I'm going to do a modern-day version of Chladni's experiment.
This is a Chladni plate.
It's just a flat metal sheet that's held in the middle.
And if I hit it…
(Dull clang)
makes a sound that doesn't sound very pleasant.
It's certainly not nearly as nice as Big Ben.
But that sound has a lot in common with the sound of Big Ben
because it's made up of lots of different frequencies.
And Ernst Chladni came up with a really clever way
of picking apart where that sound comes from.
So he started with a plate like this
and he sprinkled sand on top.
So I'm going to do that.
And then he set the plate vibrating.
And I'm going to do that with a signal generator here
that's going to move the middle of the plate up and down.
And the number on the front
here is a number of times every second that vibration is going to happen.
So at the moment it's 240.
So if I turn this on.
(Harsh humming)
So it's not a pleasant noise.
You can see the sand is dancing about on the plate
but it's not too exciting so far.
But what happens if you turn the frequency up is quite different.
And suddenly at this frequency here – 264 hertz –
you can see this beautiful pattern pops up in the sand on the top of the plate.
And what this is giving away is that the plate is vibrating in a shape
and the sand is showing us what shape that is.
What's happening is that the plate is bending like this
and at the parts of the plate
that are moving a lot, the sand's getting bounced away.
And the parts of the plate that are between a bit
that's going up and a bit that's going down, don't move at all.
And so the sand accumulates in those places.
So what Chladni had found is a really clever trick for seeing the shape of the vibration,
even though he couldn't see it with his eyes.
The vibration pattern revealed by the sand
occurs at what's known as a natural frequency of the metal plate.
This is a specific frequency at which the plate
naturally vibrates and produces sound.
And this is part of what's making up the sound when I hit the plate.
But it's not all of it, because if you keep turning the frequency up,
there's more to see.
And so here we are up at 426 hertz.
And suddenly, out of that mess, there's another pattern
of vibration, beautiful pattern on the plate here.
Chladni's experiment reveals how a simple object –
this metal plate – can produce a complex sound.
Because it doesn't just vibrate at one frequency;
it has many natural frequencies,
each corresponding to a different pattern of vibration
more elaborate than the one before.
When you hit the plate, what happens is that lots of those vibration
patterns all happen at the same time, one on top of the other.
Each one contributes their natural frequency to the mix.
And that combination is what makes up the sound that you hear.
Every object that vibrates has its own combination
of natural frequencies, determined by its physical characteristics.
And together, these frequencies form a unique acoustic signature.
So there's a beautiful relationship
between an object and the sound that it produces.
When you hear a sound, you're hearing messages
about the thing that created it – its size, its shape,
what it's made from, even how the object was made.