
This is What Happens When You Run Water Through a 24hz Sine Wave - alexholehouse
http://www.thisiscolossal.com/2013/03/this-is-what-happens-when-you-run-water-through-a-24hz-sine-wave/
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noonespecial
"This is What Happens When You Run Water Through a 24hz Sine Wave"

I'm not sure this is the best title. "What happens when you wiggle a hose at a
frequency close to a camera's frame rate and then film the water coming out of
it" might be better.

At the most basic level, this is just the game you used to play with a hose
when you were a kid by waving the end and producing ribbons of water in the
air. The video just added a camera trick to photograph the ribbon in the same
place during each oscillation appearing to freeze it in place.

Initially I thought this was going to be much cooler, actually using the
speaker to move the air through which the water was travelling to produce an
effect. I was hoping for an awesome standing wave demo or something.

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tolmasky
Do our eyes have a simplistic frame-rate thus allowing us to reproduce the
effect "in real life"?

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jeremyswank
Our eyes don't have a 'frame rate' at all. Light falls on the retina
continuously and is continuously processed, if I understand correctly.

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lutusp
Actually, the human visual system has a particular image repetition frequency,
above which we interpret the input as continuous. This is called the flicker
fusion threshold:

<http://en.wikipedia.org/wiki/Flicker_fusion_threshold>

Now one may ask -- how do movies, running at 24 frames per second, appear to
be continuous, since 24 Hz is below the FFT? The answer is that a movie
projector splits each 24 Hz frame into two frames with a moving shutter,
resulting in an apparent frame rate of 48 Hz, high enough to appear
continuous.

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jeremyswank
I am familiar with flicker fusion, and many of the theories that account for
it, as well as Purkyně's pionering work in these matters. But my sense of it
is that it is a neurological phenomenon, the brain and the eye working
together (happy to be corrected, though).

So I still conclude that light falls continunously on the retina, and is
continuously processed. But, I will further nuance this, by stating that, in
the very contrived and special circumstance of very similar images in
sequence, representing motion, appearing in rapid progression at a certain
controlled rate, are spontaneously perceived as motion (an illusion) by the
brain and the eye working together to make sense of a visual phenomenon that
does not exist in nature, but only under artificial circumstances.

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lutusp
> But my sense of it is that it is a neurological phenomenon, the brain and
> the eye working together ...

Yes, that's correct -- it's both.

> So I still conclude that light falls continunously on the retina, and is
> continuously processed.

Yes, unless it's periodically interrupted, as with a flashing source. In that
case, it's a discontinuous flow of visual information that we must assemble
into something meaningful.

> to make sense of a visual phenomenon that does not exist in nature, but only
> under artificial circumstances.

People are wired by natural selection to fuse a discontinuous series of images
into an apparent continuous sequence on the ground that this makes us more fit
to survive -- so it does exist in nature, it's not artificial.

Consider a primitive man, or one of humanity's predecessors in the long
history of evolution, watching a prey animal running through a forest. Our
distant relative sees the prey animal between two closely spaced trees at one
moment, then sees him between two other trees a bit later. With this limited
information, he is able to infer (a) the prey's speed, and (b) where he will
be five seconds from now, where he intends to be. This is all based on our
ability to ... wait for it ... fuse many discontinuous images together.

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jared314
Petapixel[0] has a better explanation. It is an illusion created by the
synchronized frame rate and oscillation. They have used the same trick with a
strobe light for live effects.

[0] [http://www.petapixel.com/2012/04/24/sound-and-frame-rates-
us...](http://www.petapixel.com/2012/04/24/sound-and-frame-rates-used-to-make-
water-travel-backwards/)

~~~
DanBC
YouTube has a bunch of videos showing strobe light version. Here are two short
videos - the second one shows the effect nicely.

(<http://www.youtube.com/watch?v=OtxlQTmx1LE>)

(<http://www.youtube.com/watch?v=ZRlNOyxWWf8>)

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nostromo
This blew my mind even more because the water looks completely frozen:
<http://www.youtube.com/watch?v=mODqQvlrgIQ>

I'm still amazed that the water comes out so uniformly.

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tedsanders
I bet this would work much better with glycerin or glycerin/water mixtures.
Water has a low viscosity, meaning it's relatively easy to induce turbulence.
And turbulence, while not exactly chaotic, is somewhat random and nonperiodic.

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raghus
Or this <http://www.youtube.com/watch?v=mODqQvlrgIQ>

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justjimmy
Oh come on, you can't show that video w/o filming what happens when you try to
touch the stream!

*The reverse flow is quite mind boggling :O

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alexholehouse
I feel like there's some information theoretic approach which could be
leveraged here, and maybe in similar systems - ie. just from the video we can
work out

\- The difference in video frame rate vs sound frequency based on the period
of the wave

\- Maybe the structure of the wave itself based on the waveform, although
maybe not.

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darkarmani
Nyquist-Shannon:
[http://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_samplin...](http://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem)

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mvanveen
Wait, if I'm interpreting the Shannon-Nyquist theorem properly here, doesn't
that mean since the signal is 24 Hz that we'd want a 48 FPS frame rate to
accurately get the signal?

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j-g-faustus
If you want to reproduce the signal, yes.

But in this case they don't want that. They want the illusion that the water
is standing still, which you get by sampling the signal at the exact same
point in every cycle. So to the sampler/viewer it looks like there is no
signal at all.

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ChuckMcM
A _much_ more interesting effect is putting water and cornstarch on a woofer
connected to an oscillator [1]. No special camera tricks required!

[1] <http://www.youtube.com/watch?v=3zoTKXXNQIU>

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kbenson
It seems someone else was reading What If[1] today as well...

[1]: <http://what-if.xkcd.com/36/>

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seanalltogether
Do you have to do something special with the camera to turn off motion blur
and simply take snapshots in order to achieve this effect?

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panic
Kind of. From the description in the YouTube video:

 _Set up your camera and switch it to 24 fps. The higher the shutter speed the
better the results. But also keep in the mind that the higher your shutter
speed, the more light you need._

Motion blur isn't something you "turn off" -- it's the natural effect of light
being collected over time. The faster the shutter speed, the less time the
camera spends collecting light each frame. That means both less motion blur
and a darker image.

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helloamar
Awesome example. Lot of information posted on this comment, that's why I love
HN.

