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Sound And Water


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Want me to explain? Or do you prefer to work it out?

Please do John.

 

I realise that it has something to do with the resonant frequency of water, but the amount of flow displacement is incredible. The shape even looks like a sine wave. It can’t be though can it?

 

So, (ahem..) spill it!

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Very cool!  I don't think the water is moving backwards/up.  I'm guessing it's an optical illusion like the "wagon wheel effect" when videos of spoked wheels makes it look like the wheels are rotating backwards even as they move forward because the camera speed and rotation speed of the wheel do not sync creating a stroboscopic effect.   I'm guessing here that a similar stroboscopic effect is created based on the speed of the camera recording  vs. the speed of the water wave wiggling caused by the sound from the speaker? 

 

I agree it's not moving backwards/up. I noticed the shadows were moving up as well, which isn't possible unless the water is actually moving up, being pulled from the ground. I think he just reversed the video for that part. The first part is very cool though.

Edited by Just1L
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I think the shadows could be subject to the same stroboscopic effect since they are directly tied to the water's movement.  It's hard to believe that for a "science" video they would just reverse the video for that effect.  I hope they didn't.

 

I hear ya. I'm guessing the camera itself plays a role in this effect.

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Okay,

 

so you know (I hope) that it is an optical illusion. The nozzle is moving up and down, projecting the water either closer or further away... that, in combination with the velocity of the water is what creates the basic wave shape. Add in the effect of gravity and you get the wave form growing in amplitude as it heads towards the ground.

 

Very similar to the effect where you observe a spoked wheel and depending on the speed the wheel rotates at the wheel appears to be either going backwardsm forwards or stays still. The exact speed depends on the distance between the spoke and the frame rate of the video. I understand that the same phenomenon can be observed "live", due to vibration of the eye.

 

In essence it works like this:

 

The frame rate of most video is 30 frames per second.

 

If we look at a sound wave of 30 Hertz, the wave would appear to stand still as the wave would be in exactly the same position in every frame as the sound wave repeats 30 times a second, ie it's repeat period is excatly that of an indiviual frame. So the wave appears to stand still.

 

If you then increase the frequency a little, say to 31 Hz, the wave would appear to go in reverse, ie the water appears to climb. If you go to 29 Hz the wave would appear to go forwards.

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