38 > 0:19:41Something else is involved, too.
0:19:41 > 0:19:43When you look at a stringed instrument,
0:19:43 > 0:19:45might think the string is making all the sound.
0:19:45 > 0:19:46Well, it is starting the sound
0:19:46 > 0:19:50but it is not what makes the sound so powerful and so strong.
0:19:50 > 0:19:52The string determines the pitch of the sound.
0:19:52 > 0:19:56Just the string - it would all be rather dull and quiet.
0:20:05 > 0:20:09The acoustic camera shows that the loudest sound,
0:20:09 > 0:20:10coloured in pink and red,
0:20:10 > 0:20:14isn't coming from string but from the wooden body of the violin.
0:20:18 > 0:20:20Tell me what happens to the sound after that.
0:20:20 > 0:20:23Well, once you've made a sound, you've got the source of the sound,
0:20:23 > 0:20:24it then has to be amplified.
0:20:24 > 0:20:27So the sound goes through the bridge first of all,
0:20:27 > 0:20:29which connects the string to the body of the violin,
0:20:29 > 0:20:31and then the actual wooden plates are all vibrating
0:20:31 > 0:20:34and they're amplifying the sound.
0:20:34 > 0:20:36So the important thing is that the thin string
0:20:36 > 0:20:38can't push on the air very much by itself,
0:20:38 > 0:20:41but once you've got a great, big, large, wooden, flat plate,
0:20:41 > 0:20:43that can push quite hard.
0:20:43 > 0:20:44Yes. Every musical instrument
0:20:44 > 0:20:46has resonances at heart and in the violin,
0:20:46 > 0:20:49it's actually the wood body that is the resonator.
0:20:52 > 0:20:56The wooden body of the violin is what's called a sound resonator.
0:20:56 > 0:21:00It transforms the sound of the vibration from the string,
0:21:00 > 0:21:04picking up and enhancing certain natural frequencies
0:21:04 > 0:21:06whilst damping down others.
0:21:10 > 0:21:12ORCHESTRA TUNES UP
0:21:15 > 0:21:18Most musical instruments have a resonator.
0:21:18 > 0:21:21The pipes of an organ, the bore of a clarinet
0:21:21 > 0:21:22and the body of a cello.
0:21:23 > 0:21:26It's what amplifies and sculpts the sound,
0:21:26 > 0:21:29giving the instrument a far richer acoustic signature.
0:21:38 > 0:21:43But the ultimate ability to shape sound doesn't belong to a musical
0:21:43 > 0:21:46instrument. It belongs to us.
0:21:46 > 0:21:50SHE SINGS: O Mio Babbino Caro by Puccini
0:21:51 > 0:21:53It's the human voice.
0:22:04 > 0:22:06As a professional opera singer,
0:22:06 > 0:22:11Lesley Garrett has exquisite control over the sound her voice produces.
0:22:12 > 0:22:14She can produce a range of sounds
0:22:14 > 0:22:18far greater than any man-made musical instrument,
0:22:18 > 0:22:22and at a volume that can compete with an entire orchestra.
0:22:25 > 0:22:30To see how Lesley is able to create such extraordinary sounds,
0:22:30 > 0:22:34we have come first to Harley Street in London to meet throat specialist
0:22:34 > 0:22:36consultant surgeon John Rubin.
0:22:36 > 0:22:40But this is so precious that I do have it checked regularly.
0:22:40 > 0:22:44John has looked after me for many decades now and kept me going.
0:22:45 > 0:22:48John is going to use a laryngoscope to allow us
0:22:48 > 0:22:50to look at Lesley's larynx,
0:22:50 > 0:22:52where the sound of her singing voice begins.
0:22:52 > 0:22:54Just give me a nice, bright forward...
0:22:54 > 0:22:57- SHE SINGS NOTE - Lovely, lovely.
0:22:57 > 0:23:01Now, I'm going to ask you if I may, to show me the tip of your tongue.
0:23:02 > 0:23:04Now, smiley face.
0:23:04 > 0:23:06Get ready. Take a little...
0:23:06 > 0:23:09HE SINGS A NOTE AND SHE REPEATS IT
0:23:16 > 0:23:18THEY SING A HIGHER NOTE
0:23:22 > 0:23:24Terrific.
0:23:24 > 0:23:27The larynx produces vibrations in air,
0:23:27 > 0:23:30the origin of the sound we hear.
0:23:30 > 0:23:32So these are your vocal folds.
0:23:32 > 0:23:34So these two white stripes down here.
0:23:34 > 0:23:37These two white stripes are Lesley's vocal folds.
0:23:37 > 0:23:39So we can see them of opening and closing as she sings.
0:23:39 > 0:23:41Exactly.
0:23:41 > 0:23:44It is the opening and closing that actually breaks up the air
0:23:44 > 0:23:46and makes sound.
0:23:46 > 0:23:48Now in Lesley's instance,
0:23:48 > 0:23:50she can make her vocal folds vibrate
0:23:50 > 0:23:55anywhere from about 80 times per second probably to over 1,000.
0:23:55 > 0:23:58- Wow, I didn't know I could do that. - It's really amazing.
0:23:58 > 0:24:03There are various sets of muscles and I have to, almost unconsciously,
0:24:03 > 0:24:07arrange those muscles so that my larynx is in the perfect position
0:24:07 > 0:24:11for the amount of pressure I'm choosing to exert upon it.
0:24:11 > 0:24:14And that is what we call the onset of tone.
0:24:14 > 0:24:17So if I was just going to move my larynx without air,
0:24:17 > 0:24:19it would sound like this...
0:24:19 > 0:24:22ALMOST SILENT BREATHS There is almost nothing there.
0:24:22 > 0:24:24But then if I were to introduce air, it would sound like this.
0:24:24 > 0:24:26SHE SINGS LOUDLY
0:24:26 > 0:24:27Like that, you know.
0:24:27 > 0:24:30You did that with so much volume, so quickly, it's astonishing.
0:24:30 > 0:24:32Just that tiny little thing.
0:24:32 > 0:24:34HE SINGS A NOTE AND SHE REPEATS IT
0:24:34 > 0:24:37The vocal folds create the initial vibration in the air.
0:24:42 > 0:24:46Yet, as remarkable as Lesley's vocal folds are,
0:24:46 > 0:24:48just like the strings of a violin,
0:24:48 > 0:24:51they are not producing the sound we hear.
0:24:51 > 0:24:55It is her resonator that is the key to her extraordinary voice.
0:25:11 > 0:25:13We've come to University College London
0:25:13 > 0:25:15to meet Professor Sophie Scott,
0:25:15 > 0:25:19who's going to reveal what makes the human resonators so special.
0:25:19 > 0:25:22So what we're going to do today is I'm going to take you through to our
0:25:22 > 0:25:25MRI machine and what we're going to do is use it to image
0:25:25 > 0:25:28Lesley's vocal tract, and that should tell us something about
0:25:28 > 0:25:32what is happening for you when you are singing so beautifully.
0:25:32 > 0:25:33I cannot tell you how excited I am about this.
0:25:33 > 0:25:36It's sort of like the answer to the ultimate mystery.
0:25:36 > 0:25:40For 40 years I've been singing and never really quite understood
0:25:40 > 0:25:41what is going on in my throat.
0:25:41 > 0:25:43None of us can. None of us can see it.
0:25:43 > 0:25:46It's not like we're pianists and we can see what is going on,
0:25:46 > 0:25:47so this is so exciting.
0:25:49 > 0:25:53The sound resonator of Lesley's voice is her throat and mouth.
0:25:53 > 0:25:58And this is what the MRI machine is going to image as she sings.
0:25:58 > 0:26:01Lesley, can you sing for me the vowels
0:26:01 > 0:26:06ee, eh, ah, oh, euh?
0:26:07 > 0:26:09LESLEY SINGS
0:26:13 > 0:26:14The whole thing is moving.
0:26:14 > 0:26:16It's quite extraordinary.
0:26:16 > 0:26:20The MRI shows how, for each of the different vowel sounds,
0:26:20 > 0:26:24Lesley's mouth and throat change shape,
0:26:24 > 0:26:28amplifying the vibrations in air produced by her vocal folds
0:26:28 > 0:26:31and sculpting them into the sound we hear.
0:26:33 > 0:26:36So what we saw with the laryngoscopy right down here
0:26:36 > 0:26:39is just the very beginning of making sound
0:26:39 > 0:26:41and then there's all this shaping that goes on up here,
0:26:41 > 0:26:44that actually determines what we hear.
0:26:44 > 0:26:46Exactly, so
0:26:46 > 0:26:49all the work being done above the voice box, the larynx,
0:26:49 > 0:26:52is essentially changing the spectral characteristics of the noise that
0:26:52 > 0:26:55you're making down there. You're making a noise here and then you're
0:26:55 > 0:26:57continuously changing it up here.
0:26:57 > 0:26:59Particularly, as you can see,
0:26:59 > 0:27:01by exactly how the tongue has been positioned
0:27:01 > 0:27:04and how the tongue is moving.
0:27:07 > 0:27:09Lesley, that was absolutely beautiful.
0:27:09 > 0:27:10Thank you.
0:27:10 > 0:27:13- So, should we do "I Dreamed A Dream"?- OK.
0:27:13 > 0:27:19# I dreamed a dream in time gone by... #
0:27:19 > 0:27:23The MRI reveals the secret of our resonator.
0:27:23 > 0:27:26And like the sound resonator of a musical instrument,
0:27:26 > 0:27:28the vocal resonator isn't fixed.
0:27:28 > 0:27:31It's incredibly flexible.
0:27:31 > 0:27:33Through the movement of the tongue in particular
0:27:33 > 0:27:35and the jaw, lips and throat,
0:27:35 > 0:27:40it can be manipulated to form a myriad of different shapes.
0:27:40 > 0:27:43Look how open that is. It's extraordinary.
0:27:43 > 0:27:45And with a trained singer like Lesley,
0:27:45 > 0:27:48the range of movement is truly amazing.
0:27:49 > 0:27:51The tongue is basically like an octopus tentacle.
0:27:51 > 0:27:54It just deforms in all these different directions.
0:27:54 > 0:27:57This is such a flexible, adaptive instrument, isn't it?
0:27:57 > 0:27:59That is a surprise to me, I must admit.
0:27:59 > 0:28:02It takes you so long to coordinate all that to the level
0:28:02 > 0:28:05that we can project a beautiful sound,
0:28:05 > 0:28:07a sound that will hopefully make people cry or laugh,
0:28:07 > 0:28:12to the back of a 2,000-seater auditorium without amplification,
0:28:12 > 0:28:14it's something that requires massive training
0:28:14 > 0:28:16and now I can see why it did.
0:28:18 > 0:28:23# I had a dream my life would be
0:28:23 > 0:28:24# So different... #
0:28:24 > 0:28:27Sound begins as a simple vibration.
0:28:27 > 0:28:34# So different now from what it seemed... #
0:28:34 > 0:28:39But it is how this initial vibration are sculpted by the resonator that
0:28:39 > 0:28:44lies behind our mastery and control of sound.
0:28:44 > 0:28:46# The dream
0:28:49 > 0:29:00# I dreamed. #
0:29:10 > 0:29:12APPLAUSE
0:29:18 > 0:29:23Music is an obvious way in which sound can have an impact on us.
0:29:23 > 0:29:25But there's a type of sound
0:29:25 > 0:29:28that makes an impact in a very different way.
0:29:28 > 0:29:31It is a type of sound that doesn't play by the rules
0:29:31 > 0:29:33of any of the sounds we've heard so far.
0:29:37 > 0:29:39WHIP CRACKS
0:29:39 > 0:29:41This is a thing that is entirely new to me.
0:29:41 > 0:29:45It is a bullwhip, and Lila here is about to have a go at teaching me
0:29:45 > 0:29:46how to crack it.
0:29:46 > 0:29:48OK, so whip cracking.
0:29:48 > 0:29:52- Safety goggles.- Good idea.
0:29:52 > 0:29:54Right, so these are bullwhips.
0:29:54 > 0:29:56This is the bit that makes the sound.
0:29:58 > 0:30:01So behind you, turn sideways slightly.
0:30:01 > 0:30:02Yes.
0:30:04 > 0:30:06I hit myself in the head.
0:30:06 > 0:30:09FAINT CLICKING
0:30:09 > 0:30:10CRACK!
0:30:10 > 0:30:13So I think I'm doing all right, and then you're coming along behind
0:30:13 > 0:30:15with this enormous noise.
0:30:20 > 0:30:22- Oh!- That was it, yeah. - We're in business.
0:30:22 > 0:30:24Just try and get that...
0:30:24 > 0:30:26That was a good one.
0:30:26 > 0:30:28Shall we finish on a high?
0:30:29 > 0:30:33The sound comes that comes from this whip is something special.
0:30:33 > 0:30:35It's different. We're not hearing a shape.
0:30:35 > 0:30:38It hasn't got specific frequencies associated with it.
0:30:38 > 0:30:40And it's also fantastically loud.
0:30:40 > 0:30:43All of that sound is coming just from that tiny bit on the end
0:30:43 > 0:30:46and yet it echoed around this entire space.
0:30:46 > 0:30:49Right at the point this sound forms, it isn't even a wave.
0:30:49 > 0:30:51This is something different.
0:30:52 > 0:30:56The key to what makes this type of sound different and so loud
0:30:56 > 0:30:58is how it's generated.
0:31:02 > 0:31:04And to see how that happens,
0:31:04 > 0:31:07we need the help of physicist Dr Daniel Eakins.
0:31:07 > 0:31:09- This is Lila.- Hi, nice to meet you.
0:31:09 > 0:31:11So what have we got here?
0:31:11 > 0:31:12What does the set-up do?
0:31:12 > 0:31:14This is known as a Schlieren imaging set-up,
0:31:14 > 0:31:18and what it allows us to do is detect very small, minute changes
0:31:18 > 0:31:22in the way light refracts through gas as it is heated, for example.
0:31:23 > 0:31:26- There you go. Yes. - It's pretty, isn't it?- Yes.
0:31:26 > 0:31:30The Schlieren camera is able to detect distortions in light
0:31:30 > 0:31:33created by changes in air temperature and pressure.
0:31:33 > 0:31:37What we are going to try to do is have it
0:31:37 > 0:31:40so that when the whip, or when the end of the whip
0:31:40 > 0:31:43is at its highest speed,
0:31:43 > 0:31:46that that's within the field of view of the Schlieren camera.
0:31:46 > 0:31:48She's got to hit that toothpick thing there?
0:31:48 > 0:31:51Yes, she has to be in the vicinity of this,
0:31:51 > 0:31:53probably within about 50 mil if you can manage, yes.
0:31:53 > 0:31:55- Can you do that?- Sure.
0:32:03 > 0:32:05Wow. If only we had that one.
0:32:05 > 0:32:06It's amazing.
0:32:06 > 0:32:08Oh, my goodness.
0:32:10 > 0:32:11This is it.
0:32:13 > 0:32:18- Oh, wow. You've done it.- OK.
0:32:18 > 0:32:20You owe me a cocktail stick. OK.
0:32:20 > 0:32:22We'll just go and have a look at the data, then.
0:32:24 > 0:32:28This slow-motion footage shows the disturbance in the air
0:32:28 > 0:32:30created by the tip of the bullwhip.
0:32:31 > 0:32:35The dark lines show where the air has been compressed together to form
0:32:35 > 0:32:37concentrated pressure fronts.
0:32:39 > 0:32:42The strands of the bullwhip create pressure fronts that travel
0:32:42 > 0:32:43at phenomenal speed.
0:32:45 > 0:32:47This is what creates the sound.
0:32:50 > 0:32:54It looks like it is moving at around 364 metres per second.
0:32:54 > 0:32:59So the speed of sound in air is about 343 metres a second,
0:32:59 > 0:33:01so this is going faster than the speed of sound.
0:33:01 > 0:33:03It is a supersonic disturbance.
0:33:05 > 0:33:08The reason this sound can travel at supersonic speed
0:33:08 > 0:33:12is because it's not a wave but a shock front.
0:33:13 > 0:33:15For a fraction of a second,
0:33:15 > 0:33:18it has enormous energy that punches through the air
0:33:18 > 0:33:22with such force that the air molecules can't oscillate
0:33:22 > 0:33:24back and forth as a wave.
0:33:24 > 0:33:28The one distinguishing feature of a shock is that it is like an impulse.
0:33:28 > 0:33:30It is an instantaneous change in pressure.
0:33:30 > 0:33:33So the reason that such a tiny thing can make such a loud sound
0:33:33 > 0:33:36is because it's barrelling into the air and so there's
0:33:36 > 0:33:37far more volume given out.
0:33:37 > 0:33:40- That's right.- So you've been breaking the sound barrier, Lila.
0:33:40 > 0:33:41So cool!
0:33:43 > 0:33:45THUNDER RUMBLES AND CRASHES
0:33:46 > 0:33:48From the crack of a lightning bolt...
0:33:49 > 0:33:50..to the bang of a gunshot...
0:33:51 > 0:33:54..and the blast of an explosion,
0:33:54 > 0:33:58the loudest sounds on the planet all originate as shock fronts.
0:34:05 > 0:34:08Nasa scientists have used the same Schlieren technique
0:34:08 > 0:34:12to image the shock fronts created by supersonic aircraft,
0:34:12 > 0:34:17by filming the aircraft flying in front of the sun.
0:34:17 > 0:34:21Three, two, one, mark.
0:34:22 > 0:34:24The aircraft is moving faster
0:34:24 > 0:34:26than the speed at which sound waves travel.
0:34:28 > 0:34:31Because of this, the air molecules in front of the aircraft
0:34:31 > 0:34:34get shoved out of the way with such ferocity
0:34:34 > 0:34:37that there's no time for normal sound waves to form.
0:34:38 > 0:34:41Instead, a pattern of shock fronts are created.
0:34:42 > 0:34:46This is the origin of the sonic boom.
0:34:46 > 0:34:48BOOM
0:34:57 > 0:34:59SIREN, ENGINES AND CHURCH BELLS
0:34:59 > 0:35:01For all of the fascinating science
0:35:01 > 0:35:04behind the sounds we are familiar with in our daily lives,
0:35:04 > 0:35:09these are only a tiny fraction of the sounds that fill our planet.
0:35:13 > 0:35:17There are entire worlds of sound that remain hidden from us.
0:35:17 > 0:35:20Places where sound can behave in very different ways.
0:35:22 > 0:35:26And perhaps the most intriguing of these is the ocean.
0:35:28 > 0:35:32Two-thirds of our planet is covered by water.
0:35:32 > 0:35:35And yet apart from the sound of the waves,
0:35:35 > 0:35:39it's a world that appears to us here on land as silent.
0:35:45 > 0:35:47When I put my hand in the water here,
0:35:47 > 0:35:50I'm touching a different acoustic world.
0:35:50 > 0:35:53And that's because both sides of the water surface act like
0:35:53 > 0:35:57an acoustic mirror. Sound coming from beneath bounces off the air
0:35:57 > 0:36:00and goes back into the water and all the sound up here
0:36:00 > 0:36:03bounces off the water and goes back into the air.
0:36:03 > 0:36:06So I can put my hand into this acoustic world,
0:36:06 > 0:36:08but I can't hear it.
0:36:10 > 0:36:12The acoustic mirror effect ensures
0:36:12 > 0:36:16that sound travelling in water can't escape into the air.
0:36:18 > 0:36:20So the only way to experience
0:36:20 > 0:36:22how sound behaves differently in the ocean,
0:36:22 > 0:36:26and to see the profound effect this has on life,
0:36:26 > 0:36:28is to enter the underwater acoustic world.
0:36:30 > 0:36:32- Hello.- Hello.
0:36:32 > 0:36:33- How are you doing?- I am all right.
0:36:33 > 0:36:37'I have come to meet Dr Steve Simpson, who is a marine biologist
0:36:37 > 0:36:38'and he's going to reveal
0:36:38 > 0:36:41'just how differently sound behaves underwater.'
0:37:00 > 0:37:03- So what have we got here? - So here we have got...
0:37:03 > 0:37:05The plastic bucket of science.
0:37:05 > 0:37:07The plastic bucket of science, absolutely.
0:37:07 > 0:37:11- So we've got a hydrophone here. - So that's our underwater microphone.
0:37:11 > 0:37:12This is our ear, basically, underwater.
0:37:12 > 0:37:16And then we have a recorder that allows us to be able to take the
0:37:16 > 0:37:19recordings through the whole of our snorkel and have I have wired up
0:37:19 > 0:37:21a speaker inside a cup.
0:37:21 > 0:37:24So while we're snorkelling about on the surface of the water,
0:37:24 > 0:37:26we'll be able to hear what's going on below.
0:37:26 > 0:37:28- Exactly, yeah.- All right. Let's give it a go.
0:37:47 > 0:37:52'Part of the reason that sound is so different in water compared to air
0:37:52 > 0:37:56'is that water is 1,000 times more dense.
0:37:56 > 0:38:00'One consequence is that it takes more energy to start a vibration
0:38:00 > 0:38:01'in the first place.
0:38:03 > 0:38:06'Sea creatures have evolved specific means to create sound
0:38:06 > 0:38:08'in this much denser medium.'
0:38:12 > 0:38:15- Here you go. You take this. - So this is the listening device?
0:38:15 > 0:38:17- There's your ear and here's a hydrophone.- OK.
0:38:20 > 0:38:22CRACKLING
0:38:26 > 0:38:28- How was that?- I can hear popcorn.
0:38:28 > 0:38:30It sounds like snapping shrimp to me.
0:38:30 > 0:38:33It is the soundtrack of the ocean, that's right.
0:38:33 > 0:38:36Snapping shrimp overcome the difficulty of producing sound
0:38:36 > 0:38:40in water by snapping their claws together really fast...
0:38:43 > 0:38:44..causing bubbles to implode.
0:38:46 > 0:38:49So it is kind of a grating, scraping noise.
0:38:51 > 0:38:52And this is the sound
0:38:52 > 0:38:55of a sea urchin scratching seaweed off the rock.
0:38:55 > 0:38:57SCRAPING
0:39:03 > 0:39:06Water transmits sound much more effectively than air.
0:39:12 > 0:39:17In fact, sound travels much further in water than light does,
0:39:17 > 0:39:20something that life under the waves takes full advantage of.
0:39:23 > 0:39:27I've got a recording of a soldier fish. So this is a coral reef fish.
0:39:27 > 0:39:29Spends its day living in a cave
0:39:29 > 0:39:31then goes out at night looking for shrimp
0:39:31 > 0:39:32that come out of the sand to feed.
0:39:33 > 0:39:35And when it finds the food...
0:39:35 > 0:39:36BOOMING GRUNTS
0:39:40 > 0:39:42It's a very big, deep noise, isn't it?
0:39:42 > 0:39:44It's like a deep trumpeting sound.
0:39:44 > 0:39:46- How big is the fish?- So the fish would be about this sort of size.
0:39:46 > 0:39:48- It's quite a small fish. - A small fish to make
0:39:48 > 0:39:50- a lot of noise, that's right. - That's really impressive.
0:39:50 > 0:39:53What sort of distances are these sounds travelling underwater?
0:39:53 > 0:39:57So with a hydrophone like this, if you're out in the open ocean,
0:39:57 > 0:39:59you'd hear a coral reef from up to 25km away.
0:39:59 > 0:40:02So it really is a cacophony of noise.
0:40:03 > 0:40:06And we think that fish can hear the sound from hundreds of metres,
0:40:06 > 0:40:08some species for kilometres.
0:40:08 > 0:40:11So it's almost in the ocean as though sound and light have swapped
0:40:11 > 0:40:14places. Sound is much more useful underwater than light is.
0:40:14 > 0:40:18Yes. So you might be able to see 30 metres in really clear water,
0:40:18 > 0:40:21but you can hear for hundreds of metres or kilometres.
0:40:21 > 0:40:23So it becomes an information channel
0:40:23 > 0:40:26that works over much larger distances.
0:40:28 > 0:40:33The distances over which sound can travel underwater are truly amazing.
0:40:33 > 0:40:37The sounds made by whales can carry for thousands of kilometres...
0:40:38 > 0:40:41..travelling across almost entire oceans.
0:40:42 > 0:40:46Yet because these sounds remain locked beneath the water surface,
0:40:46 > 0:40:48they never reach our ears.
0:40:58 > 0:41:00We can't hear underwater sounds
0:41:00 > 0:41:03because we are not a part of that acoustic world.
0:41:03 > 0:41:06However, there is a whole class of sounds that we don't hear
0:41:06 > 0:41:09for a completely different reason,
0:41:09 > 0:41:13because their frequency lies outside our range of hearing.
0:41:14 > 0:41:17And yet it is these sound that turn out to deliver
0:41:17 > 0:41:19the most fascinating insights.
0:41:19 > 0:41:24It is easy to take the huge range of human hearing for granted
0:41:24 > 0:41:26but it is worth spending a moment on.
0:41:26 > 0:41:28The piano is a really good way to demonstrate it.
0:41:28 > 0:41:33This is middle C here and that is at 262 hertz,
0:41:33 > 0:41:35which means 262 cycles every second.
0:41:35 > 0:41:39And the lovely thing about a piano is that you can go up in octaves,
0:41:41 > 0:41:44and every octave involves a doubling of frequencies.
0:41:44 > 0:41:49So the highest note in the piano, this C here is 4,186 hertz.
0:41:49 > 0:41:51It doesn't stop there.
0:41:51 > 0:41:53If we were to build our piano outwards
0:41:53 > 0:41:56to the edge of the human hearing range,
0:41:56 > 0:42:00we come all the way up here, which is 19.9 kilohertz -
0:42:00 > 0:42:02a gigantic number.
0:42:02 > 0:42:05And it also carries on down the other end.
0:42:05 > 0:42:07The lowest C on the piano is this one,
0:42:07 > 0:42:09with a frequency of 32 hertz.
0:42:09 > 0:42:13And if we were to carry on our piano to the limit of human hearing,
0:42:13 > 0:42:16we would get down here. This one is 20.6 hertz.
0:42:16 > 0:42:19So this piano, with all its extra keys,
0:42:19 > 0:42:22represents the full range of human hearing.
0:42:24 > 0:42:28'This is our rich but ultimately limited experience of sound...
0:42:30 > 0:42:33'..because the full spectrum of sound frequencies
0:42:33 > 0:42:36'extends way beyond what we can hear.'
0:42:49 > 0:42:53These sounds that lie outside our range of hearing
0:42:53 > 0:42:58hold the key to a world where sound gives life extraordinary powers,
0:42:58 > 0:43:02and opens new windows onto our planet and even the universe.
0:43:27 > 0:43:30I'm in the middle of a huge pod of dolphins.
0:43:30 > 0:43:32There must be hundreds of them out here.
0:43:34 > 0:43:36These dolphins are hunters.
0:43:36 > 0:43:39They're using high-frequency sounds to locate their prey.
0:43:44 > 0:43:48Most of the clicks and whistles that these dolphins produce
0:43:48 > 0:43:50are way beyond the range of our hearing.
0:43:50 > 0:43:52HIGH-PITCHED WHISTLING
0:43:52 > 0:43:55This is the realm of ultrasound -
0:43:55 > 0:43:58sound at frequencies above what we can here.
0:43:59 > 0:44:02What I can hear are whistling noises but they are calls.
0:44:02 > 0:44:05Most of them are at higher frequencies than I can hear.
0:44:05 > 0:44:07So I'm just hearing a tiny, tiny bit at the bottom
0:44:07 > 0:44:10and it's still really loud.
0:44:14 > 0:44:17Ultrasound is key to the dolphin's hunting ability.
0:44:18 > 0:44:23Because ultrasound has a very high frequency and a small wavelength,
0:44:23 > 0:44:27it reflects off small, fast-moving objects
0:44:27 > 0:44:30that audible sound waves would pass over.
0:44:30 > 0:44:33The dolphin creates short pulses of ultrasound and then listens
0:44:33 > 0:44:35for the echoes and, from this,
0:44:35 > 0:44:38creates a detailed image of its surroundings,
0:44:38 > 0:44:40enabling it to catch its prey.
0:44:44 > 0:44:48These animals are operating in a sound range that is outside
0:44:48 > 0:44:49what we can perceive
0:44:49 > 0:44:53and it really highlights how much more there is out there.
0:44:56 > 0:44:58Dolphins are not alone
0:44:58 > 0:45:01in using ultrasound as a second form of sight.
0:45:02 > 0:45:05Bats use it for their version of echolocation...
0:45:09 > 0:45:11..and we use ultrasound for medical imaging.
0:45:13 > 0:45:16Pulses of ultrasound can penetrate the skin and reflect off
0:45:16 > 0:45:19different tissues.
0:45:19 > 0:45:21Fluid, muscle and bone.
0:45:22 > 0:45:26And these echoes are recorded and displayed as an image,
0:45:26 > 0:45:29enabling us to see the foetus inside the womb.
0:45:39 > 0:45:42At the other end of the sound spectrum
0:45:42 > 0:45:45lies an even more mysterious and unfamiliar group of sounds.
0:45:48 > 0:45:51This is the realm of infrasound -
0:45:51 > 0:45:54sounds that are too deep for us to hear.
0:45:54 > 0:45:57And as we learn to decode these sounds,
0:45:57 > 0:46:00they give us a greater understanding of our planet
0:46:00 > 0:46:03an offer us the potential to save thousands of lives.
0:46:05 > 0:46:09Infrasound lets us listen in on the geological world,
0:46:09 > 0:46:11and, if you want to listen to infrasound,
0:46:11 > 0:46:12this is the place to come.
0:46:16 > 0:46:18This is Stromboli,
0:46:18 > 0:46:21one of the most active volcanoes on the planet.
0:46:23 > 0:46:27It has been erupting almost continuously for over 1,000 years.
0:46:35 > 0:46:37I've come here to meet some scientists
0:46:37 > 0:46:39whose research has helped reveal
0:46:39 > 0:46:42that this volcano, although we can't hear it,
0:46:42 > 0:46:44creates an extraordinary sound.
0:46:49 > 0:46:52The sound is created in a two-stage process
0:46:52 > 0:46:54that starts with the spectacular
0:46:54 > 0:46:57explosions of magma from within the volcano.
0:47:02 > 0:47:05And this is what Dr Jacopo Taddeucci
0:47:05 > 0:47:07and Dr Jorn Sesterhenn are studying.
0:47:07 > 0:47:11So basically, we use a high-speed camera to take footage
0:47:11 > 0:47:14of what happens at the vent of the volcano.
0:47:14 > 0:47:16So can we see some of these videos?
0:47:16 > 0:47:17Yeah, sure.
0:47:17 > 0:47:19OK.
0:47:19 > 0:47:21This is the eruption.
0:47:21 > 0:47:22And then you see the bombs...
0:47:22 > 0:47:26that are these particles flying here.
0:47:26 > 0:47:29- So these big lumps flying up into the sky.- Exactly.
0:47:29 > 0:47:31And how fast are they going?
0:47:31 > 0:47:33They can go up to 400 metres per second.
0:47:33 > 0:47:36So that's very, very fast.
0:47:36 > 0:47:39It is faster than sound in the air.
0:47:39 > 0:47:41It is a supersonic eruption.
0:47:44 > 0:47:46In this processed image,
0:47:46 > 0:47:50the dark lines travelling ahead of the molten rock are the sound waves
0:47:50 > 0:47:52created by the supersonic eruption.
0:47:52 > 0:47:55So there is a rush of gas and particles.
0:47:55 > 0:47:58Coming out very fast, even supersonic.
0:47:58 > 0:48:01- This makes the sound.- There is a very powerful eruption of gas and
0:48:01 > 0:48:04particles and it is just pushing on the air around it and sending out
0:48:04 > 0:48:06- sound waves.- Exactly.
0:48:07 > 0:48:10The eruption creates a supersonic shock front...
0:48:10 > 0:48:11BOOM
0:48:13 > 0:48:16..that we hear as an explosion.
0:48:16 > 0:48:18So far, so conventional.
0:48:23 > 0:48:25But this is just the first stage
0:48:25 > 0:48:28of the creation of a far more surprising sound -
0:48:28 > 0:48:32an infrasound that is well below our range of hearing.
0:48:33 > 0:48:35Detecting it isn't easy.
0:48:38 > 0:48:41- Hello.- Welcome.
0:48:41 > 0:48:44So this is Stromboli.
0:48:44 > 0:48:47'The way that this infrasound is created depends on how the sound
0:48:47 > 0:48:50'of the eruption is shaped by the crater.
0:48:52 > 0:48:55'This is what Dr Jeffrey Johnson has been studying.'
0:48:55 > 0:48:56It's loud, isn't it?
0:48:58 > 0:49:01That second reverberation,
0:49:01 > 0:49:03that is effectively a sound wave
0:49:03 > 0:49:05oscillating back and forth in this giant, giant pit.
0:49:05 > 0:49:09So a load of sound just washed past us that we couldn't hear
0:49:09 > 0:49:10but that was what you were measuring.
0:49:10 > 0:49:13Right. We could hear a component of that but not all of it.
0:49:13 > 0:49:16- And I would like to show you what the signals look like.- Cool.
0:49:20 > 0:49:24Fractions of a second after the explosive supersonic eruption,
0:49:24 > 0:49:27a second sound carries on -
0:49:27 > 0:49:29a pure tone of infrasound.
0:49:32 > 0:49:34Since we can't hear it directly,
0:49:34 > 0:49:37we need the help of a bit of audio trickery.
0:49:37 > 0:49:43I would like you to put these on and tell me what kind of sound you hear.
0:49:43 > 0:49:44SQUEAKING
0:49:46 > 0:49:49I hear the world's angriest mosquito.
0:49:49 > 0:49:50That's what it should sound like.
0:49:50 > 0:49:56This box produces a 700 hertz tone that is being frequency modulated by
0:49:56 > 0:49:58infrasound produced by the volcano.
0:49:58 > 0:50:01So what you should be hearing is a constant tone and there when
0:50:01 > 0:50:03there is an infrasound signal,
0:50:03 > 0:50:06it deflects that tone to higher and lower frequencies.
0:50:07 > 0:50:11'We can't hear the infrasound directly.
0:50:11 > 0:50:13'Instead, Jeff's apparatus is set up
0:50:13 > 0:50:16'so that when the infrasound passes by,
0:50:16 > 0:50:20'it changes the pitch of the constant buzzing sound.
0:50:20 > 0:50:23'Whenever the angry bee sound wobbles,
0:50:23 > 0:50:25'it's because it has been hit by infrasound.'
0:50:25 > 0:50:27There we go.
0:50:27 > 0:50:30And you can see a huge deflection corresponding to that explosion,
0:50:30 > 0:50:33and that was about a 2-3 hertz tone that I just observed.
0:50:36 > 0:50:40This distinct 2-3 hertz tone is part of the unique
0:50:40 > 0:50:45infrasound signature produced by Stromboli.
0:50:45 > 0:50:48It's created when the sound of the explosion
0:50:48 > 0:50:52from the base of the crater reverberates around the walls
0:50:52 > 0:50:55of one of the volcano's cavernous vents.
0:50:55 > 0:50:58This vent acts as a sound resonator,
0:50:58 > 0:51:02sculpting the noise of the explosion into a single tone.
0:51:02 > 0:51:05So the whole volcano is a giant musical instrument.
0:51:05 > 0:51:08The moment of explosion is like the hammer hitting a bell.
0:51:08 > 0:51:11That's what starts everything but then the shape of the musical
0:51:11 > 0:51:14instrument itself means the sound goes on for a little bit longer.
0:51:14 > 0:51:17That's right. And the size of that vent,
0:51:17 > 0:51:19how deep it is, how wide it is,
0:51:19 > 0:51:22will dictate the tone that is produced by that crater.
0:51:25 > 0:51:28Because Stromboli's craters are so big,
0:51:28 > 0:51:32the sound they produce is incredibly low-frequency infrasound.
0:51:35 > 0:51:38Scientists believe all active volcanoes like Stromboli
0:51:38 > 0:51:41have their own unique infrasound signature...
0:51:42 > 0:51:46..determined by the shape of the volcano vent acting as a resonator.
0:51:48 > 0:51:50And just as for a musical instrument,
0:51:50 > 0:51:53if the resonator changes shape,
0:51:53 > 0:51:57for example, because lava rises up within the vent,
0:51:57 > 0:51:59then the volcano sings a different sound.
0:52:06 > 0:52:10This means that we could listen to volcanoes around the world
0:52:10 > 0:52:16and, by monitoring their infrasound, better forecast a major eruption
0:52:16 > 0:52:19and that would buy precious time for people living nearby to escape
0:52:19 > 0:52:21with their lives.
0:52:26 > 0:52:29You might think that by the time we've explored the deep notes of
0:52:29 > 0:52:33Stromboli, the story of infrasound would have reached its limit.
0:52:33 > 0:52:35And yet it hasn't.
0:52:42 > 0:52:45To explore the extreme limits of infrasound,
0:52:45 > 0:52:48we need to leave our planet behind.
0:52:52 > 0:52:54It's long been assumed that
0:52:54 > 0:52:56in the emptiness of space there is no sound,
0:52:56 > 0:53:00because there's nothing for sound to travel through.
0:53:02 > 0:53:04'But, as impossible as it seems,
0:53:04 > 0:53:07'infrasound could be playing a fundamental role
0:53:07 > 0:53:10'in shaping the structure of the universe.'
0:53:13 > 0:53:16We're used to the idea of our busy bustling world down here
0:53:16 > 0:53:17being noisy.
0:53:17 > 0:53:21But when we look up at the night sky, we assume it's silent.
0:53:21 > 0:53:24No-one has ever heard sound from space.
0:53:24 > 0:53:28But in this building, there is a man who thinks he has seen it.
0:53:31 > 0:53:32'Professor Andrew Fabian
0:53:32 > 0:53:34'is an astronomer at the University of Cambridge.'
0:53:35 > 0:53:38He uses telescopes to study galaxy clusters,
0:53:38 > 0:53:41the largest structures in the universe.
0:53:41 > 0:53:45And he's trying to solve a mystery concerning how they grow.
0:53:45 > 0:53:48His research has led him to make a surprising discovery.
0:53:50 > 0:53:53So, Andy, where is it that you think you've seen sound in space?
0:53:53 > 0:53:56We're looking in the consolation of Perseus at what is known
0:53:56 > 0:53:58as the Perseus cluster of galaxies.
0:53:58 > 0:54:01When you have a cluster like this, which has got an enormous mass,
0:54:01 > 0:54:03it tends to...
0:54:03 > 0:54:08drag all the matter in and squeeze it and it makes it very hot
0:54:08 > 0:54:13and this hot stuff is known as the intra-cluster medium,
0:54:13 > 0:54:17is what we study in X-rays with an X-ray telescope.
0:54:17 > 0:54:20So in between all the bright galaxies here there is other stuff.
0:54:20 > 0:54:22Exactly.
0:54:22 > 0:54:26'And it turns out there is more than space than meets the eye.'
0:54:26 > 0:54:29Let's go to an X-ray image.
0:54:29 > 0:54:31It is completely different.
0:54:31 > 0:54:33So it is definitely the same bit of sky we're looking at.
0:54:33 > 0:54:36It is the same bit of sky but what we're seeing here is
0:54:36 > 0:54:39the gas between the galaxies.
0:54:39 > 0:54:43'This intra-cluster medium, shown here in orange,
0:54:43 > 0:54:47'is a cloud of gas that blankets the entire Perseus cluster.
0:54:47 > 0:54:50'At one particle every few centimetres,
0:54:50 > 0:54:55'the gas is far too diffuse to carry sound that we can hear.
0:54:55 > 0:55:00'But infra-sound can boldly go where no other sound can.'
0:55:00 > 0:55:02What makes you think there is actually sound there?
0:55:02 > 0:55:05Well, now we are going to look at the same region
0:55:05 > 0:55:08with a specially adapted image from the X-rays.
0:55:13 > 0:55:15And what we see is a whole set of ripples.
0:55:15 > 0:55:17And they are really clear.
0:55:17 > 0:55:18Really clear shapes.
0:55:18 > 0:55:23Yes. Where they are bright is where the gas is denser and it looks
0:55:23 > 0:55:26very much as though we've got
0:55:26 > 0:55:29a pressure wave which is propagating outwards.
0:55:29 > 0:55:32In other words, a sound wave.
0:55:32 > 0:55:34'If Andy is right,
0:55:34 > 0:55:38'what we're looking at is a snapshot of a wave of infrasound,
0:55:38 > 0:55:40'travelling through the intra-cluster gas
0:55:40 > 0:55:42'of the Perseus cluster.'
0:55:44 > 0:55:46So what is the scale of this image?
0:55:46 > 0:55:49The spacing between the ripples
0:55:49 > 0:55:51is about the diameter of our galaxy.
0:55:51 > 0:55:54- So gigantic.- So it's gigantic.
0:55:54 > 0:55:57And if you were to wait on one ripple,
0:55:57 > 0:56:00sit there and wait for the next ripple to come past you,
0:56:00 > 0:56:03- how long would that take? - Ten million years.
0:56:03 > 0:56:05So you need patience for this game.
0:56:05 > 0:56:10- Indeed, yes.- What could possibly cause ripples of sound that big?
0:56:10 > 0:56:13Well, I think it is coming from the centre,
0:56:13 > 0:56:15and there there's a massive black hole.
0:56:18 > 0:56:22It generates an enormous amount of energy in the material
0:56:22 > 0:56:24just before it's swallowed,
0:56:24 > 0:56:27and that energy is pushing out into the surrounding gas.
0:56:27 > 0:56:30So we think of black holes sucking stuff in,
0:56:30 > 0:56:32but the way that material moves around them,
0:56:32 > 0:56:35sometimes they can also spit it out.
0:56:35 > 0:56:38Indeed. And this could solve one of the problems,
0:56:38 > 0:56:41a puzzle that is associated with the centre of these clusters.
0:56:41 > 0:56:45These galaxies we're looking at here are the biggest galaxies
0:56:45 > 0:56:48in the universe. And they would be yet bigger,
0:56:48 > 0:56:50they could be up to ten times bigger
0:56:50 > 0:56:54in terms of numbers of stars, if this process was not operating.
0:56:57 > 0:56:59These ripples would be the lowest frequency sound
0:56:59 > 0:57:04ever detected in the universe - a pure tone of infrasound,
0:57:04 > 0:57:09one million billion times lower than the limit of human hearing.
0:57:10 > 0:57:12If Andy's theory is correct,
0:57:12 > 0:57:17infrasound plays a significant role in controlling the size of galaxies.
0:57:26 > 0:57:29The mysterious sounds of a black hole
0:57:29 > 0:57:32and the unique voice of a volcano...
0:57:35 > 0:57:39..are a fascinating glimpse into a new world of sound,
0:57:39 > 0:57:41beyond our human experience.
0:57:42 > 0:57:46As we explore more of these exciting soundscapes,
0:57:46 > 0:57:51it's clear that sound will become an even more powerful tool for
0:57:51 > 0:57:55understanding our world and even our universe.
0:58:01 > 0:58:03Next time, I will be investigating
0:58:03 > 0:58:08the incredible ways in which we use, control and manipulate sound...
0:58:10 > 0:58:12..helping us to survive...
0:58:15 > 0:58:18..to explore the world around us...
0:58:20 > 0:58:23..and to make the invisible visible.
0:58:23 > 0:58:26If you want to find out more about the science of sound
0:58:26 > 0:58:28and how we hear sound, go to...
0:58:31 > 0:58:33..and follow the links to the Open University.