(whistles)
- This is a bird water whistle, or a wobbling whistle.
And it sounds just like a normal whistle (whistles)
until you put water in it.
And then it sounds like this (whistles)
It kind of sounds like a bird.
I mean, I dunno what bird, but it's giving bird vibes.
I wanted to understand how the thing works.
So, of course, I built a 2D transparent version
because that's what I do in these scenarios.
Interestingly, I think the 2D version does a good job
of demonstrating how the water turns it
from a normal whistle into a bird whistle.
And I'll demonstrate that soon.
But what it doesn't do
is explain how whistles in general work.
And I really wanted to understand that.
And man, it turns out like there's a lot to say
about how whistles work.
It's not massively straightforward.
For a start, there's more than one type of whistle.
Like with this, it's the sort of standard whistle.
You know, you'd look at that and you go, oh, it's a whistle.
It's what you see on certain wind instruments.
Like this slide whistle (whistles)
You've got like a hole there.
And then there's a sort of wedge-shaped thing.
And you've got this thing channel that directs the air
onto the edge of that wedge shaped bit.
But it's not the only type of whistle.
I'm gonna go out on a limb and say,
that's probably the most common type of whistle,
but there are reasonably common whistles
that work in a completely different way.
The most surprising one for me is a kettle whistle.
So stove top kettles need some kind of thing
to indicate that the water has boiled
unlike an electric kettle, that turns off by itself.
I made a whole video about that
where I tried to boil pure alcohol in a kettle.
But anyway, (whistles)
Now the weird thing is that it's so simple, right?
It's just two holes, one in front of the other.
So you've got this disc here with a hole in it.
You've got this disc here with a hole in it, and that's it.
And it makes the listening sound.
And clearly, it's a very different mechanism
to what I'm gonna call standard whistle setup
because well, for one thing,
the pitch increases steadily with flow rate.
(whistles)
Which isn't true for a standard whistle.
(whistles)
The pitch does go up as I increase the airflow,
but it goes up stepwise.
It's transitioning between harmonics of the instrument.
And then, of course, there's this type of whistle,
helm halts resonator, and a really weird one is this one
(whistles)
Which doesn't work like any of the other ones,
you might call it a worldly tube.
But my favorite name for it is the Corrugaphone,
it's corrugated.
And it makes a sound for the Corrugaphone.
I want to explain the kettle whistle first
because I think it's like the first step
to understanding the standard whistle.
So it turns out that when you pass air through a single hole
so you get a jet of air, that's inherently unstable
and actually with nothing else going on,
you might find that the Airstream from a hole,
just a single hole not a double one like this,
just from a single hole might start to oscillate up
and down.
Why would that happen?
Here's a sort of hand wave the explanation
of why a jet come from a hole may oscillate up and down.
The airstream that leaves the hole,
I'm representing with a spring here
and to buy some random perturbation,
you might imagine that the Airstream could move over
to one side slightly.
So then the air pressure is lower here than it is here
cause the air is being stretched here and squashed here.
And we know that air moves from areas of high pressure
to low pressure,
so we should expect the stream to switch over
to the other side.
It's carried past the midpoint by its momentum.
So now the areas of high and low pressure have switched.
So we should expect the air stream
to move over to the other side again.
It's gonna go backwards and forwards
and perhaps by some positive feedback, the aptitude goes up
until some equilibrium is reached.
In other words, boiling,
that's my intuitive explanation anyway.
The truth is gonna be more complicated than that.
So air passes through this hole,
which creates an oscillating Airstream on the other side
that oscillating airstream then impinges on this hole
on the way out, and that amplifies the effect.
So you've got an Airstream coming out of here
that is oscillating wildly, not just in direction,
but in strength as well as the airstream
that's impinging on that hole is moving side to side.
That's the principle of it.
And the frequency of the sound
depends on the speed of the jet.
(whistles)
With a traditional whistle, the setup is slightly different.
So you have this jet of air, you know,
you blow into that hole,
the jet of air comes out of the other end of that hole,
but instead of impinging on another hole
like it does with a kettle whistle, impinges on a wedge,
interestingly, you don't need
the rest of the instrument here.
Like if you have a jet of air
and you put a wedge in front of it,
you can get a whistling sound.
Pairing the oscillation of the airstream coming from a hole
with a wedge shape in front of the airstream,
drastically increases the amplitude
of the sound that you get.
So there'll be a slight oscillation in the airstream
that hits the wedge and is amplified by vortex shedding
causing the airstream to move up over the wedge.
then the impinging airstream moves down a little
and so it flips to the underside of the wedge
and it keeps flipping back and forth like that.
When you have just an airstream and a wedge, again,
the pitch is dependent on the geometry of the hole,
the speed of the air and the geometry of the wedge.
That's based on literature, by the way,
I attempted to make a sound by blowing air over a wedge
and I wasn't able to, but what I read in the literature
was that actually a typical instrument blowing speeds
that frequency generated from just a hole and a wedge
is way above the frequency of what humans can hear.
And it's also quite quiet, but if you couple a hole, a wedge
and a cavity, like in this case,
then you get an audible pitch and it's much louder.
And it turns out that when you have this setup,
the three things all together,
actually the pitch is no longer dependent
on the speed of the airflow
and the geometry of the hole and the wedge,
it's now dependent on the geometry of the cavity
that you've coupled it with,
which is how the slide whistle can be so effective
at accompanying someone falling over.
(whistles)
When there's a cavity attached,
the air inside feeds back to the wedge.
So if the airstream happens to be directed below the wedge,
then the pressure increases inside the cavity.
But the air inside is an elastic medium, it's springy.
So when the pressure built up,
it pushes back and that causes the airstream
to be diverted above the wedge.
But then if the airstream's to be diverted above the wedge,
that lowers the pressure in the cavity.
And again, because it's an elastic medium, it's springy,
it's gonna pull back.
And so the airstream gets pulled to below the wedge again,
that keeps happening back and forth
and you hear that as a whistling sound.
So the air inside of the cavity
has a natural resonating frequency
which is down to the shape and size of the cavity
and the springiness of the air
and that's the frequency that you hear.
It's a bit like the air inside this syringe here.
Look, it's got this natural bounciness to it,
a natural frequency to it, the same thing, oh, gosh.
The same thing happens with this whistle,
but how does the water change things?
Let's see what happens with the 2D transparent version.
(whistles)
So the air that you blow into the whistle
not only activates the whistle itself,
but it also pushes against the water that's in the vessel
and so you end up blowing bubbles through the vessel
and as the Airstream bubbles through the liquid,
it's constantly changing the shape of the cavity.
And in the case of a whistle using this design,
the shape and size of the cavity is crucial.
It's the shape and size of the cavity
that defines the frequency.
It's the resonating frequency of the air in that cavity
that you hear when you blow one of these whistles.
And so that's why you get this sometimes predictable,
sometimes chaotic variation in pitch.
It depends entirely on the movement of the water
as you bubble air through it.
It actually reminds me a little bit
of how a sports whistle works like this one.
Ignoring the ball for a second.
These whistles have quite a nice explanation.
You can imagine a jet of air passing underneath the wedge
being directed in this circular fashion,
around the inside of the whistle
and then hitting the incoming stream.
And when it does that, it interrupts the stream.
But, of course, once the stream is interrupted,
there's nothing to interrupt it
because there's nothing to travel around the inside.
And so the airstream comes back again.
When it comes back, it travels around the loop
and it interrupts the airstream.
This repeated interruption of the airstream
is the tone that you hear.
Interestingly, that means that the pitch of the whistle
depends on the flow rate.
Like the faster you blow air through the whistle,
the more frequently it will interrupt itself.
(whistles)
Which means, again, this whistle is different
in the way it operates to a standard whistle.
When you add the ball, well,
that repeatedly interrupts the whistle itself.
So you get this trill sound, you know, on closer inspection
maybe the sports whistle doesn't have that much in common
with the water whistle,
except the fact that the internal geometry
is always changing for both whistles.
But I mean, I'd made a transparent version
and I wanted to show you.
So, yeah, that's how a bunch of different whistles work.
On the subject of interesting whistles,
I think I figured out what's going on (whistles)
in one of these things
and I didn't have to smash it to figure it out
which is a bonus.
I mean the process of making the transparent 2D version
at the moment, I reckon that video will be out
in maybe like three weeks, four weeks.
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