(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. So if you wanna make sure
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