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EXPLOSION
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This is Stromboli, one of the most active volcanoes in the world.
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And a few times every hour,
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it sends out huge explosions of lava and ash.
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And of course we expect noise to go with those explosions
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but along with the sounds we can hear,
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there are also sounds we can't.
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Because this volcano, like many others,
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is, in effect, a gigantic musical instrument.
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Only now are scientists understanding
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how strange and spectacular
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the world of sound really is.
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It is easy to take sound for granted.
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Sound is noise...
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it's music...
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..it is the spoken word.
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But it is far more than just a soundtrack to our lives.
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CRACK!
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The more we've discovered about the physics of sound...
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BOOM
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..the more astonishing the secrets it's revealed.
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HIGH-PITCHED SQUEAKING I hear the word's angriest mosquito.
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In this series, I'm going to investigate the nature of sound -
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what it is...
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BELL CLANGS
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I can feel that through my feet. it's really cool.
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'..what it tells us...'
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Just the quality of the sound says something is not right here.
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'..and how we use it...'
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ENGINE ROARS
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Certainly heard him.
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'..allowing us to see the world and even the universe
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'in new and exciting ways.'
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Every sound is created for a reason.
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Every sound has a story to tell.
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BIRDS SING AND BEES BUZZ
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CRACKLING
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Listening to a tree seems like an odd thing to do
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but this tree isn't silent.
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Even through a stethoscope like this, I can hear creaking and
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groaning as the branches move in the wind,
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and there are other sounds in there that I can't quite hear with this.
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Crackling, popping sounds.
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POPPING AND CRACKLING
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It happens because the tree
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is drawing water up from its roots to its leaves
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and, on a hot sunny day like this, as that water travels through
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the tiny tubes round the outside of the tree,
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bubbles form, and those are what are making the crackling noise.
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So although you wouldn't know it by looking at it,
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that crackling noise could tell you that this tree is thirsty.
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Before we can unlock all the secrets of sound...
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..we need to understand it at a fundamental level.
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So in this programme,
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I'm going to explore what sound is and how it's made.
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First, it would help if I could turn a sound
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into something we can actually see.
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This is a very special space.
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It is called a hemi-anechoic chamber,
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and what means is that all the walls and ceiling have these funny shapes
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on them that are absorbing sound so it's really quiet in here.
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It's the perfect environment to isolate a pure sound
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and observe its effects.
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All I need is this small army of candles and a speaker.
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To make this work, I need the sound to be really loud
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so I'm going to wear ear defenders.
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DEEP RUMBLE This is a really deep sound -
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you can see the speaker going in and out.
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And what you can see is that the candles are vibrating -
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this very, very fast vibration.
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'The individual candle flames are showing the movement in the air
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'caused by the speaker.'
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What's happening is that the speaker here
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is producing enormous amounts of sound by pushing on the air.
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And that push pushes on the air next to it
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which pushes on the air next to it,
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and it travels out across the candles.
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'The candle flames are flickering back and forth 20 times per second,
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'or at 20 hertz.
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'This is the frequency of the sound we are hearing.'
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I'm going to turn it up.
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NOISE RISES IN PITCH
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If I increase the frequency, the candle flames flicker even faster.
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And what you can see is that the candles are all flickering
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but they are all flickering together.
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This is synchronised movement.
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They are all moving forwards and backwards together.
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'So what we're seeing is the sound.
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'The movement of the speaker causes the air molecules to oscillate
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'back and forth at a specific frequency.
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'These oscillations travel through the air as sound waves
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'and they are picked up by our ears.'
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A loudspeaker is actually a very unusual way of making sound
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because it's artificially manufactured
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to generate any sound you like.
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Most sound is much more interesting.
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That's because, unlike the loudspeaker,
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most objects create a specific sound that's unique to them...
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..and this is ultimately at the heart
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of why sound is such a rich source of information about the world.
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To understand how an object produces its own unique sound...
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ENGINE ROARS
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..we need a clear and simple sound source.
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For me, one of the most beautiful examples of this is a sound that has
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been ringing out across our cities for centuries.
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BELL CLANGS
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The sound of church bells
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is one of the most distinctive sounds of Britain.
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And I learned to ring bells as a kid,
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so I have certainly spent a lot of time in bell towers,
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but there is one bell that I've never seen.
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It's not only the most famous bell in this country,
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but the most famous bell in the world.
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It's just up there and it's the one we all know as Big Ben.
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'The sound of Big Ben is instantly recognisable.
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'It's an apparently simple sound
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'but also one that's rich and melodious.
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'Analysing how Big Ben's sound is created
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'reveals something remarkable about the relationship between
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'an object and the sound it produces.'
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So this is it.
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This gigantic bell is Big Ben.
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And all sorts of things have changed in the 150 years
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since the Victorians hung it here. But the sound is exactly the same.
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And now I am up here, I can see it in action for the first time.
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'Alongside Big Ben,
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'there are four other smaller bells that hang in the belfry.'
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BELLS CHIME
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'These play the famous Westminster chimes.'
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BELLS PLAY WESTMINSTER CHIMES
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'It is only after this is finished that Big Ben itself is heard.'
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LOUD CLANG REVERBERATES
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It is an incredible amount of sound.
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I could feel that through my feet.
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That's really cool.
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The way that the bell makes sound is that this huge 200kg hammer
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hits the side and that sets the metal vibrating.
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And as it pushes out, it pushes into the air,
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sending pressure waves outwards.
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And those are the sound waves.
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But all of this doesn't just happen at one frequency.
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The huge richness of the sound that Big Ben makes
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comes from many frequencies all happening at the same time.
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So how does one bell produce many different frequencies?
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And what makes them sound so good together?
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The first scientist to try and unpick the frequencies
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within an object's sound
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was the German physicist and amateur musician, Ernst Chladni.
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Chladni devised a special experiment that enabled him
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to study how even the simplest of objects
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can produce a complex sound...
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CYMBAL CRASHES
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..made up of many different frequencies.
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I'm going to do a modern-day version of Chladni's experiment.
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This is a Chladni plate.
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It's just a flat metal sheet that's held in the middle.
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And if I hit it... FLAT CLANG
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..it makes it a sound that doesn't sound very pleasant.
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Certainly not nearly as nice as Big Ben.
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But that sound has a lot in common with the sound of Big Ben
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because it's made up of lots of different frequencies.
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And Ernst Chladni came up with
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a really clever way of picking apart
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where that sound comes from. So he started with a plate like this.
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And he sprinkled sand on top, so I'm going to do that.
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And then he set the plate vibrating.
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And I'm going to do that with a signal generator here
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that's going to move the middle of the plate up and down.
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And the number on the front here is the number of times every second
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that vibration is going to happen - so at the moment it's 240.
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So if I turn this on...
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WHINING HUM
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So it's not a pleasant noise.
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You can see the sand is dancing about in the plate
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but it's not too exciting so far.
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But what happens if you turn the frequency up is quite different.
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HUM INCREASES IN PITCH
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And suddenly at this frequency here, 264 hertz,
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you can see this beautiful pattern pops up in the sand
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of the top of the plate.
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And what this is giving away
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is that the plate is vibrating in a shape
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and the sand is showing us what shape that is.
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What's happening is that the plate is bending like this,
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and at the parts of the plate that are moving a lot,
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the sand is getting bounced away.
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And the parts of the plate that are between a bit that is going up
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and a bit that is going down, don't move at all,
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and so the sand accumulates in those places.
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So what Chladni had found was a really clever trick
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for seeing the shape of the vibration,
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even though he couldn't see it with his eyes.
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The vibration pattern revealed by the sand occurs at what is known
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as a natural frequency of the metal plate.
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This is a specific frequency at which the plate naturally vibrates
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and produces sound.
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And this is part of what's making up the sound when I hit the plate.
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But it's not all of it, because if you keep turning the frequency up,
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there's more to see.
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SOUND INCREASES IN PITCH
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And so here we are up at 426 hertz
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and suddenly, out of that mess,
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there's another pattern of vibration,
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beautiful pattern on the plate here.
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Chladni's experiment reveals how a simple object, this metal plate,
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can produce a complex sound...
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..because it doesn't vibrate at one frequency.
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It has many natural frequencies...
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HIGH-PITCHED RINGING
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..each corresponding to a different pattern of vibration,
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more elaborate than the one before.
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When you hit the plate, what happens is that lots of those vibration
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patterns all happen at the same time, one on top of the other.
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Each one contributes their natural frequency to the mix,
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and that combination is what makes up the sound that you hear.
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Every object that vibrates
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has its own combination of natural frequencies
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determined by its physical characteristics.
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And together, these frequencies form a unique acoustic signature.
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So there's a beautiful relationship between an object and the sound that
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it produces. When you hear sound,
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you are hearing messages about the thing that created it.
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Its size, its shape, what it's made from, even how the object was made.
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And natural frequencies are the key to understanding
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one of the most fascinating mysteries
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about the sounds we encounter in our daily lives.
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Why do some sounds seem rough and unpleasant,
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whilst other sounds like Big Ben
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seem more attractive to the human ear?
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To find the answer,
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we need a way to reveal the exact natural frequencies
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of Big Ben.
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Sprinkling sand isn't going to work for a bell.
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But a team of scientists from the University of Leicester
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are recreating Chladni's experiment using state-of-the-art technology.
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Tell me about the measurements you're making here.
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You've got these lasers around. What are they doing?
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We've got two laser Doppler vibrometers
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pointed at the surface of Big Ben.
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That allows us to measure the motion of the surface,
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the vibration of the surface, directly,
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but without touching the bell.
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So by a tiny change in the laser light,
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you can find out how quickly
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the surface of the bell is moving in and out.
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That's right. We are going to characterise that,
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and be able to show that for all of the natural frequencies of the bell.
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BELLS CHIME
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Across three hours, Martin's two lasers scan Big Ben as it chimes.
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The aim is to discover the bell's different natural frequencies
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and patterns of vibration.
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BIG BEN CHIMES
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Just as with Chladni's plate,
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every time the hammer strikes the bell, the metal vibrates at many
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different natural frequencies,
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each corresponding to a different pattern of vibration.
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Together, these make up
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the distinctive, melodious sound that we hear.
259
00:15:54,920 --> 00:15:56,520
Tell me what we're looking at.
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00:15:56,520 --> 00:15:59,240
We've got an average of the entire chime of Big Ben,
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00:15:59,240 --> 00:16:02,960
and from that you can see a number of different dominant frequencies,
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00:16:02,960 --> 00:16:06,040
and some subordinate frequencies that all go together to make up
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00:16:06,040 --> 00:16:07,880
the characteristic sound of Big Ben.
264
00:16:07,880 --> 00:16:10,440
So those are some at the front here which are really obvious.
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They are much bigger than the others.
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Yeah, particularly 199 hertz and the 336 hertz
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really dominate the character.
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00:16:18,840 --> 00:16:21,240
So each of these natural frequencies corresponds
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to a different vibration pattern on the bell.
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00:16:24,200 --> 00:16:27,000
That's right. To give the note and the colour
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that is the sound of Big Ben.
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This animation is showing the lowest natural frequency of Big Ben.
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95 hertz.
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At the bell's higher natural frequencies,
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the animation shows that it vibrates in more complex patterns.
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It's this mixture of frequencies
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that make up Big Ben's acoustic signature.
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CLANG!
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But knowing the frequencies reveals something else that helps explain
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why we perceive this to be a melodious sound.
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00:17:05,400 --> 00:17:08,040
Because underpinning the difference natural frequencies
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is a mathematical relationship.
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00:17:11,440 --> 00:17:13,440
The sound of Big Ben isn't random.
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Some of its natural frequencies are lined up in a harmonic relationship,
285
00:17:17,160 --> 00:17:19,840
and that's what gives the bell is harmonious sound.
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Some of Big Ben's natural frequencies
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are simple ratios of one another.
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For example, this natural frequency is almost precisely half
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of this one.
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When frequencies are mathematically related like this,
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in what's called a harmonic relationship,
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the human ear finds them pleasant.
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00:17:43,280 --> 00:17:47,040
And in the UK, most bells are specifically tuned to be like this.
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00:17:48,520 --> 00:17:52,280
If we change the shape of the bell or the material it is made from,
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the sound would change.
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And so when we listen to something like a bell,
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what we're hearing is its structure.
298
00:18:04,480 --> 00:18:07,440
So far, the world of sound seems relatively simple.
299
00:18:08,560 --> 00:18:11,120
An object vibrates to make a distinctive sound.
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00:18:13,480 --> 00:18:16,200
And if these vibrations are specially tuned,
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then we can turn sound into something beautiful.
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00:18:18,600 --> 00:18:22,360
ORCHESTRA PLAYS A WALTZ BY JOHANN STRAUSS
303
00:18:22,360 --> 00:18:26,480
But there is more to the beauty of sound than tuning an object.
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There is often something else involved in the production of sound.
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00:18:29,800 --> 00:18:33,040
Something that adds complexity and richness.
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00:18:33,040 --> 00:18:35,320
Something that, exploited to the full,
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can create sounds that stir the soul.
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00:18:48,120 --> 00:18:51,800
With the help of acoustics expert Professor Trevor Cox,
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00:18:51,800 --> 00:18:54,720
a violinist and a special camera,
310
00:18:54,720 --> 00:18:58,240
we're going to explore the way that some sounds are produced
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and how it can be more complex than it might first appear.
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00:19:02,440 --> 00:19:04,400
This is a fantastic toy.
313
00:19:04,400 --> 00:19:05,600
It's an acoustic camera.
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00:19:05,600 --> 00:19:07,880
It's got a little camera right in the middle looking at me,
315
00:19:07,880 --> 00:19:10,720
and then a ring of microphones around the outside.
316
00:19:10,720 --> 00:19:13,040
And they are very directional.
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00:19:13,040 --> 00:19:16,920
And so if I clap up here you can see the sound is coming from up here,
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00:19:16,920 --> 00:19:19,320
the rest of the time you can see it coming from my mouth,
319
00:19:19,320 --> 00:19:22,360
so you can identify where the sound is coming from.
320
00:19:24,880 --> 00:19:27,360
In a musical instrument like a violin,
321
00:19:27,360 --> 00:19:29,880
the initial vibration comes from the string...
322
00:19:32,120 --> 00:19:34,520
..but although the string is vibrating,
323
00:19:34,520 --> 00:19:37,080
it is not directly producing the sound that we hear.
324
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Something else is involved, too.
325
00:19:41,280 --> 00:19:43,040
When you look at a stringed instrument,
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might think the string is making all the sound.
327
00:19:45,080 --> 00:19:46,720
Well, it is starting the sound
328
00:19:46,720 --> 00:19:50,200
but it is not what makes the sound so powerful and so strong.
329
00:19:50,200 --> 00:19:52,800
The string determines the pitch of the sound.
330
00:19:52,800 --> 00:19:56,520
Just the string - it would all be rather dull and quiet.
331
00:20:05,920 --> 00:20:09,000
The acoustic camera shows that the loudest sound,
332
00:20:09,000 --> 00:20:10,880
coloured in pink and red,
333
00:20:10,880 --> 00:20:14,720
isn't coming from string but from the wooden body of the violin.
334
00:20:18,080 --> 00:20:20,120
Tell me what happens to the sound after that.
335
00:20:20,120 --> 00:20:23,000
Well, once you've made a sound, you've got the source of the sound,
336
00:20:23,000 --> 00:20:24,680
it then has to be amplified.
337
00:20:24,680 --> 00:20:27,040
So the sound goes through the bridge first of all,
338
00:20:27,040 --> 00:20:29,400
which connects the string to the body of the violin,
339
00:20:29,400 --> 00:20:31,720
and then the actual wooden plates are all vibrating
340
00:20:31,720 --> 00:20:34,600
and they're amplifying the sound.
341
00:20:34,600 --> 00:20:36,760
So the important thing is that the thin string
342
00:20:36,760 --> 00:20:38,720
can't push on the air very much by itself,
343
00:20:38,720 --> 00:20:41,720
but once you've got a great, big, large, wooden, flat plate,
344
00:20:41,720 --> 00:20:43,000
that can push quite hard.
345
00:20:43,000 --> 00:20:44,520
Yes. Every musical instrument
346
00:20:44,520 --> 00:20:46,640
has resonances at heart and in the violin,
347
00:20:46,640 --> 00:20:49,080
it's actually the wood body that is the resonator.
348
00:20:52,520 --> 00:20:56,920
The wooden body of the violin is what's called a sound resonator.
349
00:20:56,920 --> 00:21:00,960
It transforms the sound of the vibration from the string,
350
00:21:00,960 --> 00:21:04,160
picking up and enhancing certain natural frequencies
351
00:21:04,160 --> 00:21:06,560
whilst damping down others.
352
00:21:10,120 --> 00:21:12,720
ORCHESTRA TUNES UP
353
00:21:15,360 --> 00:21:18,200
Most musical instruments have a resonator.
354
00:21:18,200 --> 00:21:21,240
The pipes of an organ, the bore of a clarinet
355
00:21:21,240 --> 00:21:22,880
and the body of a cello.
356
00:21:23,920 --> 00:21:26,800
It's what amplifies and sculpts the sound,
357
00:21:26,800 --> 00:21:29,760
giving the instrument a far richer acoustic signature.
358
00:21:38,920 --> 00:21:43,320
But the ultimate ability to shape sound doesn't belong to a musical
359
00:21:43,320 --> 00:21:46,160
instrument. It belongs to us.
360
00:21:46,160 --> 00:21:50,440
SHE SINGS: O Mio Babbino Caro by Puccini
361
00:21:51,720 --> 00:21:53,360
It's the human voice.
362
00:22:04,160 --> 00:22:06,240
As a professional opera singer,
363
00:22:06,240 --> 00:22:11,320
Lesley Garrett has exquisite control over the sound her voice produces.
364
00:22:12,480 --> 00:22:14,680
She can produce a range of sounds
365
00:22:14,680 --> 00:22:18,320
far greater than any man-made musical instrument,
366
00:22:18,320 --> 00:22:22,480
and at a volume that can compete with an entire orchestra.
367
00:22:25,680 --> 00:22:30,160
To see how Lesley is able to create such extraordinary sounds,
368
00:22:30,160 --> 00:22:34,080
we have come first to Harley Street in London to meet throat specialist
369
00:22:34,080 --> 00:22:36,320
consultant surgeon John Rubin.
370
00:22:36,320 --> 00:22:40,680
But this is so precious that I do have it checked regularly.
371
00:22:40,680 --> 00:22:44,400
John has looked after me for many decades now and kept me going.
372
00:22:45,680 --> 00:22:48,360
John is going to use a laryngoscope to allow us
373
00:22:48,360 --> 00:22:50,000
to look at Lesley's larynx,
374
00:22:50,000 --> 00:22:52,560
where the sound of her singing voice begins.
375
00:22:52,560 --> 00:22:54,720
Just give me a nice, bright forward...
376
00:22:54,720 --> 00:22:57,440
- SHE SINGS NOTE
- Lovely, lovely.
377
00:22:57,440 --> 00:23:01,160
Now, I'm going to ask you if I may, to show me the tip of your tongue.
378
00:23:02,600 --> 00:23:04,960
Now, smiley face.
379
00:23:04,960 --> 00:23:06,960
Get ready. Take a little...
380
00:23:06,960 --> 00:23:09,520
HE SINGS A NOTE AND SHE REPEATS IT
381
00:23:16,080 --> 00:23:18,760
THEY SING A HIGHER NOTE
382
00:23:22,640 --> 00:23:24,280
Terrific.
383
00:23:24,280 --> 00:23:27,880
The larynx produces vibrations in air,
384
00:23:27,880 --> 00:23:30,000
the origin of the sound we hear.
385
00:23:30,000 --> 00:23:32,120
So these are your vocal folds.
386
00:23:32,120 --> 00:23:34,760
So these two white stripes down here.
387
00:23:34,760 --> 00:23:37,080
These two white stripes are Lesley's vocal folds.
388
00:23:37,080 --> 00:23:39,760
So we can see them of opening and closing as she sings.
389
00:23:39,760 --> 00:23:41,280
Exactly.
390
00:23:41,280 --> 00:23:44,600
It is the opening and closing that actually breaks up the air
391
00:23:44,600 --> 00:23:46,400
and makes sound.
392
00:23:46,400 --> 00:23:48,240
Now in Lesley's instance,
393
00:23:48,240 --> 00:23:50,040
she can make her vocal folds vibrate
394
00:23:50,040 --> 00:23:55,160
anywhere from about 80 times per second probably to over 1,000.
395
00:23:55,160 --> 00:23:58,200
- Wow, I didn't know I could do that.
- It's really amazing.
396
00:23:58,200 --> 00:24:03,280
There are various sets of muscles and I have to, almost unconsciously,
397
00:24:03,280 --> 00:24:07,080
arrange those muscles so that my larynx is in the perfect position
398
00:24:07,080 --> 00:24:11,240
for the amount of pressure I'm choosing to exert upon it.
399
00:24:11,240 --> 00:24:14,120
And that is what we call the onset of tone.
400
00:24:14,120 --> 00:24:17,680
So if I was just going to move my larynx without air,
401
00:24:17,680 --> 00:24:19,840
it would sound like this...
402
00:24:19,840 --> 00:24:22,080
ALMOST SILENT BREATHS There is almost nothing there.
403
00:24:22,080 --> 00:24:24,880
But then if I were to introduce air, it would sound like this.
404
00:24:24,880 --> 00:24:26,280
SHE SINGS LOUDLY
405
00:24:26,280 --> 00:24:27,480
Like that, you know.
406
00:24:27,480 --> 00:24:30,360
You did that with so much volume, so quickly, it's astonishing.
407
00:24:30,360 --> 00:24:32,040
Just that tiny little thing.
408
00:24:32,040 --> 00:24:34,680
HE SINGS A NOTE AND SHE REPEATS IT
409
00:24:34,680 --> 00:24:37,920
The vocal folds create the initial vibration in the air.
410
00:24:42,760 --> 00:24:46,480
Yet, as remarkable as Lesley's vocal folds are,
411
00:24:46,480 --> 00:24:48,560
just like the strings of a violin,
412
00:24:48,560 --> 00:24:51,840
they are not producing the sound we hear.
413
00:24:51,840 --> 00:24:55,680
It is her resonator that is the key to her extraordinary voice.
414
00:25:11,240 --> 00:25:13,760
We've come to University College London
415
00:25:13,760 --> 00:25:15,840
to meet Professor Sophie Scott,
416
00:25:15,840 --> 00:25:19,840
who's going to reveal what makes the human resonators so special.
417
00:25:19,840 --> 00:25:22,560
So what we're going to do today is I'm going to take you through to our
418
00:25:22,560 --> 00:25:25,760
MRI machine and what we're going to do is use it to image
419
00:25:25,760 --> 00:25:28,840
Lesley's vocal tract, and that should tell us something about
420
00:25:28,840 --> 00:25:32,000
what is happening for you when you are singing so beautifully.
421
00:25:32,000 --> 00:25:33,880
I cannot tell you how excited I am about this.
422
00:25:33,880 --> 00:25:36,920
It's sort of like the answer to the ultimate mystery.
423
00:25:36,920 --> 00:25:40,360
For 40 years I've been singing and never really quite understood
424
00:25:40,360 --> 00:25:41,880
what is going on in my throat.
425
00:25:41,880 --> 00:25:43,520
None of us can. None of us can see it.
426
00:25:43,520 --> 00:25:46,040
It's not like we're pianists and we can see what is going on,
427
00:25:46,040 --> 00:25:47,520
so this is so exciting.
428
00:25:49,560 --> 00:25:53,880
The sound resonator of Lesley's voice is her throat and mouth.
429
00:25:53,880 --> 00:25:58,480
And this is what the MRI machine is going to image as she sings.
430
00:25:58,480 --> 00:26:01,360
Lesley, can you sing for me the vowels
431
00:26:01,360 --> 00:26:06,440
ee, eh, ah, oh, euh?
432
00:26:07,560 --> 00:26:09,040
LESLEY SINGS
433
00:26:13,400 --> 00:26:14,960
The whole thing is moving.
434
00:26:14,960 --> 00:26:16,480
It's quite extraordinary.
435
00:26:16,480 --> 00:26:20,720
The MRI shows how, for each of the different vowel sounds,
436
00:26:20,720 --> 00:26:24,480
Lesley's mouth and throat change shape,
437
00:26:24,480 --> 00:26:28,720
amplifying the vibrations in air produced by her vocal folds
438
00:26:28,720 --> 00:26:31,120
and sculpting them into the sound we hear.
439
00:26:33,120 --> 00:26:36,600
So what we saw with the laryngoscopy right down here
440
00:26:36,600 --> 00:26:39,000
is just the very beginning of making sound
441
00:26:39,000 --> 00:26:41,800
and then there's all this shaping that goes on up here,
442
00:26:41,800 --> 00:26:44,440
that actually determines what we hear.
443
00:26:44,440 --> 00:26:46,000
Exactly, so
444
00:26:46,000 --> 00:26:49,200
all the work being done above the voice box, the larynx,
445
00:26:49,200 --> 00:26:52,680
is essentially changing the spectral characteristics of the noise that
446
00:26:52,680 --> 00:26:55,480
you're making down there. You're making a noise here and then you're
447
00:26:55,480 --> 00:26:57,640
continuously changing it up here.
448
00:26:57,640 --> 00:26:59,080
Particularly, as you can see,
449
00:26:59,080 --> 00:27:01,320
by exactly how the tongue has been positioned
450
00:27:01,320 --> 00:27:04,120
and how the tongue is moving.
451
00:27:07,400 --> 00:27:09,400
Lesley, that was absolutely beautiful.
452
00:27:09,400 --> 00:27:10,880
Thank you.
453
00:27:10,880 --> 00:27:13,680
- So, should we do "I Dreamed A Dream"?
- OK.
454
00:27:13,680 --> 00:27:19,120
# I dreamed a dream in time gone by... #
455
00:27:19,120 --> 00:27:23,000
The MRI reveals the secret of our resonator.
456
00:27:23,000 --> 00:27:26,160
And like the sound resonator of a musical instrument,
457
00:27:26,160 --> 00:27:28,560
the vocal resonator isn't fixed.
458
00:27:28,560 --> 00:27:31,200
It's incredibly flexible.
459
00:27:31,200 --> 00:27:33,920
Through the movement of the tongue in particular
460
00:27:33,920 --> 00:27:35,560
and the jaw, lips and throat,
461
00:27:35,560 --> 00:27:40,720
it can be manipulated to form a myriad of different shapes.
462
00:27:40,720 --> 00:27:43,480
Look how open that is. It's extraordinary.
463
00:27:43,480 --> 00:27:45,800
And with a trained singer like Lesley,
464
00:27:45,800 --> 00:27:48,120
the range of movement is truly amazing.
465
00:27:49,120 --> 00:27:51,760
The tongue is basically like an octopus tentacle.
466
00:27:51,760 --> 00:27:54,320
It just deforms in all these different directions.
467
00:27:54,320 --> 00:27:57,200
This is such a flexible, adaptive instrument, isn't it?
468
00:27:57,200 --> 00:27:59,360
That is a surprise to me, I must admit.
469
00:27:59,360 --> 00:28:02,080
It takes you so long to coordinate all that to the level
470
00:28:02,080 --> 00:28:05,080
that we can project a beautiful sound,
471
00:28:05,080 --> 00:28:07,960
a sound that will hopefully make people cry or laugh,
472
00:28:07,960 --> 00:28:12,080
to the back of a 2,000-seater auditorium without amplification,
473
00:28:12,080 --> 00:28:14,800
it's something that requires massive training
474
00:28:14,800 --> 00:28:16,920
and now I can see why it did.
475
00:28:18,480 --> 00:28:23,200
# I had a dream my life would be
476
00:28:23,200 --> 00:28:24,800
# So different... #
477
00:28:24,800 --> 00:28:27,840
Sound begins as a simple vibration.
478
00:28:27,840 --> 00:28:34,160
# So different now from what it seemed... #
479
00:28:34,160 --> 00:28:39,000
But it is how this initial vibration are sculpted by the resonator that
480
00:28:39,000 --> 00:28:44,040
lies behind our mastery and control of sound.
481
00:28:44,040 --> 00:28:46,040
# The dream
482
00:28:49,560 --> 00:29:00,960
# I dreamed. #
483
00:29:10,640 --> 00:29:12,040
APPLAUSE
484
00:29:18,560 --> 00:29:23,040
Music is an obvious way in which sound can have an impact on us.
485
00:29:23,040 --> 00:29:25,240
But there's a type of sound
486
00:29:25,240 --> 00:29:28,920
that makes an impact in a very different way.
487
00:29:28,920 --> 00:29:31,640
It is a type of sound that doesn't play by the rules
488
00:29:31,640 --> 00:29:33,920
of any of the sounds we've heard so far.
489
00:29:37,880 --> 00:29:39,600
WHIP CRACKS
490
00:29:39,600 --> 00:29:41,960
This is a thing that is entirely new to me.
491
00:29:41,960 --> 00:29:45,520
It is a bullwhip, and Lila here is about to have a go at teaching me
492
00:29:45,520 --> 00:29:46,880
how to crack it.
493
00:29:46,880 --> 00:29:48,760
OK, so whip cracking.
494
00:29:48,760 --> 00:29:52,200
- Safety goggles.
- Good idea.
495
00:29:52,200 --> 00:29:54,600
Right, so these are bullwhips.
496
00:29:54,600 --> 00:29:56,760
This is the bit that makes the sound.
497
00:29:58,760 --> 00:30:01,200
So behind you, turn sideways slightly.
498
00:30:01,200 --> 00:30:02,480
Yes.
499
00:30:04,200 --> 00:30:06,240
I hit myself in the head.
500
00:30:06,240 --> 00:30:09,200
FAINT CLICKING
501
00:30:09,200 --> 00:30:10,680
CRACK!
502
00:30:10,680 --> 00:30:13,440
So I think I'm doing all right, and then you're coming along behind
503
00:30:13,440 --> 00:30:15,480
with this enormous noise.
504
00:30:20,240 --> 00:30:22,600
- Oh!
- That was it, yeah.
- We're in business.
505
00:30:22,600 --> 00:30:24,800
Just try and get that...
506
00:30:24,800 --> 00:30:26,720
That was a good one.
507
00:30:26,720 --> 00:30:28,520
Shall we finish on a high?
508
00:30:29,880 --> 00:30:33,680
The sound comes that comes from this whip is something special.
509
00:30:33,680 --> 00:30:35,960
It's different. We're not hearing a shape.
510
00:30:35,960 --> 00:30:38,320
It hasn't got specific frequencies associated with it.
511
00:30:38,320 --> 00:30:40,160
And it's also fantastically loud.
512
00:30:40,160 --> 00:30:43,240
All of that sound is coming just from that tiny bit on the end
513
00:30:43,240 --> 00:30:46,720
and yet it echoed around this entire space.
514
00:30:46,720 --> 00:30:49,960
Right at the point this sound forms, it isn't even a wave.
515
00:30:49,960 --> 00:30:51,600
This is something different.
516
00:30:52,760 --> 00:30:56,880
The key to what makes this type of sound different and so loud
517
00:30:56,880 --> 00:30:58,840
is how it's generated.
518
00:31:02,560 --> 00:31:04,360
And to see how that happens,
519
00:31:04,360 --> 00:31:07,440
we need the help of physicist Dr Daniel Eakins.
520
00:31:07,440 --> 00:31:09,800
- This is Lila.
- Hi, nice to meet you.
521
00:31:09,800 --> 00:31:11,640
So what have we got here?
522
00:31:11,640 --> 00:31:12,960
What does the set-up do?
523
00:31:12,960 --> 00:31:14,960
This is known as a Schlieren imaging set-up,
524
00:31:14,960 --> 00:31:18,560
and what it allows us to do is detect very small, minute changes
525
00:31:18,560 --> 00:31:22,040
in the way light refracts through gas as it is heated, for example.
526
00:31:23,200 --> 00:31:26,640
- There you go. Yes.
- It's pretty, isn't it?
- Yes.
527
00:31:26,640 --> 00:31:30,240
The Schlieren camera is able to detect distortions in light
528
00:31:30,240 --> 00:31:33,360
created by changes in air temperature and pressure.
529
00:31:33,360 --> 00:31:37,560
What we are going to try to do is have it
530
00:31:37,560 --> 00:31:40,240
so that when the whip, or when the end of the whip
531
00:31:40,240 --> 00:31:43,080
is at its highest speed,
532
00:31:43,080 --> 00:31:46,120
that that's within the field of view of the Schlieren camera.
533
00:31:46,120 --> 00:31:48,680
She's got to hit that toothpick thing there?
534
00:31:48,680 --> 00:31:51,040
Yes, she has to be in the vicinity of this,
535
00:31:51,040 --> 00:31:53,560
probably within about 50 mil if you can manage, yes.
536
00:31:53,560 --> 00:31:55,240
- Can you do that?
- Sure.
537
00:32:03,760 --> 00:32:05,440
Wow. If only we had that one.
538
00:32:05,440 --> 00:32:06,920
It's amazing.
539
00:32:06,920 --> 00:32:08,360
Oh, my goodness.
540
00:32:10,600 --> 00:32:11,640
This is it.
541
00:32:13,120 --> 00:32:18,320
- Oh, wow. You've done it.
- OK.
542
00:32:18,320 --> 00:32:20,080
You owe me a cocktail stick. OK.
543
00:32:20,080 --> 00:32:22,640
We'll just go and have a look at the data, then.
544
00:32:24,640 --> 00:32:28,280
This slow-motion footage shows the disturbance in the air
545
00:32:28,280 --> 00:32:30,240
created by the tip of the bullwhip.
546
00:32:31,200 --> 00:32:35,280
The dark lines show where the air has been compressed together to form
547
00:32:35,280 --> 00:32:37,040
concentrated pressure fronts.
548
00:32:39,200 --> 00:32:42,400
The strands of the bullwhip create pressure fronts that travel
549
00:32:42,400 --> 00:32:43,840
at phenomenal speed.
550
00:32:45,440 --> 00:32:47,720
This is what creates the sound.
551
00:32:50,680 --> 00:32:54,560
It looks like it is moving at around 364 metres per second.
552
00:32:54,560 --> 00:32:59,120
So the speed of sound in air is about 343 metres a second,
553
00:32:59,120 --> 00:33:01,720
so this is going faster than the speed of sound.
554
00:33:01,720 --> 00:33:03,600
It is a supersonic disturbance.
555
00:33:05,760 --> 00:33:08,920
The reason this sound can travel at supersonic speed
556
00:33:08,920 --> 00:33:12,400
is because it's not a wave but a shock front.
557
00:33:13,680 --> 00:33:15,280
For a fraction of a second,
558
00:33:15,280 --> 00:33:18,480
it has enormous energy that punches through the air
559
00:33:18,480 --> 00:33:22,200
with such force that the air molecules can't oscillate
560
00:33:22,200 --> 00:33:24,800
back and forth as a wave.
561
00:33:24,800 --> 00:33:28,040
The one distinguishing feature of a shock is that it is like an impulse.
562
00:33:28,040 --> 00:33:30,040
It is an instantaneous change in pressure.
563
00:33:30,040 --> 00:33:33,360
So the reason that such a tiny thing can make such a loud sound
564
00:33:33,360 --> 00:33:36,240
is because it's barrelling into the air and so there's
565
00:33:36,240 --> 00:33:37,720
far more volume given out.
566
00:33:37,720 --> 00:33:40,400
- That's right.
- So you've been breaking the sound barrier, Lila.
567
00:33:40,400 --> 00:33:41,640
So cool!
568
00:33:43,440 --> 00:33:45,120
THUNDER RUMBLES AND CRASHES
569
00:33:46,240 --> 00:33:48,240
From the crack of a lightning bolt...
570
00:33:49,320 --> 00:33:50,920
..to the bang of a gunshot...
571
00:33:51,920 --> 00:33:54,400
..and the blast of an explosion,
572
00:33:54,400 --> 00:33:58,560
the loudest sounds on the planet all originate as shock fronts.
573
00:34:05,040 --> 00:34:08,080
Nasa scientists have used the same Schlieren technique
574
00:34:08,080 --> 00:34:12,400
to image the shock fronts created by supersonic aircraft,
575
00:34:12,400 --> 00:34:17,000
by filming the aircraft flying in front of the sun.
576
00:34:17,000 --> 00:34:21,360
Three, two, one, mark.
577
00:34:22,840 --> 00:34:24,520
The aircraft is moving faster
578
00:34:24,520 --> 00:34:26,960
than the speed at which sound waves travel.
579
00:34:28,400 --> 00:34:31,840
Because of this, the air molecules in front of the aircraft
580
00:34:31,840 --> 00:34:34,360
get shoved out of the way with such ferocity
581
00:34:34,360 --> 00:34:37,440
that there's no time for normal sound waves to form.
582
00:34:38,720 --> 00:34:41,640
Instead, a pattern of shock fronts are created.
583
00:34:42,880 --> 00:34:46,600
This is the origin of the sonic boom.
584
00:34:46,600 --> 00:34:48,120
BOOM
585
00:34:57,840 --> 00:34:59,720
SIREN, ENGINES AND CHURCH BELLS
586
00:34:59,720 --> 00:35:01,400
For all of the fascinating science
587
00:35:01,400 --> 00:35:04,960
behind the sounds we are familiar with in our daily lives,
588
00:35:04,960 --> 00:35:09,480
these are only a tiny fraction of the sounds that fill our planet.
589
00:35:13,120 --> 00:35:17,400
There are entire worlds of sound that remain hidden from us.
590
00:35:17,400 --> 00:35:20,960
Places where sound can behave in very different ways.
591
00:35:22,760 --> 00:35:26,000
And perhaps the most intriguing of these is the ocean.
592
00:35:28,880 --> 00:35:32,520
Two-thirds of our planet is covered by water.
593
00:35:32,520 --> 00:35:35,280
And yet apart from the sound of the waves,
594
00:35:35,280 --> 00:35:39,320
it's a world that appears to us here on land as silent.
595
00:35:45,560 --> 00:35:47,440
When I put my hand in the water here,
596
00:35:47,440 --> 00:35:50,280
I'm touching a different acoustic world.
597
00:35:50,280 --> 00:35:53,280
And that's because both sides of the water surface act like
598
00:35:53,280 --> 00:35:57,360
an acoustic mirror. Sound coming from beneath bounces off the air
599
00:35:57,360 --> 00:36:00,600
and goes back into the water and all the sound up here
600
00:36:00,600 --> 00:36:03,720
bounces off the water and goes back into the air.
601
00:36:03,720 --> 00:36:06,840
So I can put my hand into this acoustic world,
602
00:36:06,840 --> 00:36:08,080
but I can't hear it.
603
00:36:10,280 --> 00:36:12,360
The acoustic mirror effect ensures
604
00:36:12,360 --> 00:36:16,680
that sound travelling in water can't escape into the air.
605
00:36:18,040 --> 00:36:20,120
So the only way to experience
606
00:36:20,120 --> 00:36:22,920
how sound behaves differently in the ocean,
607
00:36:22,920 --> 00:36:26,000
and to see the profound effect this has on life,
608
00:36:26,000 --> 00:36:28,840
is to enter the underwater acoustic world.
609
00:36:30,800 --> 00:36:32,360
- Hello.
- Hello.
610
00:36:32,360 --> 00:36:33,920
- How are you doing?
- I am all right.
611
00:36:33,920 --> 00:36:37,720
'I have come to meet Dr Steve Simpson, who is a marine biologist
612
00:36:37,720 --> 00:36:38,920
'and he's going to reveal
613
00:36:38,920 --> 00:36:41,360
'just how differently sound behaves underwater.'
614
00:37:00,840 --> 00:37:03,920
- So what have we got here?
- So here we have got...
615
00:37:03,920 --> 00:37:05,160
The plastic bucket of science.
616
00:37:05,160 --> 00:37:07,520
The plastic bucket of science, absolutely.
617
00:37:07,520 --> 00:37:11,160
- So we've got a hydrophone here.
- So that's our underwater microphone.
618
00:37:11,160 --> 00:37:12,960
This is our ear, basically, underwater.
619
00:37:12,960 --> 00:37:16,920
And then we have a recorder that allows us to be able to take the
620
00:37:16,920 --> 00:37:19,760
recordings through the whole of our snorkel and have I have wired up
621
00:37:19,760 --> 00:37:21,560
a speaker inside a cup.
622
00:37:21,560 --> 00:37:24,000
So while we're snorkelling about on the surface of the water,
623
00:37:24,000 --> 00:37:26,160
we'll be able to hear what's going on below.
624
00:37:26,160 --> 00:37:28,720
- Exactly, yeah.
- All right. Let's give it a go.
625
00:37:47,920 --> 00:37:52,600
'Part of the reason that sound is so different in water compared to air
626
00:37:52,600 --> 00:37:56,600
'is that water is 1,000 times more dense.
627
00:37:56,600 --> 00:38:00,080
'One consequence is that it takes more energy to start a vibration
628
00:38:00,080 --> 00:38:01,480
'in the first place.
629
00:38:03,400 --> 00:38:06,640
'Sea creatures have evolved specific means to create sound
630
00:38:06,640 --> 00:38:08,400
'in this much denser medium.'
631
00:38:12,520 --> 00:38:15,160
- Here you go. You take this.
- So this is the listening device?
632
00:38:15,160 --> 00:38:17,760
- There's your ear and here's a hydrophone.
- OK.
633
00:38:20,560 --> 00:38:22,120
CRACKLING
634
00:38:26,080 --> 00:38:28,200
- How was that?
- I can hear popcorn.
635
00:38:28,200 --> 00:38:30,400
It sounds like snapping shrimp to me.
636
00:38:30,400 --> 00:38:33,240
It is the soundtrack of the ocean, that's right.
637
00:38:33,240 --> 00:38:36,960
Snapping shrimp overcome the difficulty of producing sound
638
00:38:36,960 --> 00:38:40,440
in water by snapping their claws together really fast...
639
00:38:43,080 --> 00:38:44,880
..causing bubbles to implode.
640
00:38:46,880 --> 00:38:49,120
So it is kind of a grating, scraping noise.
641
00:38:51,640 --> 00:38:52,880
And this is the sound
642
00:38:52,880 --> 00:38:55,840
of a sea urchin scratching seaweed off the rock.
643
00:38:55,840 --> 00:38:57,280
SCRAPING
644
00:39:03,360 --> 00:39:06,960
Water transmits sound much more effectively than air.
645
00:39:12,320 --> 00:39:17,040
In fact, sound travels much further in water than light does,
646
00:39:17,040 --> 00:39:20,160
something that life under the waves takes full advantage of.
647
00:39:23,840 --> 00:39:27,840
I've got a recording of a soldier fish. So this is a coral reef fish.
648
00:39:27,840 --> 00:39:29,280
Spends its day living in a cave
649
00:39:29,280 --> 00:39:31,280
then goes out at night looking for shrimp
650
00:39:31,280 --> 00:39:32,840
that come out of the sand to feed.
651
00:39:33,920 --> 00:39:35,240
And when it finds the food...
652
00:39:35,240 --> 00:39:36,720
BOOMING GRUNTS
653
00:39:40,400 --> 00:39:42,480
It's a very big, deep noise, isn't it?
654
00:39:42,480 --> 00:39:44,040
It's like a deep trumpeting sound.
655
00:39:44,040 --> 00:39:46,480
- How big is the fish?
- So the fish would be about this sort of size.
656
00:39:46,480 --> 00:39:48,200
- It's quite a small fish.
- A small fish to make
657
00:39:48,200 --> 00:39:50,520
- a lot of noise, that's right.
- That's really impressive.
658
00:39:50,520 --> 00:39:53,640
What sort of distances are these sounds travelling underwater?
659
00:39:53,640 --> 00:39:57,000
So with a hydrophone like this, if you're out in the open ocean,
660
00:39:57,000 --> 00:39:59,400
you'd hear a coral reef from up to 25km away.
661
00:39:59,400 --> 00:40:02,000
So it really is a cacophony of noise.
662
00:40:03,120 --> 00:40:06,480
And we think that fish can hear the sound from hundreds of metres,
663
00:40:06,480 --> 00:40:08,240
some species for kilometres.
664
00:40:08,240 --> 00:40:11,160
So it's almost in the ocean as though sound and light have swapped
665
00:40:11,160 --> 00:40:14,520
places. Sound is much more useful underwater than light is.
666
00:40:14,520 --> 00:40:18,080
Yes. So you might be able to see 30 metres in really clear water,
667
00:40:18,080 --> 00:40:21,440
but you can hear for hundreds of metres or kilometres.
668
00:40:21,440 --> 00:40:23,640
So it becomes an information channel
669
00:40:23,640 --> 00:40:26,040
that works over much larger distances.
670
00:40:28,480 --> 00:40:33,240
The distances over which sound can travel underwater are truly amazing.
671
00:40:33,240 --> 00:40:37,680
The sounds made by whales can carry for thousands of kilometres...
672
00:40:38,760 --> 00:40:41,640
..travelling across almost entire oceans.
673
00:40:42,720 --> 00:40:46,640
Yet because these sounds remain locked beneath the water surface,
674
00:40:46,640 --> 00:40:48,640
they never reach our ears.
675
00:40:58,440 --> 00:41:00,360
We can't hear underwater sounds
676
00:41:00,360 --> 00:41:03,600
because we are not a part of that acoustic world.
677
00:41:03,600 --> 00:41:06,760
However, there is a whole class of sounds that we don't hear
678
00:41:06,760 --> 00:41:09,400
for a completely different reason,
679
00:41:09,400 --> 00:41:13,440
because their frequency lies outside our range of hearing.
680
00:41:14,560 --> 00:41:17,320
And yet it is these sound that turn out to deliver
681
00:41:17,320 --> 00:41:19,720
the most fascinating insights.
682
00:41:19,720 --> 00:41:24,160
It is easy to take the huge range of human hearing for granted
683
00:41:24,160 --> 00:41:26,080
but it is worth spending a moment on.
684
00:41:26,080 --> 00:41:28,400
The piano is a really good way to demonstrate it.
685
00:41:28,400 --> 00:41:33,000
This is middle C here and that is at 262 hertz,
686
00:41:33,000 --> 00:41:35,760
which means 262 cycles every second.
687
00:41:35,760 --> 00:41:39,120
And the lovely thing about a piano is that you can go up in octaves,
688
00:41:41,160 --> 00:41:44,120
and every octave involves a doubling of frequencies.
689
00:41:44,120 --> 00:41:49,000
So the highest note in the piano, this C here is 4,186 hertz.
690
00:41:49,000 --> 00:41:51,200
It doesn't stop there.
691
00:41:51,200 --> 00:41:53,840
If we were to build our piano outwards
692
00:41:53,840 --> 00:41:56,600
to the edge of the human hearing range,
693
00:41:56,600 --> 00:42:00,440
we come all the way up here, which is 19.9 kilohertz -
694
00:42:00,440 --> 00:42:02,200
a gigantic number.
695
00:42:02,200 --> 00:42:05,080
And it also carries on down the other end.
696
00:42:05,080 --> 00:42:07,520
The lowest C on the piano is this one,
697
00:42:07,520 --> 00:42:09,680
with a frequency of 32 hertz.
698
00:42:09,680 --> 00:42:13,320
And if we were to carry on our piano to the limit of human hearing,
699
00:42:13,320 --> 00:42:16,680
we would get down here. This one is 20.6 hertz.
700
00:42:16,680 --> 00:42:19,480
So this piano, with all its extra keys,
701
00:42:19,480 --> 00:42:22,320
represents the full range of human hearing.
702
00:42:24,040 --> 00:42:28,120
'This is our rich but ultimately limited experience of sound...
703
00:42:30,320 --> 00:42:33,400
'..because the full spectrum of sound frequencies
704
00:42:33,400 --> 00:42:36,160
'extends way beyond what we can hear.'
705
00:42:49,880 --> 00:42:53,440
These sounds that lie outside our range of hearing
706
00:42:53,440 --> 00:42:58,440
hold the key to a world where sound gives life extraordinary powers,
707
00:42:58,440 --> 00:43:02,800
and opens new windows onto our planet and even the universe.
708
00:43:27,880 --> 00:43:30,320
I'm in the middle of a huge pod of dolphins.
709
00:43:30,320 --> 00:43:32,920
There must be hundreds of them out here.
710
00:43:34,280 --> 00:43:36,440
These dolphins are hunters.
711
00:43:36,440 --> 00:43:39,600
They're using high-frequency sounds to locate their prey.
712
00:43:44,440 --> 00:43:48,160
Most of the clicks and whistles that these dolphins produce
713
00:43:48,160 --> 00:43:50,400
are way beyond the range of our hearing.
714
00:43:50,400 --> 00:43:52,320
HIGH-PITCHED WHISTLING
715
00:43:52,320 --> 00:43:55,720
This is the realm of ultrasound -
716
00:43:55,720 --> 00:43:58,480
sound at frequencies above what we can here.
717
00:43:59,840 --> 00:44:02,840
What I can hear are whistling noises but they are calls.
718
00:44:02,840 --> 00:44:05,280
Most of them are at higher frequencies than I can hear.
719
00:44:05,280 --> 00:44:07,880
So I'm just hearing a tiny, tiny bit at the bottom
720
00:44:07,880 --> 00:44:10,160
and it's still really loud.
721
00:44:14,600 --> 00:44:17,840
Ultrasound is key to the dolphin's hunting ability.
722
00:44:18,840 --> 00:44:23,760
Because ultrasound has a very high frequency and a small wavelength,
723
00:44:23,760 --> 00:44:27,080
it reflects off small, fast-moving objects
724
00:44:27,080 --> 00:44:30,080
that audible sound waves would pass over.
725
00:44:30,080 --> 00:44:33,640
The dolphin creates short pulses of ultrasound and then listens
726
00:44:33,640 --> 00:44:35,960
for the echoes and, from this,
727
00:44:35,960 --> 00:44:38,800
creates a detailed image of its surroundings,
728
00:44:38,800 --> 00:44:40,520
enabling it to catch its prey.
729
00:44:44,360 --> 00:44:48,160
These animals are operating in a sound range that is outside
730
00:44:48,160 --> 00:44:49,920
what we can perceive
731
00:44:49,920 --> 00:44:53,200
and it really highlights how much more there is out there.
732
00:44:56,880 --> 00:44:58,280
Dolphins are not alone
733
00:44:58,280 --> 00:45:01,080
in using ultrasound as a second form of sight.
734
00:45:02,360 --> 00:45:05,200
Bats use it for their version of echolocation...
735
00:45:09,040 --> 00:45:11,600
..and we use ultrasound for medical imaging.
736
00:45:13,560 --> 00:45:16,720
Pulses of ultrasound can penetrate the skin and reflect off
737
00:45:16,720 --> 00:45:19,200
different tissues.
738
00:45:19,200 --> 00:45:21,080
Fluid, muscle and bone.
739
00:45:22,480 --> 00:45:26,680
And these echoes are recorded and displayed as an image,
740
00:45:26,680 --> 00:45:29,840
enabling us to see the foetus inside the womb.
741
00:45:39,680 --> 00:45:42,160
At the other end of the sound spectrum
742
00:45:42,160 --> 00:45:45,760
lies an even more mysterious and unfamiliar group of sounds.
743
00:45:48,560 --> 00:45:51,560
This is the realm of infrasound -
744
00:45:51,560 --> 00:45:54,600
sounds that are too deep for us to hear.
745
00:45:54,600 --> 00:45:57,040
And as we learn to decode these sounds,
746
00:45:57,040 --> 00:46:00,240
they give us a greater understanding of our planet
747
00:46:00,240 --> 00:46:03,280
an offer us the potential to save thousands of lives.
748
00:46:05,320 --> 00:46:09,000
Infrasound lets us listen in on the geological world,
749
00:46:09,000 --> 00:46:11,080
and, if you want to listen to infrasound,
750
00:46:11,080 --> 00:46:12,440
this is the place to come.
751
00:46:16,840 --> 00:46:18,920
This is Stromboli,
752
00:46:18,920 --> 00:46:21,600
one of the most active volcanoes on the planet.
753
00:46:23,880 --> 00:46:27,960
It has been erupting almost continuously for over 1,000 years.
754
00:46:35,000 --> 00:46:37,160
I've come here to meet some scientists
755
00:46:37,160 --> 00:46:39,120
whose research has helped reveal
756
00:46:39,120 --> 00:46:42,440
that this volcano, although we can't hear it,
757
00:46:42,440 --> 00:46:44,440
creates an extraordinary sound.
758
00:46:49,000 --> 00:46:52,120
The sound is created in a two-stage process
759
00:46:52,120 --> 00:46:54,440
that starts with the spectacular
760
00:46:54,440 --> 00:46:57,160
explosions of magma from within the volcano.
761
00:47:02,000 --> 00:47:05,040
And this is what Dr Jacopo Taddeucci
762
00:47:05,040 --> 00:47:07,440
and Dr Jorn Sesterhenn are studying.
763
00:47:07,440 --> 00:47:11,440
So basically, we use a high-speed camera to take footage
764
00:47:11,440 --> 00:47:14,400
of what happens at the vent of the volcano.
765
00:47:14,400 --> 00:47:16,120
So can we see some of these videos?
766
00:47:16,120 --> 00:47:17,480
Yeah, sure.
767
00:47:17,480 --> 00:47:19,600
OK.
768
00:47:19,600 --> 00:47:21,240
This is the eruption.
769
00:47:21,240 --> 00:47:22,840
And then you see the bombs...
770
00:47:22,840 --> 00:47:26,040
that are these particles flying here.
771
00:47:26,040 --> 00:47:29,120
- So these big lumps flying up into the sky.
- Exactly.
772
00:47:29,120 --> 00:47:31,120
And how fast are they going?
773
00:47:31,120 --> 00:47:33,920
They can go up to 400 metres per second.
774
00:47:33,920 --> 00:47:36,200
So that's very, very fast.
775
00:47:36,200 --> 00:47:39,280
It is faster than sound in the air.
776
00:47:39,280 --> 00:47:41,000
It is a supersonic eruption.
777
00:47:44,880 --> 00:47:46,480
In this processed image,
778
00:47:46,480 --> 00:47:50,720
the dark lines travelling ahead of the molten rock are the sound waves
779
00:47:50,720 --> 00:47:52,880
created by the supersonic eruption.
780
00:47:52,880 --> 00:47:55,520
So there is a rush of gas and particles.
781
00:47:55,520 --> 00:47:58,240
Coming out very fast, even supersonic.
782
00:47:58,240 --> 00:48:01,520
- This makes the sound.
- There is a very powerful eruption of gas and
783
00:48:01,520 --> 00:48:04,960
particles and it is just pushing on the air around it and sending out
784
00:48:04,960 --> 00:48:06,480
- sound waves.
- Exactly.
785
00:48:07,640 --> 00:48:10,880
The eruption creates a supersonic shock front...
786
00:48:10,880 --> 00:48:11,920
BOOM
787
00:48:13,680 --> 00:48:16,560
..that we hear as an explosion.
788
00:48:16,560 --> 00:48:18,200
So far, so conventional.
789
00:48:23,000 --> 00:48:25,160
But this is just the first stage
790
00:48:25,160 --> 00:48:28,400
of the creation of a far more surprising sound -
791
00:48:28,400 --> 00:48:32,240
an infrasound that is well below our range of hearing.
792
00:48:33,840 --> 00:48:35,440
Detecting it isn't easy.
793
00:48:38,520 --> 00:48:41,000
- Hello.
- Welcome.
794
00:48:41,000 --> 00:48:44,240
So this is Stromboli.
795
00:48:44,240 --> 00:48:47,880
'The way that this infrasound is created depends on how the sound
796
00:48:47,880 --> 00:48:50,400
'of the eruption is shaped by the crater.
797
00:48:52,320 --> 00:48:55,400
'This is what Dr Jeffrey Johnson has been studying.'
798
00:48:55,400 --> 00:48:56,920
It's loud, isn't it?
799
00:48:58,880 --> 00:49:01,040
That second reverberation,
800
00:49:01,040 --> 00:49:03,000
that is effectively a sound wave
801
00:49:03,000 --> 00:49:05,960
oscillating back and forth in this giant, giant pit.
802
00:49:05,960 --> 00:49:09,120
So a load of sound just washed past us that we couldn't hear
803
00:49:09,120 --> 00:49:10,720
but that was what you were measuring.
804
00:49:10,720 --> 00:49:13,400
Right. We could hear a component of that but not all of it.
805
00:49:13,400 --> 00:49:16,160
- And I would like to show you what the signals look like.
- Cool.
806
00:49:20,000 --> 00:49:24,640
Fractions of a second after the explosive supersonic eruption,
807
00:49:24,640 --> 00:49:27,640
a second sound carries on -
808
00:49:27,640 --> 00:49:29,840
a pure tone of infrasound.
809
00:49:32,240 --> 00:49:34,520
Since we can't hear it directly,
810
00:49:34,520 --> 00:49:37,960
we need the help of a bit of audio trickery.
811
00:49:37,960 --> 00:49:43,120
I would like you to put these on and tell me what kind of sound you hear.
812
00:49:43,120 --> 00:49:44,560
SQUEAKING
813
00:49:46,240 --> 00:49:49,080
I hear the world's angriest mosquito.
814
00:49:49,080 --> 00:49:50,680
That's what it should sound like.
815
00:49:50,680 --> 00:49:56,440
This box produces a 700 hertz tone that is being frequency modulated by
816
00:49:56,440 --> 00:49:58,520
infrasound produced by the volcano.
817
00:49:58,520 --> 00:50:01,480
So what you should be hearing is a constant tone and there when
818
00:50:01,480 --> 00:50:03,320
there is an infrasound signal,
819
00:50:03,320 --> 00:50:06,560
it deflects that tone to higher and lower frequencies.
820
00:50:07,720 --> 00:50:11,120
'We can't hear the infrasound directly.
821
00:50:11,120 --> 00:50:13,880
'Instead, Jeff's apparatus is set up
822
00:50:13,880 --> 00:50:16,560
'so that when the infrasound passes by,
823
00:50:16,560 --> 00:50:20,880
'it changes the pitch of the constant buzzing sound.
824
00:50:20,880 --> 00:50:23,280
'Whenever the angry bee sound wobbles,
825
00:50:23,280 --> 00:50:25,560
'it's because it has been hit by infrasound.'
826
00:50:25,560 --> 00:50:27,440
There we go.
827
00:50:27,440 --> 00:50:30,240
And you can see a huge deflection corresponding to that explosion,
828
00:50:30,240 --> 00:50:33,680
and that was about a 2-3 hertz tone that I just observed.
829
00:50:36,800 --> 00:50:40,920
This distinct 2-3 hertz tone is part of the unique
830
00:50:40,920 --> 00:50:45,320
infrasound signature produced by Stromboli.
831
00:50:45,320 --> 00:50:48,360
It's created when the sound of the explosion
832
00:50:48,360 --> 00:50:52,480
from the base of the crater reverberates around the walls
833
00:50:52,480 --> 00:50:55,080
of one of the volcano's cavernous vents.
834
00:50:55,080 --> 00:50:58,000
This vent acts as a sound resonator,
835
00:50:58,000 --> 00:51:02,120
sculpting the noise of the explosion into a single tone.
836
00:51:02,120 --> 00:51:05,160
So the whole volcano is a giant musical instrument.
837
00:51:05,160 --> 00:51:08,360
The moment of explosion is like the hammer hitting a bell.
838
00:51:08,360 --> 00:51:11,320
That's what starts everything but then the shape of the musical
839
00:51:11,320 --> 00:51:14,600
instrument itself means the sound goes on for a little bit longer.
840
00:51:14,600 --> 00:51:17,520
That's right. And the size of that vent,
841
00:51:17,520 --> 00:51:19,480
how deep it is, how wide it is,
842
00:51:19,480 --> 00:51:22,560
will dictate the tone that is produced by that crater.
843
00:51:25,240 --> 00:51:28,320
Because Stromboli's craters are so big,
844
00:51:28,320 --> 00:51:32,800
the sound they produce is incredibly low-frequency infrasound.
845
00:51:35,080 --> 00:51:38,560
Scientists believe all active volcanoes like Stromboli
846
00:51:38,560 --> 00:51:41,280
have their own unique infrasound signature...
847
00:51:42,520 --> 00:51:46,240
..determined by the shape of the volcano vent acting as a resonator.
848
00:51:48,200 --> 00:51:50,800
And just as for a musical instrument,
849
00:51:50,800 --> 00:51:53,080
if the resonator changes shape,
850
00:51:53,080 --> 00:51:57,040
for example, because lava rises up within the vent,
851
00:51:57,040 --> 00:51:59,720
then the volcano sings a different sound.
852
00:52:06,480 --> 00:52:10,520
This means that we could listen to volcanoes around the world
853
00:52:10,520 --> 00:52:16,240
and, by monitoring their infrasound, better forecast a major eruption
854
00:52:16,240 --> 00:52:19,920
and that would buy precious time for people living nearby to escape
855
00:52:19,920 --> 00:52:21,280
with their lives.
856
00:52:26,720 --> 00:52:29,840
You might think that by the time we've explored the deep notes of
857
00:52:29,840 --> 00:52:33,960
Stromboli, the story of infrasound would have reached its limit.
858
00:52:33,960 --> 00:52:35,960
And yet it hasn't.
859
00:52:42,840 --> 00:52:45,920
To explore the extreme limits of infrasound,
860
00:52:45,920 --> 00:52:48,880
we need to leave our planet behind.
861
00:52:52,720 --> 00:52:54,360
It's long been assumed that
862
00:52:54,360 --> 00:52:56,920
in the emptiness of space there is no sound,
863
00:52:56,920 --> 00:53:00,240
because there's nothing for sound to travel through.
864
00:53:02,520 --> 00:53:04,960
'But, as impossible as it seems,
865
00:53:04,960 --> 00:53:07,680
'infrasound could be playing a fundamental role
866
00:53:07,680 --> 00:53:10,160
'in shaping the structure of the universe.'
867
00:53:13,400 --> 00:53:16,640
We're used to the idea of our busy bustling world down here
868
00:53:16,640 --> 00:53:17,880
being noisy.
869
00:53:17,880 --> 00:53:21,440
But when we look up at the night sky, we assume it's silent.
870
00:53:21,440 --> 00:53:24,840
No-one has ever heard sound from space.
871
00:53:24,840 --> 00:53:28,040
But in this building, there is a man who thinks he has seen it.
872
00:53:31,280 --> 00:53:32,480
'Professor Andrew Fabian
873
00:53:32,480 --> 00:53:34,840
'is an astronomer at the University of Cambridge.'
874
00:53:35,880 --> 00:53:38,880
He uses telescopes to study galaxy clusters,
875
00:53:38,880 --> 00:53:41,920
the largest structures in the universe.
876
00:53:41,920 --> 00:53:45,960
And he's trying to solve a mystery concerning how they grow.
877
00:53:45,960 --> 00:53:48,960
His research has led him to make a surprising discovery.
878
00:53:50,160 --> 00:53:53,680
So, Andy, where is it that you think you've seen sound in space?
879
00:53:53,680 --> 00:53:56,480
We're looking in the consolation of Perseus at what is known
880
00:53:56,480 --> 00:53:58,960
as the Perseus cluster of galaxies.
881
00:53:58,960 --> 00:54:01,800
When you have a cluster like this, which has got an enormous mass,
882
00:54:01,800 --> 00:54:03,880
it tends to...
883
00:54:03,880 --> 00:54:08,840
drag all the matter in and squeeze it and it makes it very hot
884
00:54:08,840 --> 00:54:13,080
and this hot stuff is known as the intra-cluster medium,
885
00:54:13,080 --> 00:54:17,040
is what we study in X-rays with an X-ray telescope.
886
00:54:17,040 --> 00:54:20,680
So in between all the bright galaxies here there is other stuff.
887
00:54:20,680 --> 00:54:22,000
Exactly.
888
00:54:22,000 --> 00:54:26,040
'And it turns out there is more than space than meets the eye.'
889
00:54:26,040 --> 00:54:29,120
Let's go to an X-ray image.
890
00:54:29,120 --> 00:54:31,120
It is completely different.
891
00:54:31,120 --> 00:54:33,440
So it is definitely the same bit of sky we're looking at.
892
00:54:33,440 --> 00:54:36,400
It is the same bit of sky but what we're seeing here is
893
00:54:36,400 --> 00:54:39,640
the gas between the galaxies.
894
00:54:39,640 --> 00:54:43,360
'This intra-cluster medium, shown here in orange,
895
00:54:43,360 --> 00:54:47,800
'is a cloud of gas that blankets the entire Perseus cluster.
896
00:54:47,800 --> 00:54:50,800
'At one particle every few centimetres,
897
00:54:50,800 --> 00:54:55,320
'the gas is far too diffuse to carry sound that we can hear.
898
00:54:55,320 --> 00:55:00,200
'But infra-sound can boldly go where no other sound can.'
899
00:55:00,200 --> 00:55:02,880
What makes you think there is actually sound there?
900
00:55:02,880 --> 00:55:05,720
Well, now we are going to look at the same region
901
00:55:05,720 --> 00:55:08,960
with a specially adapted image from the X-rays.
902
00:55:13,200 --> 00:55:15,880
And what we see is a whole set of ripples.
903
00:55:15,880 --> 00:55:17,240
And they are really clear.
904
00:55:17,240 --> 00:55:18,440
Really clear shapes.
905
00:55:18,440 --> 00:55:23,960
Yes. Where they are bright is where the gas is denser and it looks
906
00:55:23,960 --> 00:55:26,120
very much as though we've got
907
00:55:26,120 --> 00:55:29,760
a pressure wave which is propagating outwards.
908
00:55:29,760 --> 00:55:32,640
In other words, a sound wave.
909
00:55:32,640 --> 00:55:34,600
'If Andy is right,
910
00:55:34,600 --> 00:55:38,240
'what we're looking at is a snapshot of a wave of infrasound,
911
00:55:38,240 --> 00:55:40,800
'travelling through the intra-cluster gas
912
00:55:40,800 --> 00:55:42,280
'of the Perseus cluster.'
913
00:55:44,400 --> 00:55:46,640
So what is the scale of this image?
914
00:55:46,640 --> 00:55:49,120
The spacing between the ripples
915
00:55:49,120 --> 00:55:51,560
is about the diameter of our galaxy.
916
00:55:51,560 --> 00:55:54,720
- So gigantic.
- So it's gigantic.
917
00:55:54,720 --> 00:55:57,840
And if you were to wait on one ripple,
918
00:55:57,840 --> 00:56:00,640
sit there and wait for the next ripple to come past you,
919
00:56:00,640 --> 00:56:03,120
- how long would that take?
- Ten million years.
920
00:56:03,120 --> 00:56:05,440
So you need patience for this game.
921
00:56:05,440 --> 00:56:10,360
- Indeed, yes.
- What could possibly cause ripples of sound that big?
922
00:56:10,360 --> 00:56:13,720
Well, I think it is coming from the centre,
923
00:56:13,720 --> 00:56:15,600
and there there's a massive black hole.
924
00:56:18,840 --> 00:56:22,440
It generates an enormous amount of energy in the material
925
00:56:22,440 --> 00:56:24,320
just before it's swallowed,
926
00:56:24,320 --> 00:56:27,760
and that energy is pushing out into the surrounding gas.
927
00:56:27,760 --> 00:56:30,520
So we think of black holes sucking stuff in,
928
00:56:30,520 --> 00:56:32,440
but the way that material moves around them,
929
00:56:32,440 --> 00:56:35,000
sometimes they can also spit it out.
930
00:56:35,000 --> 00:56:38,520
Indeed. And this could solve one of the problems,
931
00:56:38,520 --> 00:56:41,960
a puzzle that is associated with the centre of these clusters.
932
00:56:41,960 --> 00:56:45,120
These galaxies we're looking at here are the biggest galaxies
933
00:56:45,120 --> 00:56:48,600
in the universe. And they would be yet bigger,
934
00:56:48,600 --> 00:56:50,640
they could be up to ten times bigger
935
00:56:50,640 --> 00:56:54,280
in terms of numbers of stars, if this process was not operating.
936
00:56:57,080 --> 00:56:59,960
These ripples would be the lowest frequency sound
937
00:56:59,960 --> 00:57:04,960
ever detected in the universe - a pure tone of infrasound,
938
00:57:04,960 --> 00:57:09,240
one million billion times lower than the limit of human hearing.
939
00:57:10,640 --> 00:57:12,760
If Andy's theory is correct,
940
00:57:12,760 --> 00:57:17,280
infrasound plays a significant role in controlling the size of galaxies.
941
00:57:26,720 --> 00:57:29,720
The mysterious sounds of a black hole
942
00:57:29,720 --> 00:57:32,360
and the unique voice of a volcano...
943
00:57:35,880 --> 00:57:39,600
..are a fascinating glimpse into a new world of sound,
944
00:57:39,600 --> 00:57:41,360
beyond our human experience.
945
00:57:42,640 --> 00:57:46,760
As we explore more of these exciting soundscapes,
946
00:57:46,760 --> 00:57:51,720
it's clear that sound will become an even more powerful tool for
947
00:57:51,720 --> 00:57:55,080
understanding our world and even our universe.
948
00:58:01,360 --> 00:58:03,720
Next time, I will be investigating
949
00:58:03,720 --> 00:58:08,680
the incredible ways in which we use, control and manipulate sound...
950
00:58:10,320 --> 00:58:12,040
..helping us to survive...
951
00:58:15,960 --> 00:58:18,080
..to explore the world around us...
952
00:58:20,120 --> 00:58:23,440
..and to make the invisible visible.
953
00:58:23,440 --> 00:58:26,040
If you want to find out more about the science of sound
954
00:58:26,040 --> 00:58:28,360
and how we hear sound, go to...
955
00:58:31,080 --> 00:58:33,840
..and follow the links to the Open University.
79400
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