The hose moves in all examples - but you can't see it moving in the first example - what you see is a result of basically a different form of stroboscope behavior.

The video camera has a 24 fps rate - it is taking 24 still shots per second. The reason you don't see the hose moving in the first example is that the subwoofer is moving it at exactly the same rate as the frame rate of the camera - so at 24 Hz, the position of the hose is precisely the same place for each still shot so it appears that it is not moving, just like using a strobe timing light on a the crank pulley on a car - it looks like the mark isn't moving, but that's only because you see it at the same location on each rotation due to the srobe light.

Now, when you slow things down a bit, then the hose doesn't quite make it to the same location it did for the previous shot and you see what is an aliased image of a slow moving wave towards the hose. Speed it up faster than 24 Hz, and the opposite is the case.

What this demonstrates is a visual analogy to sampling aliasing in audio - so it totally belongs in this forum.

Also similar to when you used to see CRT flashing with slow moving images up or down the screen, with film shots of said CRT - the slight mismatch of the CRT refresh rate with the frame rate of the camera would alias and yield those images. Likewise when wheels on cars are filmed and it appears as if the wheel is slowly rotating compared to what the speed should actually look like. All the same phenomenon.

-Scott.

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