Original subtitles

Breaking the sound barrier.

What does that actually mean?

Sound's one of those weird, everyday things

that we all know about but no one really understands.

Well, one thing I do know is that if you're

trying to break the sound barrier,

you need to be going really fast.

So I'm thinking that this might be a good place to start.

(upbeat music)

(jet plane whooshes)

In this episode, I'm going to attempt to break

the sound barrier and create a sonic boom.

And I'll try to break glass using

just the sound of the human voice.

(graphics whirring)

I've always been fascinated with how the world works.

And I've got lots of questions about why things happen.

To find out the answers, I'm gonna blow stuff up.

That's 370.

That's 390.

Oh my God.

(upbeat techno music)

(jet plane whooshes)

We're currently traveling at just under 400 knots.

That's, that's just under 700 kilometers per hour.

But if I'm gonna break the sound barrier,

that's 1,200 kilometers per hour.

So I'm gonna have to figure out a way to go a bit faster.

(chuckles) Ah!

We take sound for granted, it's just always there.

But it wasn't until recently

that we began to understand it.

Ernst Chladni was an 18th century

musician and scientist.

And he was the first to prove something

that had previously only been a theory.

The slightly weird idea that sound is a wave,

just like the waves at a beach.

What I'm going to do is set the tone to a low frequency,

and that's playing through the speaker

with a metal plate on top.

And now I'm gonna sprinkle some salt

onto the metal plate and watch what happens.

(bright music)

What this experiment proves is that sound travels in waves.

These waves physically move through the air,

and each of these different sounds

that I'm creating makes a unique sound wave.

And so every time I make a new sound,

(tone ringing)

a new sound wave creates a new pattern in the salt.

Every frequency produces a different pattern.

It's kind of like playing a musical instrument.

There's no real purpose to these

patterns produced in the salt,

but it's just oddly, strangely beautiful.

And a little painful.

(tone ringing)

(footsteps crunching)

Sound waves travel through the air and vibrate our eardrums.

Like the sound of these footsteps.

But interestingly, we can't all hear the same waves.

To see how this works, it's time to go back to school.

In this classroom, we've set up a student's phone

to receive texts with a ringtone of 16,000 hertz,

which is a very, very high-pitched sound.

Yvonne is young, so she should be

able to hear high frequencies.

But in theory, her teacher Mr. Bogaart, in his 50s,

won't be able to hear the sound

the text makes when it arrives.

Right, let's see if the experiment works.

(phone beeping)

(cheerful music)

People, Fijian people, Maori people.

Yvonne heard the high frequency tone no problem.

But why is it that a lot of you,

like Mr. Bogaart, heard nothing at all?

And who have a left out?

Human hearing is within the range of 20 to 20,000 hertz.

So what you're listening to at the moment is a whiny sound

which is becoming higher and higher pitched.

But, as you age, you lose the ability

to hear those high frequencies.

And that's because ironically, even though

over the course of your life your ears become hairier,

you actually damage the small hairs inside your ears

with things like too much Def Leppard

when you're a teenager, and screaming

children when you're a parent.

So, at 8,000 hertz (radio squealing)

most of us can still hear it.

But some of you, in your 70s, won't be able

to hear this, just the sound of my voice.

At 12,000 a lot of people in their 50s have dropped out.

At 14,000 hertz a lot of 40-somethings have dropped out.

15,800 hertz, that's me gone, I can't here a thing.

At 18,000 hertz, the only people who can

probably still hear this will be 20 or below.

I could keep going higher and higher,

but your TV speakers won't put out

any more than about 20,000 hertz.

So there would be no point

because the only thing it would do is annoy your cat,

which can hear up to about 64,000 hertz.

(bright music)

(gear clicking)

If I'm going to break the sound barrier,

I'm going to need to figure out the speed of sound.

Thought someone else would have done

this before we got here, that's all right.

Because breaking the sound barrier is shorthand

for traveling faster than sound itself.

Okay, so that's 600 meters from our start

position to our final flag position.

Now all I need is a bunch of little kids with flags.

(cheerful music)

All right.

So if you all sort of form up about here,

and we'll give you a science briefing.

Bring it over.

All right, let's just huddle up around here.

Okay everyone, listen up.

So what we're gonna do is we're gonna

measure the speed of sound.

So line up five meters apart,

take a flag or make a loud noise.

You'll hold the flag up.

With 150 kids and me in charge,

this has now become an experiment in chaos.

(children chattering)

All right, we're basically just getting them

into the group position has proven difficult.

(laughs) We'll try a line in a minute.

♪ Stop, look and listen. ♪

♪ Stop, look and listen. ♪

We need some child psychology experience,

but as soon as I start talking

the teacher just rolls his eyes,

and then he steps in and takes over.

So, the experiment's simple.

I'll make a loud noise, and the kids will raise

their flags when they hear the noise,

and we should be able to witness

sound traveling over 600 meters.

Now, all of the children to form a line with their flags,

and all I have to do is make a really loud noise.

(upbeat music)

(cymbals crashing)

600 meters away, the flag stays down.

Sound travels by causing air molecules

to vibrate back and forth.

When I hit the cymbals, the molecules in front of them

bump into the one in front, and into the next,

and the next, and so on.

But each bump causes a loss of energy.

Sound is just energy on a journey.

But like all journeys, it has to come to an end.

Just like ours did about halfway down that line.

So I'm gonna need something with a lot more energy.

(air horn blares)

Success, the last flag goes up.

So the maths is pretty basic.

If we divide the distance, 600 meters,

by the time it took, 1.75 seconds,

we get the speed of sound at 343 meters per second.

And yeah, I know our line was a bit wonky,

but let's not split hairs.

So we know that sound travels at 343 meters per second.

But what's the sound barrier, and how do we break it?

Well, I can show you that using

this Ford Falcon and a siren.

All right, Gary?

(siren wails)

So, the sound's quite consistent at the moment,

but something really weird happens

when the vehicle's moving at speed.

We've set up a microphone about

halfway down this dragway track,

and this will capture what's known as the Doppler effect.

Traveling in this completely outrageous car,

the sound I hear remains constant.

But at the microphone midway down the track,

the siren sounds very different.

(car engine rumbling) (siren wailing)

That's the Doppler effect.

(upbeat rock music)

When the car is stopped, the sound waves

spread out equally in all directions.

But as the car moves, the waves at the front

are squeezed together, and those behind

become further apart.

The closer together, the higher the pitch.

(siren wailing) (car engine rumbling)

So what's this got to do with breaking the sound barrier?

(siren wails)

Well, if this car could go faster,

say 1,200 kilometers per hour,

it would burst through the sound waves,

producing a powerful shockwave as the squashed air

behind it rapidly spreads out, the so-called sonic boom.

(siren wailing)

Next, I'll try to make my own sonic boom.

Hopefully without destroying some very expensive equipment.

And then I'll use sound to try and break stuff.

(logo whirring)

I'm trying to break the sound barrier.

And it turns out one way to do it

is with this, cowboy style. (whip cracks)

The first thing that ever broke

the sound barrier was probably one of these.

Although some paleontologists believe

it may actually have been a dinosaur,

a diplodocus whipping her giant tail.

If I could crack the whip, the sound you'd hear

is a small sonic boom, as the very tip

breaks the sound barrier.

It's pretty simple really.

I'm gonna crack the whip in front of this board,

which will measure the distance it travels,

and then we'll record it with this camera,

which lets you see things in super slow motion.

Like this, and this,

and this.

(water splashes)

And hopefully, the supersonic crack of the whip.

(bright music)

Right, well there's a couple of things

that are immediately clear to me.

One, a whip is a terrifying bit of equipment to use.

And two, this isn't working.

So I need a different plan.

Graham Hardy first picked up a bullwhip as a kid.

And maybe he can help us break the sound barrier,

or at the very least, crack the whip.

(whip cracks)

(upbeat rock music)

Here's my cunning plan, Graham.

My theory is, it's all about

a decent target.

We have decided to use a balloon as a target for Graham,

because when the tip of the whip

breaks the sound barrier, it will also pop the balloon.

Which means we can find the exact moment in the footage.

Great stuff, thanks very much.

(whip cracks)

(whip cracks)

Now, when the balloon bursts,

we know that's approximately where the sonic boom occurred.

Gian Schmidt is a sound expert

who'll be helping us decipher the images.

It burst the balloon, looks fantastic.

I think we're just too close.

So it looks fantastic, but scientifically

of little value to us.

That's right, we just can't see

where the whip's cracking.

Okay, so we need another one but wider?

(gentle music)

(whip cracks)

That close enough?

And that's why you never mix whips and water balloons.

(whip cracks)

(upbeat music)

Right, well this looks good to me.

But Gian spots a flaw with the shot.

See that one, again, that looked fantastic,

and he clearly hits the balloon.

Look at that.

There's still no crack, so it's obviously

not breaking the sound barrier there.

So it looks great, but again,

because we didn't see the crack,

you can't measure the speed, and so, scientifically,

not of much value, do another one.

Okay, great. Okay, all right.

We'll do one more.

(whip cracks)

Okay, that's got to be it, a good solid crack.

Our whip needs to be traveling at 343 meters per second

to break the sound barrier.

With our perfect shot, we can see that it's taking

the whip six frames to move through

our five centimeter square.

And because we know the camera is filming

10,000 frames per second, we get 85 meters per second,

well short of the speed of sound.

It's so much energy being released at that point.

So what we can see there, Gian,

is as it's dropping down through the squares,

you're saying the very end butt

is moving at greater than the speed of sound.

The main part of the whip itself is not moving

as fast as the speed of sound.

It's that the end cracks.

Gian thinks the tip breaking the sound barrier

is in between the frames, and so we're only capturing

the slower moving body of the whip.

We heard the sound barrier being broken,

but we certainly didn't see it.

It's time to get a bit Rambo.

Surely there must be a more impressive way

to break the sound barrier.

Turns out there is, and all you need

is the Thames Valley Pistol Club.

For most of my career as a clinical psychologist,

I didn't spend a lot of time firing guns.

But in the name of science, I'm willing to give it a go.

(bright music)

This is a .22 caliber rifle.

And it's the kind of thing that people use

for shooting rabbits on a farm or target shooting.

The way that I'm gonna measure the speed of the bullet

is with a ballistic chronograph.

And what happens is you fire the bullet

through these three sensors,

and that tells me how fast it's going.

The only thing I have to be particularly

careful about is not to shoot

the delicate and expensive scientific instruments.

(bright music)

(gun bangs)

(printer squeals)

So the chronograph says 313 meters per second.

But that's still 32 meters per second too slow.

So I'm going to need a bigger gun.

(gun bangs)

Jesus!

That's not just, that's a cannon!

It moved my jeans.

It was like (imitates gun banging).

Richard Munt, the owner of this monster,

makes sure it's accurately sighted.

And now it's my turn.

Once again, we've set up the chronograph

to measure the speed of the bullet.

The .22 didn't break the sound barrier.

But this thing was designed to put a bullet

through an engine block two kilometers away.

So I'm gonna fire it, and we'll see

if we can pick up that sonic boom.

(gun bangs)

If I'm going to be shooting a massive gun,

I kinda want to look like an action hero.

Not pulling a stupid face.

And there's a fly mucking around.

So we're going to do it again.

Not in the name of science, but in the name of vanity.

(gun bangs)

(gun bangs)

We're hoping this bullet will travel faster

than the speed of sound, punching through

its own sound waves and creating a sonic boom.

Or, to be more accurate, given it's a small piece

of metal and not a jet plane, a sonic crack.

That was more like it.

Let's check the chronograph.

And remember, our target is 343 meters per second

if we want to break the sound barrier.

So that's 953 meters per second,

which is almost three times the speed of sound.

So we've broken the sound barrier,

but we can't hear it because the gun's too loud.

The solution, use a gun with a silencer,

which will mute the sound of the bullet firing,

meaning we should be able to hear

the sonic boom of the speeding bullet.

(gun bangs)

(bullet whooshes)

That was 615 meters per second.

Even with a smaller gun, the bullet is traveling

at twice the speed of its own sound.

Now we put a microphone at the target

to help capture that sonic crack.

(gun bangs)

That's a sonic boom.

It's not dramatic, but it's still pretty cool.

That crack is caused by the sound waves

collapsing behind the bullet

which has broken through the sound barrier.

It may not be quite as grand as a supersonic jet

breaking windows on the ground,

but the science is exactly the same.

Now, I don't have a supersonic jet.

But can we use sound to break stuff in other ways?

(opera singer warbles)

(logo whirring)

(upbeat music)

If we're making a show about blowing stuff up,

can you do something really destructive with sound?

Can you break stuff?

Turns out that you can.

A glass has its own natural resonance.

And so this one,

(glass chimes)

it's 787.5 hertz.

So all I have to do is produce that same frequency

and make it loud enough, and in theory,

the glass should break.

Right, so we'll use our tone generator

to get the right frequency, 787.5 hertz.

And then what I'm gonna do

is I put the straw into the glass

'cause when that starts moving,

that will tell me that the glass is starting to resonate.

And that sound is all coming out

of our speaker with this very high-tech

short piece of PVC piping, which is going to send

all the sound directly at the glass.

(tone generator ringing)

So now, glass explosion.

Our lab boys did a day of testing trying to make this work,

and never managed to break a glass.

So I'm doubtful this will actually work.

(tone generator ringing)

Right, so we're getting quite a bit

of straw movement in there,

which means it's really starting to resonate.

So, if we just turn it up a bit,

then hopefully it should work.

(tone generator ringing)

(glass shatters)

(Nigel laughs)

(gently chiming music)

And that's how you do it.

Good God! (laughs)

I was not expecting that, but it was oddly satisfying.

This is probably how evil geniuses

get started with death rays.

Right, so that worked.

But I wanna take it to the next level.

We're going to need an opera singer.

(opera singer warbles)

We see opera singers breaking glass

with their voice in cartoons all the time.

But can it actually happen in real life?

Andrea Monroe is a professional opera singer,

but she's never broken a glass with a high note.

All right Andrea, so what I'm gonna do

is I'll give you a guide to kind of frequency,

the one we've measured for this glass.

And because it's all about safety, safety, safety,

I'll give you these goggles so that when

it explodes you don't go blind.

And then I'll leave it up to you.

Okay.

(opera singer warbles)

Andrea must hit the right frequency,

and hit it with 100 decibels.

The theory is that the frequency

of her voice will produce sound waves

which vibrate the glass so violently it'll break.

Well, I'm vibrating, I don't know why the glass isn't.

Should we try the straw, and see if that gives

you a guide about how much vibrating's going on?

(opera singer warbles)

That was 104 decibels.

The power in Andrea's voice was astounding.

It was definitely vibrating.

And the straw head started to move.

I wonder if, when you're holding it,

that's changing the way it vibrates.

Could be.

The glass may not be exploding at this point,

but she was so loud my ears were trying

to crawl back into my head.

(opera singer warbles)

So Andrea, like, we're definitely getting the volume.

I wonder if it's a different glass.

Okay.

I have this glass, which has lived a slightly harder life,

and is a bit more world weary.

And maybe we'll try that, and just try some different tones.

Sure.

(opera singer warbles)

(both laughing)

It should break.

Try that.

(opera singer warbles)

(glass shatters)

It cracked. Whoa!

(all laughing)

Whoo!

Nice work.

(all applauding)

Nice work. Oh my God.

[Nigel So breaking the sound barrier is pretty easy.

As long as you don't lose an eye.

But perhaps even better than that,

I also discovered that you can break stuff with sound,

and you can use an opera singer as a death ray.

As long as all you want to do is destroy wine glasses.

It cracked. Whoa! (laughs)

(bright music)